View source for Bacterial motility

{{Short description|Ability of bacteria to move independently using metabolic energy}}
{{good article}}
{{Use British English|date=August 2021}}
{{Use dmy dates|date=August 2021}}
{{microbial and microbot movement|taxa}}

'''Bacterial motility''' is the ability of [[bacteria]] to [[motility|move independently]] using [[metabolic]] energy. Most motility mechanisms that evolved among bacteria also evolved [[Parallel evolution|in parallel]] among the [[archaea]]. Most rod-shaped bacteria can move using their own power, which allows colonization of new environments and discovery of new resources for survival. Bacterial movement depends not only on the characteristics of the medium, but also on the use of different appendages to propel. Swarming and swimming movements are both powered by [[flagella|rotating flagella]]. Whereas [[Swarming motility|swarming]] is a multicellular 2D movement over a surface and requires the presence of [[surfactant]]s, [[#Swimming|swimming]] is movement of individual cells in liquid environments.

Other types of movement occurring on solid surfaces include twitching, gliding and sliding, which are all independent of flagella. [[Twitching motility|Twitching]] depends on the extension, attachment to a surface, and retraction of [[type IV pili]] which pull the cell forwards in a manner similar to the action of a grappling hook, providing energy to move the cell forward. [[Gliding motility|Gliding]] uses different motor complexes, such as the [[focal adhesion]] complexes of ''[[Myxococcus]]''. Unlike twitching and gliding motilities, which are active movements where the motive force is generated by the individual cell, [[Sliding (motion)|sliding]] is a passive movement. It relies on the motive force generated by the cell community due to the expansive forces caused by cell growth within the colony in the presence of surfactants, which reduce the friction between the cells and the surface. The overall movement of a bacterium can be the result of alternating tumble and swim phases. As a result, the trajectory of a bacterium swimming in a uniform environment will form a [[random walk]] with relatively straight swims interrupted by random tumbles that reorient the bacterium.

Bacteria can also exhibit [[taxis]], which is the ability to move towards or away from stimuli in their environment. In [[chemotaxis]] the overall motion of bacteria responds to the presence of chemical gradients. In [[phototaxis]] bacteria can move towards or away from light. This can be particularly useful for [[cyanobacteria]], which use light for [[photosynthesis]]. Likewise, [[magnetotactic bacteria]] align their movement with the [[Earth's magnetic field]]. Some bacteria have [[escape reaction]]s allowing them to back away from stimuli that might harm or kill. This is fundamentally different from navigation or exploration, since response times must be rapid. Escape reactions are achieved by [[action potential]]-like phenomena, and have been observed in [[biofilm]]s as well as in single cells such as [[cable bacteria]]. 

Currently there is interest in developing [[#Biohybrid microswimmers|biohybrid microswimmers]], microscopic swimmers which are part biological and part engineered by humans, such as swimming bacteria modified to carry cargo.

==Background==
In 1828, the British biologist [[Robert Brown (botanist, born 1773)|Robert Brown]] discovered the incessant jiggling motion of [[pollen]] in water and described his finding in his article "A Brief Account of Microscopical Observations…",<ref>{{cite journal |doi = 10.1080/14786445208647098|title = XXIV. On some salts and products of decomposition of pyromeconic acid|year = 1852|last1 = Brown|first1 = James F.|journal = The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science|volume = 4|issue = 24|pages = 161–168}}</ref> leading to extended scientific discussion about the origin of this motion. This enigma was resolved only in 1905, when Albert Einstein published his celebrated essay ''[[Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen]]''.<ref>{{cite journal |doi = 10.1002/andp.19053220806|title = Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen|year = 1905|last1 = Einstein|first1 = A.|journal = Annalen der Physik|volume = 322|issue = 8|pages = 549–560|bibcode = 1905AnP...322..549E|doi-access = free}}</ref> Einstein not only deduced the diffusion of suspended particles in quiescent liquids, but also suggested these findings could be used to determine particle size — in a sense, he was the world's first micro[[rheologist]].<ref name=BastosArrieta2018 />

Ever since Newton established his equations of motion, the mystery of motion on the microscale has emerged frequently in scientific history, as famously demonstrated by a couple of articles that should be discussed briefly. First, an essential concept, popularized by [[Osborne Reynolds]], is that the relative importance of [[inertia]] and [[viscosity]] for the motion of a fluid depends on certain details of the system under consideration.<ref name=BastosArrieta2018 /> The [[Reynolds number]] {{math|''Re''}}, named in his honor, quantifies this comparison as a [[dimensionless ratio]] of characteristic inertial and viscous forces:

[[File:Edward_Mills_Purcell.jpg|thumb|upright|{{center|E. M. Purcell}}]]
[[File:Purcell’s swimming scallop with reciprocal motion.png|thumb|{{center|'''Purcell’s swimming scallop'''<br /><small>"Fast or slow, it exactly retraces its trajectory and it's back where it started".<ref name=Purcell1977 /></small>}}]]

:<math>\mathrm{Re} = \frac{\rho u l}{\mu}</math>

Here, {{math|''ρ''}} represents the density of the fluid; {{math|''u''}} is a characteristic velocity of the system (for instance, the velocity of a swimming particle); {{math|''l''}} is a characteristic length scale (e.g., the swimmer size); and {{math|''μ''}} is the viscosity of the fluid. Taking the suspending fluid to be water, and using experimentally observed values for {{math|''u''}}, one can determine that inertia is important for macroscopic swimmers like fish ({{math|''Re''}} = 100), while viscosity dominates the motion of microscale swimmers like bacteria ({{math|''Re''}} = 10<sup>−4</sup>).<ref name=BastosArrieta2018 />

The overwhelming importance of viscosity for swimming at the micrometer scale has profound implications for swimming strategy. This has been discussed memorably by [[E. M. Purcell]], who invited the reader into the world of microorganisms and theoretically studied the conditions of their motion.<ref name=Purcell1977>{{cite journal |doi = 10.1119/1.10903|title = Life at low Reynolds number|year = 1977|last1 = Purcell|first1 = E. M.|journal = American Journal of Physics|volume = 45|issue = 1|pages = 3–11|bibcode = 1977AmJPh..45....3P}}</ref> In the first place, propulsion strategies of large scale swimmers often involve imparting [[momentum]] to the surrounding fluid in periodic [[Wiktionary:discrete|discrete events]], such as [[vortex shedding]], and coasting between these events through [[inertia]]. This cannot be effective for microscale swimmers like bacteria: due to the large [[viscous damping]], the inertial coasting time of a micron-sized object is on the order of 1 μs. The coasting distance of a microorganism moving at a typical speed is about 0.1 [[angstrom]]s (Å). Purcell concluded that only forces that are exerted in the present moment on a microscale body contribute to its propulsion, so a constant energy conversion method is essential.<ref name=Purcell1977 /><ref name=BastosArrieta2018 />

Microorganisms have optimized their [[Microbial metabolism|metabolism]] for continuous energy production, while purely artificial microswimmers (microrobots) must obtain energy from the environment, since their on-board-storage-capacity is very limited. As a further consequence of the continuous dissipation of energy, biological and artificial microswimmers do not obey the laws of equilibrium statistical physics, and need to be described by non-equilibrium dynamics.<ref name=BastosArrieta2018 /> Mathematically, Purcell explored the implications of low Reynolds number by taking the [[Navier-Stokes equation]] and eliminating the inertial terms:

:<math> \begin{align} \mu \nabla^2 \mathbf{u} -\boldsymbol{\nabla}p &= \boldsymbol{0} \\ \end{align}</math>

where <math>\mathbf{u}</math> is the [[velocity]] of the fluid and <math>\boldsymbol{\nabla} p</math> is the gradient of the [[pressure]]. As Purcell noted, the resulting equation — the [[Stokes flow|Stokes equation]] — contains no explicit time dependence.<ref name=Purcell1977 /> This has some important consequences for how a suspended body (e.g., a bacterium) can swim through periodic mechanical motions or deformations (e.g., of a [[flagellum]]). First, the rate of motion is practically irrelevant for the motion of the microswimmer and of the surrounding fluid: changing the rate of motion will change the scale of the velocities of the fluid and of the microswimmer, but it will not change the pattern of fluid flow. Secondly, reversing the direction of mechanical motion will simply reverse all velocities in the system. These properties of the Stokes equation severely restrict the range of feasible swimming strategies.<ref name=Purcell1977 /><ref name=BastosArrieta2018 />

As a concrete illustration, consider a [[Spherical cow|mathematical]] [[scallop]] that consists of two rigid pieces connected by a hinge. Can the "scallop" swim by periodically opening and closing the hinge? No: regardless of how the cycle of opening and closing depends on time, the scallop will always return to its starting point at the end of the cycle. Here originated the striking quote: "Fast or slow, it exactly retraces its trajectory and it's back where it started".<ref name=Purcell1977 /> In light of this [[scallop theorem]], Purcell developed approaches concerning how artificial motion at the micro scale can be generated.<ref name=BastosArrieta2018 /> This paper continues to inspire ongoing scientific discussion; for example, recent work by the [[Peer Fischer|Fischer group]] from the [[Max Planck Institute for Intelligent Systems]] experimentally confirmed that the scallop principle is only valid for [[Newtonian fluid]]s.<ref>{{cite journal |doi = 10.1038/ncomms6119|title = Swimming by reciprocal motion at low Reynolds number|year = 2014|last1 = Qiu|first1 = Tian|last2 = Lee|first2 = Tung-Chun|last3 = Mark|first3 = Andrew G.|last4 = Morozov|first4 = Konstantin I.|last5 = Münster|first5 = Raphael|last6 = Mierka|first6 = Otto|last7 = Turek|first7 = Stefan|last8 = Leshansky|first8 = Alexander M.|last9 = Fischer|first9 = Peer|journal = Nature Communications|volume = 5|page = 5119|pmid = 25369018|pmc = 4241991|bibcode = 2014NatCo...5.5119Q}}</ref><ref name=BastosArrieta2018 />

[[File:Changes in swimmer velocity and Reynolds number with length scale (trimmed).jpg|thumb|upright=2| {{center|Changes in speed and Reynolds number with length of swimmer{{hsp}}<ref name=BastosArrieta2018>{{cite journal |doi = 10.3389/frobt.2018.00097|title = Bacterial Biohybrid Microswimmers|year = 2018|last1 = Bastos-Arrieta|first1 = Julio|last2 = Revilla-Guarinos|first2 = Ainhoa|last3 = Uspal|first3 = William E.|last4 = Simmchen|first4 = Juliane|journal = Frontiers in Robotics and AI|volume = 5|page = 97|pmid = 33500976|pmc = 7805739|doi-access = free}} [[File:CC-BY icon.svg|50px]] Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>}}]]

Motile systems have developed in the natural world over time and length scales spanning several orders of magnitude, and have evolved [[anatomically]] and [[physiologically]] to attain optimal strategies for [[self-propulsion]] and overcome the implications of high [[Viscosity|viscosity forces]] and [[Brownian motion]], as shown in the diagram on the right.<ref>{{cite journal |doi = 10.1088/0034-4885/72/9/096601|title = The hydrodynamics of swimming microorganisms|year = 2009|last1 = Lauga|first1 = Eric|last2 = Powers|first2 = Thomas R.|journal = Reports on Progress in Physics|volume = 72|issue = 9|page = 096601|arxiv = 0812.2887|bibcode = 2009RPPh...72i6601L|s2cid = 3932471}}</ref><ref name=BastosArrieta2018 />

Some of the smallest known motile systems are [[motor protein]]s, i.e., proteins and protein complexes present in cells that carry out a variety of physiological functions by transducing [[chemical energy]] into [[mechanical energy]]. These motor proteins are classified as [[myosin]]s, [[kinesin]]s, or [[dynein]]s. Myosin motors are responsible for muscle contractions and the transport of [[cargo]]using [[actin filament]]s as tracks. [[Dynein motor]]s and kinesin motors, on the other hand, use [[microtubule]]s to transport [[Vesicle (biology and chemistry)|vesicles]] across the cell.<ref name="Nature's design of nanomotors">{{cite journal |doi = 10.1016/j.ejpb.2004.10.007|title = Nature's design of nanomotors|year = 2005|last1 = Vogel|first1 = Pia D.|journal = European Journal of Pharmaceutics and Biopharmaceutics|volume = 60|issue = 2|pages = 267–277|pmid = 15939237}}</ref><ref>{{cite journal |doi = 10.1039/C2NR32600K|title = Intelligent, self-powered, drug delivery systems|year = 2013|last1 = Patra|first1 = Debabrata|last2 = Sengupta|first2 = Samudra|last3 = Duan|first3 = Wentao|last4 = Zhang|first4 = Hua|last5 = Pavlick|first5 = Ryan|last6 = Sen|first6 = Ayusman|journal = Nanoscale|volume = 5|issue = 4|pages = 1273–1283|pmid = 23166050|bibcode = 2013Nanos...5.1273P}}</ref> The mechanism these protein motors use to convert chemical energy into movement depends on [[ATP hydrolysis]], which leads to a conformation modification in the globular motor domain, leading to directed motion.<ref>{{cite journal |doi = 10.1021/ar0001721|title = In Control of Motion: From Molecular Switches to Molecular Motors|year = 2001|last1 = Feringa|first1 = Ben L.|journal = Accounts of Chemical Research|volume = 34|issue = 6|pages = 504–513|pmid = 11412087|url = https://research.rug.nl/en/publications/in-control-of-motion(a0b20090-34b9-4e2d-8450-bc2afbea2fcf).html|hdl = 11370/a0b20090-34b9-4e2d-8450-bc2afbea2fcf|hdl-access = free}}</ref><ref>{{cite journal |doi = 10.1073/pnas.0913015107|title = Swimming bacteria power microscopic gears|year = 2010|last1 = Sokolov|first1 = A.|last2 = Apodaca|first2 = M. M.|last3 = Grzybowski|first3 = B. A.|last4 = Aranson|first4 = I. S.|journal = Proceedings of the National Academy of Sciences|volume = 107|issue = 3|pages = 969–974|pmid = 20080560|pmc = 2824308|bibcode = 2010PNAS..107..969S|doi-access = free}}</ref><ref name=BastosArrieta2018 />

Bacteria can be roughly divided into two fundamentally different groups, [[Gram-positive bacteria|gram-positive]] and [[gram-negative bacteria]], distinguished by the architecture of their cell envelope. In each case the cell envelope is a complex multi-layered structure that protects the cell from its environment. In gram-positive bacteria, the [[cytoplasmic membrane]] is only surrounded by a thick cell wall of [[peptidoglycan]]. By contrast, the envelope of gram-negative bacteria is more complex and consists (from inside to outside) of the cytoplasmic membrane, a thin layer of peptidoglycan, and an additional outer membrane, also called the [[lipopolysaccharide]] layer. Other bacterial cell surface structures range from disorganised slime layers to highly structured [[Bacterial capsule|capsules]]. These are made from secreted slimy or sticky [[polysaccharide]]s or proteins that provide protection for the cells and are in direct contact with the environment. They have other functions, including attachment to solid surfaces. Additionally, protein appendages can be present on the surface: [[fimbria (bacteriology)|fimbriae]] and [[Pilus|pili]] can have different lengths and diameters and their functions include adhesion and [[twitching motility]].<ref name=Madigan2017>{{Cite book|url=https://books.google.com/books?id=pxrvswEACAAJ&q=%22Brock+Biology+of+Microorganisms%22|title = Brock Biology of Microorganisms|isbn = 9781292235103|last1 = Madigan|first1 = Michael T.|last2 = Bender|first2 = Kelly S.|last3 = Buckley|first3 = Daniel H.|last4 = Brock|first4 = Thomas D.|last5 = Matthew Sattley|first5 = W.|last6 = Stahl|first6 = David Allan|date = 29 January 2018| publisher=Pearson }}</ref><ref>{{cite journal |doi = 10.1016/j.tim.2015.01.011|title = Sticky microbes: Forces in microbial cell adhesion|year = 2015|last1 = Dufrêne|first1 = Yves F.|journal = Trends in Microbiology|volume = 23|issue = 6|pages = 376–382|pmid = 25684261}}</ref><ref name=BastosArrieta2018 />

Specifically, for microorganisms that live in aqueous environments, locomotion refers to swimming, and hence the world is full of different classes of swimming microorganisms, such as bacteria, [[spermatozoa]], [[protozoa]], and [[algae]]. Bacteria move due to rotation of hair-like filaments called [[flagella]], which are anchored to a protein motor complex on the bacteria cell wall.<ref name=BastosArrieta2018 />

{{clear}}

==Movement mechanisms==
[[File:Flagellum from a gram-negative bacterium.webp|thumb|upright=1.5| {{center|Flagellum from a gram-negative bacterium<br />rotated by a [[molecular motor]] at its base{{hsp}}<ref name=Barker2017>{{cite journal | last1=Barker | first1=Clive S. | last2=Meshcheryakova | first2=Irina V. | last3=Kostyukova | first3=Alla S. | last4=Freddolino | first4=Peter L. | last5=Samatey | first5=Fadel A. | title=An intrinsically disordered linker controlling the formation and the stability of the bacterial flagellar hook | journal=BMC Biology | publisher=Springer Science and Business Media LLC | volume=15 | issue=1 | date=27 October 2017 | page=97 | issn=1741-7007 | doi=10.1186/s12915-017-0438-7| pmid=29078764 | pmc=5660449 | doi-access=free }} [[File:CC-BY icon.svg|50px]] Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>}}]]

{{further|Flagella|Pilus|Archaella|Nanomotor}}

Bacteria have two different primary mechanisms they use for movement. The flagellum is used for swimming and swarming, and the pilus (or fimbria) is used for twitching.

===Flagellum===
The [[flagellum]] (plural, flagella; a group of flagella is called a tuft) is a helical, thin and long appendage attached to the cell surface by one of its ends, performing a rotational motion to push or pull the cell.<ref name="Berg1973"/><ref name=BastosArrieta2018 /> During the rotation of the bacterial flagellar motor, which is located in the membrane, the flagella rotate at speeds between 200 and 2000 rpm, depending on the bacterial species. The hook substructure of the bacterial flagellum acts as a [[universal joint]] connecting the motor to the flagellar filament.<ref name=Barker2017 />

[[File:Bacterial flagellate configurations.jpg|thumb|upright=1.5|right| {{center|Bacterial flagellate configurations{{hsp}}<ref name=BastosArrieta2018 /> }}]]

{{multiple image
 | align = right
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 | image1    = Crenarchaeal archaellum model.jpg
 | width1    = 120
 | alt1      = 
 | caption1  = {{center|Model of an [[archaellum]]{{hsp}}<ref name="Albers"/>}}
 | image2    = Difference Between Prokaryote and Eukaryote Flagella.svg
 | width2    = 200
 | alt2      = 
 | caption2  = Prokaryotic flagella run in a rotary movement, while eukaryotic flagella run in a bending movement
}}

Prokaryotes, both bacteria and archaea, primarily use [[flagella]] for locomotion.

* Bacterial flagella are helical filaments, each with a [[rotating locomotion in living systems|rotary motor]] at its base which can turn clockwise or counterclockwise.<ref name=Silverman1>{{cite journal | vauthors = Silverman M, Simon M | title = Flagellar rotation and the mechanism of bacterial motility | journal = Nature | volume = 249 | issue = 452 | pages = 73–4 | date = May 1974 | pmid = 4598030 | doi = 10.1038/249073a0 | bibcode = 1974Natur.249...73S | s2cid = 10370084 }}</ref><ref name=Meister1>{{cite journal |title= Rapid rotation of flagellar bundles in swimming bacteria |vauthors=Meister GL, Berg HC |journal=Nature |year=1987 |volume=325 |pages=637–640 |doi= 10.1038/325637a0 |issue=6105|bibcode=1987Natur.325..637L |s2cid=4242129 }}</ref><ref name=Berg1>{{cite journal | vauthors = Berg HC, Anderson RA | title = Bacteria swim by rotating their flagellar filaments | journal = Nature | volume = 245 | issue = 5425 | pages = 380–2 | date = October 1973 | pmid = 4593496 | doi = 10.1038/245380a0 | bibcode = 1973Natur.245..380B | s2cid = 4173914 }}</ref> They provide two of several kinds of bacterial motility.<ref name=Jahn1>{{cite journal | vauthors = Jahn TL, Bovee EC | title = Movement and locomotion of microorganisms | journal = Annual Review of Microbiology | volume = 19 | pages = 21–58 | year = 1965 | pmid = 5318439 | doi = 10.1146/annurev.mi.19.100165.000321 }}</ref><ref name=Harshey1>{{cite journal | vauthors = Harshey RM | title = Bacterial motility on a surface: many ways to a common goal | journal = Annual Review of Microbiology | volume = 57 | pages = 249–73 | year = 2003 | pmid = 14527279 | doi = 10.1146/annurev.micro.57.030502.091014 }}</ref>
* Archaeal flagella are called [[archaellum|''archaella'']], and function in much the same way as bacterial flagella. Structurally the archaellum is superficially similar to a bacterial flagellum, but it differs in many details and is considered non-[[homology (biology)|homologous]].<ref name= Jarelletal>{{cite book | vauthors = Jarrell K |year=2009|chapter=Archaeal Flagella and Pili|title=Pili and Flagella: Current Research and Future Trends|publisher=Caister Academic Press|isbn= 978-1-904455-48-6}}</ref><ref name=Albers>{{cite journal | vauthors = Albers SV, Jarrell KF | title = The archaellum: how Archaea swim | journal = Frontiers in Microbiology | volume = 6 | pages = 23 | date = 27 January 2015 | pmid = 25699024 | doi = 10.3389/fmicb.2015.00023 | pmc = 4307647 | doi-access = free }}</ref>

Some eukaryotic cells also use flagella — and they can be found in some protists and plants as well as animal cells. Eukaryotic flagella are complex cellular projections that lash back and forth, rather than in a circular motion. Prokaryotic flagella use a rotary motor, and the eukaryotic flagella use a complex sliding filament system. Eukaryotic flagella are [[Adenosine triphosphate|ATP-driven]], while prokaryotic flagella can be ATP-driven (archaea) or [[Proton pump|proton-driven]] (bacteria).<ref>{{cite journal | vauthors = Streif S, Staudinger WF, Marwan W, Oesterhelt D | year = 2008 | title = Flagellar rotation in the archaeon Halobacterium salinarum depends on ATP | journal = Journal of Molecular Biology | volume = 384 | issue = 1| pages = 1–8 | doi = 10.1016/j.jmb.2008.08.057 | pmid = 18786541 }}</ref>

Different types of cell flagellation are found depending on the number and arrangement of the flagella on the cell surface, e.g., only at the cell poles or spread over the cell surface.<ref>{{Cite book|url=https://books.google.com/books?id=kM3mwD4y_TAC&q=%22Comprehensive+Handbook+of+Chemical+Bond+Energies%22|title=Comprehensive Handbook of Chemical Bond Energies|isbn=9781420007282|last1=Luo|first1=Yu-Ran|date=9 March 2007|publisher=CRC Press }}</ref> In polar flagellation, the flagella are present at one or both ends of the cell: if a single flagellum is attached at one pole, the cell is called monotrichous; if a tuft of flagella is located at one pole, the cells is lophotrichous; when flagella are present at both ends, the cell is amphitrichous. In peritrichous flagellation, the flagella are distributed in different locations around the cell surface. Nevertheless, variations within this classification can be found, like lateral and subpolar—instead of polar—monotrichous and lophotrichous flagellation.<ref name=Haya2011 /><ref name=BastosArrieta2018 />

The rotary motor model used by bacteria uses the protons of an [[electrochemical gradient]] in order to move their flagella. [[Torque]] in the flagella of bacteria is created by particles that conduct protons around the base of the flagellum. The direction of rotation of the flagella in bacteria comes from the occupancy of the proton channels along the perimeter of the flagellar motor.<ref>{{cite journal |author = Brady, Richard M. |title= Torque and switching in the bacterial flagellar motor. An electrostatic model|journal= Biophysical Journal |volume= 64 |issue= 4| year = 1993 | pages = 961–973|bibcode = 1993BpJ....64..961B |doi = 10.1016/S0006-3495(93)81462-0 |pmid= 7684268|pmc= 1262414}}</ref>

The bacterial flagellum is a protein-nanomachine that converts electrochemical energy in the form of a gradient of H+ or Na+ ions into mechanical work.<ref name=Manson1977>{{cite journal |doi = 10.1073/pnas.74.7.3060|title = A protonmotive force drives bacterial flagella|year = 1977|last1 = Manson|first1 = M. D.|last2 = Tedesco|first2 = P.|last3 = Berg|first3 = H. C.|last4 = Harold|first4 = F. M.|last5 = Van Der Drift|first5 = C.|journal = Proceedings of the National Academy of Sciences|volume = 74|issue = 7|pages = 3060–3064|pmid = 19741|pmc = 431412|bibcode = 1977PNAS...74.3060M|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1016/0014-5793(81)81178-7|title = Flagellar motors of alkalophilicbacillusare powered by an electrochemical potential gradient of Na+|year = 1981|last1 = Hirota|first1 = Norifumi|last2 = Kitada|first2 = Makio|last3 = Imae|first3 = Yasuo|journal = FEBS Letters|volume = 132|issue = 2|pages = 278–280|s2cid = 85138168|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1016/S0006-3495(97)78104-9|title = Protein turbines. I: The bacterial flagellar motor|year = 1997|last1 = Elston|first1 = T.C.|last2 = Oster|first2 = G.|journal = Biophysical Journal|volume = 73|issue = 2|pages = 703–721|pmid = 9251788|pmc = 1180968|bibcode = 1997BpJ....73..703E}}</ref> The flagellum is composed of three parts: the basal body, the hook, and the filament. The basal body is a reversible motor that spans the bacterial cell envelope. It is composed of the central rod and several rings: in Gram-negative bacteria, these are the outer L-ring ([[lipopolysaccharide]]) and P-ring ([[peptidoglycan]]), and the inner MS-ring (membrane/supramembrane) and C-ring ([[cytoplasmic]]). In Gram-positive bacteria only the inner rings are present.<ref>{{cite journal |doi = 10.1038/emboj.2011.186|title = Structural diversity of bacterial flagellar motors|year = 2011|last1 = Chen|first1 = Songye|last2 = Beeby|first2 = Morgan|last3 = Murphy|first3 = Gavin E.|last4 = Leadbetter|first4 = Jared R.|last5 = Hendrixson|first5 = David R.|last6 = Briegel|first6 = Ariane|last7 = Li|first7 = Zhuo|last8 = Shi|first8 = Jian|last9 = Tocheva|first9 = Elitza I.|last10 = Müller|first10 = Axel|last11 = Dobro|first11 = Megan J.|last12 = Jensen|first12 = Grant J.|journal = The EMBO Journal|volume = 30|issue = 14|pages = 2972–2981|pmid = 21673657|pmc = 3160247}}</ref> The Mot proteins ([[MotA]]  and [[MotB]]) surround the inner rings in the [[cytoplasmic membrane]]; ion translocation through the Mot proteins provide the energy for flagella rotation.<ref name=Manson1977 /> The [[Fli proteins]] allow reversal of the direction of rotation of the flagella in response to specific stimuli.<ref>{{cite journal |doi = 10.1128/jb.174.3.793-806.1992|title = Molecular analysis of the flagellar switch protein FliM of Salmonella typhimurium|year = 1992|last1 = Sockett|first1 = H.|last2 = Yamaguchi|first2 = S.|last3 = Kihara|first3 = M.|last4 = Irikura|first4 = V. M.|last5 = MacNab|first5 = R. M.|journal = Journal of Bacteriology|volume = 174|issue = 3|pages = 793–806|pmid = 1732214|pmc = 206156}}</ref><ref>{{cite journal |doi = 10.1073/pnas.90.19.8787|title = Phosphorylation-dependent binding of a signal molecule to the flagellar switch of bacteria|year = 1993|last1 = Welch|first1 = M.|last2 = Oosawa|first2 = K.|last3 = Aizawa|first3 = S.|last4 = Eisenbach|first4 = M.|journal = Proceedings of the National Academy of Sciences|volume = 90|issue = 19|pages = 8787–8791|pmid = 8415608|pmc = 47445|bibcode = 1993PNAS...90.8787W|doi-access = free}}</ref> The hook connects the filament to the motor protein in the base. The helical filament is composed of many copies of the protein flagellin, and it can rotate clockwise (CW) and counterclockwise (CCW).<ref>{{cite journal |doi = 10.1146/annurev.biochem.72.121801.161737|title = The Rotary Motor of Bacterial Flagella|year = 2003|last1 = Berg|first1 = Howard C.|journal = Annual Review of Biochemistry|volume = 72|pages = 19–54|pmid = 12500982}}</ref><ref>{{cite journal |doi = 10.1101/cshperspect.a000299|title = Bacterial Nanomachines: The Flagellum and Type III Injectisome|year = 2010|last1 = Erhardt|first1 = M.|last2 = Namba|first2 = K.|last3 = Hughes|first3 = K. T.|journal = Cold Spring Harbor Perspectives in Biology|volume = 2|issue = 11|pages = a000299|pmid = 20926516|pmc = 2964186}}</ref><ref>{{cite journal |doi = 10.1016/j.tim.2014.05.009|title = Building a flagellum outside the bacterial cell|year = 2014|last1 = Evans|first1 = Lewis D.B.|last2 = Hughes|first2 = Colin|last3 = Fraser|first3 = Gillian M.|journal = Trends in Microbiology|volume = 22|issue = 10|pages = 566–572|pmid = 24973293|pmc = 4183434}}</ref><ref>{{cite journal |doi = 10.1016/j.tim.2014.12.011|title = The bacterial flagellar motor and its structural diversity|year = 2015|last1 = Minamino|first1 = Tohru|last2 = Imada|first2 = Katsumi|journal = Trends in Microbiology|volume = 23|issue = 5|pages = 267–274|pmid = 25613993}}</ref><ref name=BastosArrieta2018 />

===Pilus (fimbria)===
{{multiple image
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 | caption2  = {{center|Pili can be used for [[Bacterial_motility#Twitching|twitching]].<ref name=BastosArrieta2018 /><br />They are long, sticky filaments, as shown here covering this ''[[E. coli]]'' bacterium{{hsp}}<ref name=Gross2006>{{cite journal | last=Gross | first=Liza | title=Bacterial Fimbriae Designed to Stay with the Flow | journal=PLOS Biology | publisher=Public Library of Science (PLoS) | volume=4 | issue=9 | date=29 August 2006 | issn=1545-7885 | doi=10.1371/journal.pbio.0040314 | page=e314| pmid=20076642 | pmc=1557401 | doi-access=free }} [[File:CC-BY icon.svg|50px]] Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>}}
}}

A [[pilus]] ([[Latin]] for 'hair') is a hair-like appendage found on the surface of many [[bacterium|bacteria]] and [[archaea]].<ref>{{DorlandsDict|nine/000955836|pilus}}</ref>  The terms ''pilus'' and ''[[fimbria (bacteriology)|fimbria]]'' (Latin for 'fringe') can be used interchangeably, although some researchers reserve the term ''pilus'' for the appendage required for [[bacterial conjugation]]. Dozens of these structures can exist on the bacterial and archaeal surface.

[[Twitching motility]] is a form of crawling bacterial motility used to move over surfaces. Twitching is mediated by the activity of a particular type of pilus called [[type IV pili|type IV pilus]] which extends from the cell's exterior, binds to surrounding solid substrates and retracts, pulling the cell forwards in a manner similar to the action of a [[grappling hook]].<ref name=":4">{{Cite journal|last1=Skerker|first1=J. M.|last2=Berg|first2=H. C.|date=2001-06-05|title=Direct observation of extension and retraction of type IV pili|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=98|issue=12|pages=6901–6904|doi=10.1073/pnas.121171698|issn=0027-8424|pmid=11381130|pmc=34450|bibcode=2001PNAS...98.6901S|doi-access=free}}</ref><ref>{{Cite journal|last=Mattick|first=John S.|date=2002|title=Type IV pili and twitching motility|journal=Annual Review of Microbiology|volume=56|pages=289–314|doi=10.1146/annurev.micro.56.012302.160938|issn=0066-4227|pmid=12142488}}</ref><ref>{{Cite journal|last1=Merz|first1=A. J.|last2=So|first2=M.|last3=Sheetz|first3=M. P.|date=2000-09-07|title=Pilus retraction powers bacterial twitching motility|journal=Nature|volume=407|issue=6800|pages=98–102|doi=10.1038/35024105|issn=0028-0836|pmid=10993081|bibcode=2000Natur.407...98M|s2cid=4425775}}</ref> Pili are not used just for twitching. They are also [[antigen]]ic and are required for the formation of biofilm, as they attach bacteria to host surfaces for colonisation during infection. They are fragile and constantly replaced, sometimes with pili of different composition.<ref>{{cite journal |last1=Brinton |first1=Charles |title=Electrophoresis and phage susceptibility studies on a filament-producing variant of the E. coli bacterium |journal=Biochimica et Biophysica Acta |date=1954 |volume=15 |issue=4 |pages=533–542|doi=10.1016/0006-3002(54)90011-6 |pmid=13230101 }}</ref>

===Other===
[[Gliding motility]] is a type of translocation that is independent of propulsive structures such as [[flagellum|flagella]] or [[pilus|pili]].<ref name=":0">{{cite journal|last1=Nan|first1=Beiyan|title=Bacterial Gliding Motility: Rolling Out a Consensus Model|journal=Current Biology|date=February 2017|volume=27|issue=4|pages=R154–R156|doi=10.1016/j.cub.2016.12.035|pmid=28222296|doi-access=free}}</ref> Gliding allows microorganisms to travel along the surface of low aqueous films. The mechanisms of this motility are only partially known.  Gliding motility uses a highly diverse set of different motor complexes, including e.g., the [[focal adhesion]] complexes of [[Myxococcus]].<ref name=Islam2015>{{cite journal |doi = 10.1016/j.semcdb.2015.10.033|title = The mysterious nature of bacterial surface (Gliding) motility: A focal adhesion-based mechanism in Myxococcus xanthus|year = 2015|last1 = Islam|first1 = Salim T.|last2 = Mignot|first2 = Tâm|journal = Seminars in Cell & Developmental Biology|volume = 46|pages = 143–154|pmid = 26520023}}</ref><ref>{{cite journal |doi = 10.1111/mmi.13389|title = Novel mechanisms power bacterial gliding motility|year = 2016|last1 = Nan|first1 = Beiyan|last2 = Zusman|first2 = David R.|journal = Molecular Microbiology|volume = 101|issue = 2|pages = 186–193|pmid = 27028358|pmc = 5008027}}</ref> The speed of gliding varies between organisms, and the reversal of direction is seemingly regulated by some sort of internal clock.<ref name=":1">{{cite journal|last1=Nan|first1=Beiyan|last2=McBride|first2=Mark J.|last3=Chen|first3=Jing|last4=Zusman|first4=David R.|last5=Oster|first5=George|date=February 2014|title=Bacteria that Glide with Helical Tracks|journal=Current Biology|volume=24|issue=4|pages=169–174|doi=10.1016/j.cub.2013.12.034|pmid=24556443|pmc=3964879}}</ref>
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==Modes of locomotion==
[[File:Bacterial swimming.jpg|thumb|upright=1.3|right| {{center|'''Bacterial swimming'''{{hsp}}<ref name=BastosArrieta2018 />}}]]

Most [[rod-shaped bacteria]] can move using their own power, which allows [[colonization]] of new environments and discovery of new resources for survival. Bacterial movement depends not only on the characteristics of the medium, but also on the use of different appendages to propel. Swarming and swimming movements are both powered by rotating flagella.<ref name=Berg1973>{{cite journal |doi = 10.1038/245380a0|title = Bacteria Swim by Rotating their Flagellar Filaments|year = 1973|last1 = Berg|first1 = Howard C.|last2 = Anderson|first2 = Robert A.|journal = Nature|volume = 245|issue = 5425|pages = 380–382|pmid = 4593496|bibcode = 1973Natur.245..380B|s2cid = 4173914}}</ref><ref name=Turner2010>{{cite journal |doi = 10.1038/ncomms15974|title = Light controlled 3D micromotors powered by bacteria|year = 2017|last1 = Vizsnyiczai|first1 = Gaszton|last2 = Frangipane|first2 = Giacomo|last3 = Maggi|first3 = Claudio|last4 = Saglimbeni|first4 = Filippo|last5 = Bianchi|first5 = Silvio|last6 = Di Leonardo|first6 = Roberto|journal = Nature Communications|volume = 8|page = 15974|pmid = 28656975|pmc = 5493761|bibcode = 2017NatCo...815974V}}</ref> Whereas swarming is a multicellular 2D movement over a surface and requires the presence of [[surfactant]] substances, swimming is movement of individual cells in liquid environments.<ref>Henrichsen, J. (1972) "Bacterial surface translocation: a survey and a classification". ''Bacteriol. Rev.'', '''36''': 478–503.</ref><ref name=BastosArrieta2018 />

Other types of movement occurring on solid surfaces include twitching, gliding and sliding, which are all independent of flagella. Twitching motility depends on the extension, attachment to a surface, and retraction of [[type IV pili]] which provide the energy required to push the cell forward.<ref>{{cite journal |doi = 10.1146/annurev.micro.56.012302.160938|title = Type IV Pili and Twitching Motility|year = 2002|last1 = Mattick|first1 = John S.|journal = Annual Review of Microbiology|volume = 56|pages = 289–314|pmid = 12142488}}</ref> Gliding motility uses a highly diverse set of different motor complexes, including e.g., the [[focal adhesion]] complexes of ''[[Myxococcus]]''.<ref name=Islam2015/><ref>{{cite journal |doi = 10.1111/mmi.13389|title = Novel mechanisms power bacterial gliding motility|year = 2016|last1 = Nan|first1 = Beiyan|last2 = Zusman|first2 = David R.|journal = Molecular Microbiology|volume = 101|issue = 2|pages = 186–193|pmid = 27028358|pmc = 5008027}}</ref> Unlike twitching and gliding motilities, which are active movements where the motive force is generated by the individual cell, sliding is a passive movement. It relies on the motive force generated by the cell community due to the expansive forces caused by cell growth within the colony in the presence of surfactants, which reduce the friction between the cells and the surface.<ref>{{cite journal |doi = 10.1111/1462-2920.13741|title = Sliding on the surface: Bacterial spreading without an active motor|year = 2017|last1 = Hölscher|first1 = Theresa|last2 = Kovács|first2 = Ákos T.|journal = Environmental Microbiology|volume = 19|issue = 7|pages = 2537–2545|pmid = 28370801|hdl = 11858/00-001M-0000-002D-2464-7|hdl-access = free}}</ref><ref name=BastosArrieta2018 />

===Swimming===
[[File:Swimming strategy of bacteria - run and tumble.jpg|thumb|upright=1.3|right| {{center|'''[[Run-and-tumble]]'''{{hsp}}<ref name=BastosArrieta2018 /><br />Example: ''[[Escherichia coli]]''}}]]
{{ external media
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| video1 = [https://www.youtube.com/watch?v=L-vprX2kpds&t=339s&ab_channel=JourneytotheMicrocosmos Why do bacteria move like vibrating chaos snakes?] – ''Journey to the Microcosmos''
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Many bacteria swim, propelled by rotation of the flagella outside the cell body. In contrast to [[protist flagella]], bacterial flagella are [[Wiktionary:rotor|rotors]] and — irrespective of species and type of flagellation — they have only two modes of operation: clockwise (CW) or counterclockwise (CCW) rotation. Bacterial swimming is used in [[#Bacterial taxis: Directed motion|bacterial taxis]] (mediated by specific receptors and [[signal transduction]] pathways) for the bacterium to move in a directed manner along gradients and reach more favorable conditions for life.<ref>{{cite journal |doi = 10.1017/S0033583508004691|title = Bacterial flagellar motor|year = 2008|last1 = Sowa|first1 = Yoshiyuki|last2 = Berry|first2 = Richard M.|journal = Quarterly Reviews of Biophysics|volume = 41|issue = 2|pages = 103–132|pmid = 18812014|s2cid = 3297704|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1111/j.1462-2920.2010.02383.x|title = Diversity at its best: Bacterial taxis|year = 2011|last1 = Krell|first1 = Tino|last2 = Lacal|first2 = Jesús|last3 = Muñoz-Martínez|first3 = Francisco|last4 = Reyes-Darias|first4 = José Antonio|last5 = Cadirci|first5 = Bilge Hilal|last6 = García-Fontana|first6 = Cristina|last7 = Ramos|first7 = Juan Luis|journal = Environmental Microbiology|volume = 13|issue = 5|pages = 1115–1124|pmid = 21087385|doi-access = free}}</ref> The direction of flagellar rotation is controlled by the type of molecules detected by the receptors on the surface of the cell: in the presence of an attractant gradient, the rate of smooth swimming increases, while the presence of a repellent gradient increases the rate of tumbling.<ref name=Berg2004>{{Cite book|url=https://books.google.com/books?id=9p8SBwAAQBAJ&q=%22E.+coli+in+Motion%22|title=E. Coli in Motion|isbn=9780387216386|last1=Berg|first1=Howard C.|date=11 January 2008|publisher=Springer }}</ref><ref name=BastosArrieta2018 />

The archetype of bacterial swimming is represented by the well-studied model organism ''[[Escherichia coli]]''.<ref name=BastosArrieta2018 /> With its [[peritrichous]] flagellation, ''E. coli'' performs a [[run-and-tumble]] swimming pattern, as shown in the diagram on the right. CCW rotation of the flagellar motors leads to flagellar bundle formation that pushes the cell in a forward run, parallel to the long axis of the cell. CW rotation disassembles the bundle and the cell rotates randomly (tumbling). After the tumbling event, straight swimming is recovered in a new direction.<ref name=Berg2004 /> That is, CCW rotation results in steady motion and CW rotation in tumbling; CCW rotation in a given direction is maintained longer in the presence of molecules of interest (like sugars or aminoacids).<ref name=Berg2004 /><ref name=BastosArrieta2018 />

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[[File:Swimming strategy of bacteria - forward, reverse and turning.jpg|thumb|upright=1.3|right| {{center|'''Forward, reverse and turning by buckling'''{{hsp}}<ref name=BastosArrieta2018 /><br />Example: ''[[Vibrio alginolyticus]]''<br />Adapted from Son et al., 2013{{hsp}}<ref name=Son2013>{{cite journal |doi = 10.1038/nphys2676|doi-access = free|title = Bacteria can exploit a flagellar buckling instability to change direction|year = 2013|last1 = Son|first1 = Kwangmin|last2 = Guasto|first2 = Jeffrey S.|last3 = Stocker|first3 = Roman|journal = Nature Physics|volume = 9|issue = 8|pages = 494–498|bibcode = 2013NatPh...9..494S}}</ref>}}]]

However, the type of swimming movement (propelled by rotation of flagella outside the cell body) varies significantly with the species and number/distribution of flagella on the cell body. For example, the marine bacterium ''[[Vibrio alginolyticus]]'', with its single polar flagellum, swims in a cyclic, three-step (forward, reverse, and flick) pattern. Forward swimming occurs when the flagellum pushes the cell head, while backward swimming is based on the flagellum pulling the head upon motor reversal.<ref name=BastosArrieta2018 />

[[File:Swimming strategy of bacteria - stop and coil.jpg|thumb|upright=1.3|left| {{center|'''Stop and coil'''{{hsp}}<ref name=BastosArrieta2018 /><br />Example: ''[[Rhodobacter sphaeroides]]''<br />Adapted from Armitage and Macnab, 1987;<ref name=Armitage1987>{{cite journal |doi = 10.1128/jb.169.2.514-518.1987|title = Unidirectional, intermittent rotation of the flagellum of Rhodobacter sphaeroides|year = 1987|last1 = Armitage|first1 = J. P.|last2 = MacNab|first2 = R. M.|journal = Journal of Bacteriology|volume = 169|issue = 2|pages = 514–518|pmid = 3492489|pmc = 211807}}</ref> Armitage et al., 1999.<ref name=Armitage1999>{{cite journal |doi = 10.1128/JB.181.16.4825-4833.1999|title = Transformations in Flagellar Structure of ''Rhodobacter'' sphaeroides and Possible Relationship to Changes in Swimming Speed|year = 1999|last1 = Armitage|first1 = Judith P.|last2 = Pitta|first2 = Thomas P.|last3 = Vigeant|first3 = Margot A.-S.|last4 = Packer|first4 = Helen L.|last5 = Ford|first5 = Roseanne M.|journal = Journal of Bacteriology|volume = 181|issue = 16|pages = 4825–4833|pmid = 10438751|pmc = 93968}}</ref>}}]]

[[File:Swimming strategy of bacteria - push, pull and wrap.jpg|thumb|upright=1.3| {{center|'''Push, pull and wrap'''{{hsp}}<ref name=BastosArrieta2018 /><br />Example: ''[[Pseudomonas putida]]''<br />Adapted from Hintsche et al., 2017.<ref name=Hintsche2017>{{cite journal |doi = 10.1038/s41598-017-16428-9|title = A polar bundle of flagella can drive bacterial swimming by pushing, pulling, or coiling around the cell body|year = 2017|last1 = Hintsche|first1 = Marius|last2 = Waljor|first2 = Veronika|last3 = Großmann|first3 = Robert|last4 = Kühn|first4 = Marco J.|last5 = Thormann|first5 = Kai M.|last6 = Peruani|first6 = Fernando|last7 = Beta|first7 = Carsten|journal = Scientific Reports|volume = 7|issue = 1|page = 16771|pmid = 29196650|pmc = 5711944|bibcode = 2017NatSR...716771H}}</ref>}}]]

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Besides these 180° reversals, the cells can reorient (a "flick") by an angle around 90°, referred to as turning by buckling.<ref>{{cite journal |doi = 10.1073/pnas.1011953108|title = Bacterial flagellum as a propeller and as a rudder for efficient chemotaxis|year = 2011|last1 = Xie|first1 = L.|last2 = Altindal|first2 = T.|last3 = Chattopadhyay|first3 = S.|last4 = Wu|first4 = X.-L.|journal = Proceedings of the National Academy of Sciences|volume = 108|issue = 6|pages = 2246–2251|pmid = 21205908|pmc = 3038696|doi-access = free}}</ref><ref name=Son2013 /> ''[[Rhodobacter sphaeroides]]'' with its subpolar monotrichous flagellation, represents yet another motility strategy:<ref name=Armitage1987 /><ref name=Haya2011>{{cite journal |doi = 10.1128/AEM.00286-11|title = Characterization of Lateral Flagella of Selenomonas ruminantium|year = 2011|last1 = Haya|first1 = Shohei|last2 = Tokumaru|first2 = Yuya|last3 = Abe|first3 = Naoki|last4 = Kaneko|first4 = Jun|last5 = Aizawa|first5 = Shin-Ichi|journal = Applied and Environmental Microbiology|volume = 77|issue = 8|pages = 2799–2802|pmid = 21335384|pmc = 3126368|bibcode = 2011ApEnM..77.2799H}}</ref> the flagellum only rotates in one direction, and it stops and coils against the cell body from time to time, leading to cell body reorientations,<ref name=Armitage1999 /><ref>{{cite journal |doi = 10.1073/pnas.0813164106|title = A molecular brake, not a clutch, stops the ''Rhodobacter'' sphaeroidesflagellar motor|year = 2009|last1 = Pilizota|first1 = Teuta|last2 = Brown|first2 = Mostyn T.|last3 = Leake|first3 = Mark C.|last4 = Branch|first4 = Richard W.|last5 = Berry|first5 = Richard M.|last6 = Armitage|first6 = Judith P.|journal = Proceedings of the National Academy of Sciences|volume = 106|issue = 28|pages = 11582–11587|pmid = 19571004|pmc = 2710667|bibcode = 2009PNAS..10611582P|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1098/rsif.2014.0320|title = Modelling and analysis of bacterial tracks suggest an active reorientation mechanism in Rhodobacter sphaeroides|year = 2014|last1 = Rosser|first1 = Gabriel|last2 = Baker|first2 = Ruth E.|last3 = Armitage|first3 = Judith P.|last4 = Fletcher|first4 = Alexander G.|journal = Journal of the Royal Society Interface|volume = 11|issue = 97|pmid = 24872500|pmc = 4208361}}</ref> In the soil bacterium ''[[Pseudomonas putida]]'', a tuft of helical flagella is attached to its posterior pole. ''P. putida'' alternates between three swimming modes: pushing, pulling, and wrapping.<ref name=Hintsche2017 /><ref name=BastosArrieta2018 />

In the pushing mode, the rotating flagella (assembled in a bundle or as an open tuft of individual filaments) drive the motion from the rear end of the cell body. The trajectories are either straight or, in the vicinity of a solid surface, curved to the right, due to hydrodynamic interaction of the cell with the surface. The direction of curvature indicates that pushers are driven by a left-handed helix turning in CCW direction. In the pulling mode, the rotating flagellar bundle is pointing ahead. In this case the trajectories are either straight or with a tendency to bend to the left, indicating that pullers swim by turning a left-handed helical bundle in CW direction. Finally, ''P. putida'' can swim by wrapping the filament bundle around its cell body, with the posterior pole pointing in the direction of motion. In that case, the flagellar bundle takes the form of a left-handed helix that turns in CW direction, and the trajectories are predominantly straight.<ref name=Hintsche2017 /><ref name=BastosArrieta2018 />

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===Swarming===
{{main|Swarming motility}}

[[Swarming motility]] is a rapid (2–10 μm/s) and coordinated translocation of a bacterial population across solid or semi-solid surfaces,<ref>{{Cite journal|title = Bacterial Motility on a Surface: Many Ways to a Common Goal|journal = Annual Review of Microbiology|date = 2003-01-01|pmid = 14527279|pages = 249–73|volume = 57|issue = 1|doi = 10.1146/annurev.micro.57.030502.091014|first = Rasika M.|last = Harshey}}</ref> and is an example of bacterial multicellularity and [[swarm behaviour]]. Swarming motility was first reported in 1972 by Jorgen Henrichsen.<ref>{{cite journal |url= | pmc = 408329 | title =Bacterial surface translocation: a survey and a classification | pmid=4631369 |year=1972 |last1=Henrichsen |first1=J |volume=36 |issue=4 |pages=478–503 |journal=Bacteriological Reviews| doi = 10.1128/BR.36.4.478-503.1972 }}</ref>

[[File:Swarming of bacteria.jpg|thumb|upright=1.25| {{center|'''Swarming of bacteria{{hsp}}<ref name=BastosArrieta2018 />'''<br /> <small>Adapted from: Kearns (2010){{hsp}}<ref name=Kearns2010>{{cite journal |doi = 10.1038/nrmicro2405|pmc = 3135019|title = A field guide to bacterial swarming motility|year = 2010|last1 = Kearns|first1 = Daniel B.|journal = Nature Reviews Microbiology|volume = 8|issue = 9|pages = 634–644|pmid = 20694026}}</ref></small>}}]]

The transition from swimming to swarming mobility is usually associated with an increase in the number of flagella per cell, accompanied by cell elongation.<ref name=Kearns2010 /> Experiments with ''[[Proteus mirabilis]]'' showed that swarming requires contact between cells: swarming cells move in side-by-side groups called rafts, which dynamically add or lose cells: when a cell is left behind the raft, its movement stops after a short time; when a group of cells moving in a raft make contact with a stationary cell, it is reactivated and incorporated into the raft.<ref name=Park2017>{{cite journal |doi = 10.1021/acsnano.7b03207|title = Multifunctional Bacteria-Driven Microswimmers for Targeted Active Drug Delivery|year = 2017|last1 = Park|first1 = Byung-Wook|last2 = Zhuang|first2 = Jiang|last3 = Yasa|first3 = Oncay|last4 = Sitti|first4 = Metin|journal = ACS Nano|volume = 11|issue = 9|pages = 8910–8923|pmid = 28873304}}</ref> More recently, Swiecicki and coworkers designed a polymer microfluidic system to confine ''E. coli'' cells in a quasi-two-dimensional layer of motility buffer in order to study different behaviors of cells transitioning from swimming to swarming movement.<ref name=Swiecicki2013>{{cite journal |doi = 10.1039/c3ib40130h|title = From swimming to swarming: Escherichia coli cell motility in two-dimensions|year = 2013|last1 = Swiecicki|first1 = Jean-Marie|last2 = Sliusarenko|first2 = Olesksii|last3 = Weibel|first3 = Douglas B.|journal = Integrative Biology|volume = 5|issue = 12|pages = 1490–1494|pmid = 24145500|pmc = 4222179}}</ref> For this, they forced ''E. coli'' [[planktonic]] cells into a swarming-cell-phenotype by inhibiting cell division (leading to cell elongation) and by deletion of the chemosensory system (leading to smooth swimming cells that do not tumble). The increase of bacterial density inside the channel led to the formation of progressively larger rafts. Cells colliding with the raft contributed to increase its size, while cells moving at a velocity different from the mean velocity within the raft separated from it.<ref name=Swiecicki2013 /><ref name=BastosArrieta2018 />

Cell trajectories and flagellar motion during swarming was thoroughly studied for ''E. coli'', in combination with fluorescently labeled flagella.<ref>{{cite journal |doi = 10.1128/JB.182.10.2793-2801.2000|title = Real-Time Imaging of Fluorescent Flagellar Filaments|year = 2000|last1 = Turner|first1 = Linda|last2 = Ryu|first2 = William S.|last3 = Berg|first3 = Howard C.|journal = Journal of Bacteriology|volume = 182|issue = 10|pages = 2793–2801|pmid = 10781548|pmc = 101988}}</ref><ref name=Turner2010 /> The authors described four different types of tracks during bacterial swarming: forward movement, reversals, lateral movement, and stalls.<ref name=Turner2010 /> In forward movement, the long axis of the cell, the flagellar bundle and the direction of movement are aligned, and propulsion is similar to the propulsion of a freely swimming cell. In a reversal, the flagellar bundle loosens, with the filaments in the bundle changing from their "normal form" (left-handed helices) into a "curly" form of right-handed helices with lower pitch and amplitude. Without changing its orientation, the cell body moves backwards through the loosened bundle. The bundle re-forms from curly filaments on the opposite pole of the cell body, and the filaments eventually relax back into their normal form. Lateral motion can be caused by collisions with other cells or by a motor reversal. Finally, stalled cells are paused but the flagella continue spinning and pumping fluid in front of the swarm, usually at the swarm edge.<ref name=Turner2010 /><ref name=BastosArrieta2018 />

===Twitching===
[[File:Bacterial twitching.png|thumb|upright=1.25| {{center|'''Bacterial twitching'''{{hsp}}<ref name=Jayathilake2019>{{cite journal |doi = 10.1038/s41598-019-51101-3|title = Modelling bacterial twitching in fluid flows: A CFD-DEM approach|year = 2019|last1 = Jayathilake|first1 = Pahala Gedara|last2 = Li|first2 = Bowen|last3 = Zuliani|first3 = Paolo|last4 = Curtis|first4 = Tom|last5 = Chen|first5 = Jinju|journal = Scientific Reports|volume = 9|issue = 1|page = 14540|pmid = 31601892|pmc = 6787227|bibcode = 2019NatSR...914540J}} [[File:CC-BY icon.svg|50px]] Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>}}]]
{{further|Twitching motility|Pilus|Fimbria (bacteriology)}}

[[Twitching motility]] is a form of crawling bacterial motility used to move over surfaces. Twitching is mediated by the activity of hair-like filaments called [[type IV pili]] which extend from the cell's exterior, bind to surrounding solid substrates and retract, pulling the cell forwards in a manner similar to the action of a [[grappling hook]].<ref name=":4"/><ref>{{Cite journal|last=Mattick|first=John S.|date=2002|title=Type IV pili and twitching motility|journal=Annual Review of Microbiology|volume=56|pages=289–314|doi=10.1146/annurev.micro.56.012302.160938|issn=0066-4227|pmid=12142488}}</ref><ref>{{Cite journal|last1=Merz|first1=A. J.|last2=So|first2=M.|last3=Sheetz|first3=M. P.|date=2000-09-07|title=Pilus retraction powers bacterial twitching motility|journal=Nature|volume=407|issue=6800|pages=98–102|doi=10.1038/35024105|issn=0028-0836|pmid=10993081|bibcode=2000Natur.407...98M|s2cid=4425775}}</ref> The name ''twitching motility'' is derived from the characteristic jerky and irregular motions of individual cells when viewed under the microscope.<ref>{{Cite journal|last=Henrichsen|first=J.|date=December 1972|title=Bacterial surface translocation: a survey and a classification|journal=Bacteriological Reviews|volume=36|issue=4|pages=478–503|doi=10.1128/BR.36.4.478-503.1972|issn=0005-3678|pmid=4631369|pmc=408329}}</ref>

A bacterial [[biofilm]] is a bacterial community attached into a surface through [[Extracellular polymeric substance|extracellular polymeric materials]].<ref>{{cite journal |doi = 10.1016/j.mib.2016.02.004|title = Sensational biofilms: Surface sensing in bacteria|year = 2016|last1 = o'Toole|first1 = George A.|last2 = Wong|first2 = Gerard CL|journal = Current Opinion in Microbiology|volume = 30|pages = 139–146|pmid = 26968016|pmc = 4843124}}</ref> Prior to biofilm formation, bacteria may need to deposit on the surface from their [[planktonic]] state. After bacteria deposit on surfaces they may "twitch" or crawl over the surface using appendages called [[type IV pili]] to "explore" the substratum to find suitable sites for growth and thus biofilm formation.<ref>{{cite journal |doi = 10.1016/j.tim.2015.09.002|title = How Bacteria Use Type IV Pili Machinery on Surfaces|year = 2015|last1 = Maier|first1 = Berenike|last2 = Wong|first2 = Gerard C.L.|journal = Trends in Microbiology|volume = 23|issue = 12|pages = 775–788|pmid = 26497940}}</ref><ref>{{cite journal |doi = 10.1073/pnas.1105073108|title = Bacteria use type-IV pili to slingshot on surfaces|year = 2011|last1 = Jin|first1 = F.|last2 = Conrad|first2 = J. C.|last3 = Gibiansky|first3 = M. L.|last4 = Wong|first4 = G. C. L.|journal = Proceedings of the National Academy of Sciences|volume = 108|issue = 31|pages = 12617–12622|pmid = 21768344|pmc = 3150923|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1038/srep45467|title = Emergence of complex behavior in pili-based motility in early stages of P. Aeruginosa surface adaptation|year = 2017|last1 = Brill-Karniely|first1 = Yifat|last2 = Jin|first2 = Fan|last3 = Wong|first3 = Gerard C. L.|last4 = Frenkel|first4 = Daan|last5 = Dobnikar|first5 = Jure|journal = Scientific Reports|volume = 7|page = 45467|pmid = 28393835|pmc = 5385500|bibcode = 2017NatSR...745467B}}</ref><ref>{{cite journal |doi = 10.1099/00221287-145-10-2863|title = A re-examination of twitching motility in Pseudomonas aeruginosa|year = 1999|last1 = Semmler|first1 = Annalese B. T.|last2 = Whitchurch|first2 = Cynthia B.|last3 = Mattick|first3 = John S.|journal = Microbiology|volume = 145|issue = 10|pages = 2863–2873|pmid = 10537208|doi-access = free}}</ref> [[Pilus|Pili]] emanate from bacterial surface and they can be up to several [[micrometre]]s long (though they are [[nanometre]]s in diameter).<ref>{{cite journal |doi = 10.1016/j.bpj.2016.09.050|title = Modeling and Simulating the Dynamics of Type IV Pili Extension of Pseudomonas aeruginosa|year = 2016|last1 = De Haan|first1 = Hendrick W.|journal = Biophysical Journal|volume = 111|issue = 10|pages = 2263–2273|pmid = 27851948|pmc = 5112937|bibcode = 2016BpJ...111.2263D}}</ref> Bacterial twitching occurs through cycles of [[polymerization]] and [[depolymerization]] of type IV pili.<ref>{{cite journal |doi = 10.1039/c3sm50546d |title = The bacterial type IV pilus system – a tunable molecular motor |year = 2013 |last1 = Maier |first1 = Berenike |journal = Soft Matter |volume = 9 |issue = 24 |page = 5667 |bibcode = 2013SMat....9.5667M |doi-access = free }}</ref><ref>{{cite journal |doi = 10.1073/pnas.121171698|title = Direct observation of extension and retraction of type IV pili|year = 2001|last1 = Skerker|first1 = J. M.|last2 = Berg|first2 = H. C.|journal = Proceedings of the National Academy of Sciences|volume = 98|issue = 12|pages = 6901–6904|pmid = 11381130|pmc = 34450|bibcode = 2001PNAS...98.6901S|doi-access = free}}</ref> Polymerization causes the pilus to elongate and eventually attaching into surfaces. Depolymerization makes the pilus retract and detach from the surfaces. Pili retraction produces pulling forces on the bacterium, which will be pulled in the direction of the [[vector sum]] of the pili forces, resulting in a jerky movement. A typical type IV pilus can produce a force exceeding 100 [[piconewton]]s{{hsp}}<ref>{{cite journal |doi = 10.1073/pnas.242523299|title = Single pilus motor forces exceed 100 pN|year = 2002|last1 = Maier|first1 = B.|last2 = Potter|first2 = L.|last3 = So|first3 = M.|last4 = Seifert|first4 = H. S.|last5 = Sheetz|first5 = M. P.|last6 = Sheetz|first6 = M. P.|journal = Proceedings of the National Academy of Sciences|volume = 99|issue = 25|pages = 16012–16017|pmid = 12446837|pmc = 138556|bibcode = 2002PNAS...9916012M|doi-access = free}}</ref> and then a bundle of pili can produce pulling forces up to several [[nanonewton]]s.<ref>{{cite journal |doi = 10.1371/journal.pbio.0060087|title = Cooperative Retraction of Bundled Type IV Pili Enables Nanonewton Force Generation|year = 2008|last1 = Biais|first1 = Nicolas|last2 = Ladoux|first2 = Benoît|last3 = Higashi|first3 = Dustin|last4 = So|first4 = Magdalene|last5 = Sheetz|first5 = Michael|journal = PLOS Biology|volume = 6|issue = 4|pages = e87|pmid = 18416602|pmc = 2292754 | doi-access=free }}</ref> Bacteria may use pili not only for twitching but also for cell-cell interactions,<ref>{{cite journal |doi = 10.1088/1367-2630/aa5483|title = Multiscale modeling of bacterial colonies: How pili mediate the dynamics of single cells and cellular aggregates|year = 2017|last1 = Pönisch|first1 = Wolfram|last2 = Weber|first2 = Christoph A.|last3 = Juckeland|first3 = Guido|last4 = Biais|first4 = Nicolas|last5 = Zaburdaev|first5 = Vasily|journal = New Journal of Physics|volume = 19|issue = 1|page = 015003|pmid = 34017216|pmc = 8132470|bibcode = 2017NJPh...19a5003P}}</ref><ref>{{cite journal |doi = 10.1016/j.bpj.2015.11.2513|title = Differential Interaction Forces Govern Bacterial Sorting and Stability in Early Biofilms|year = 2016|last1 = Dewenter|first1 = Lena|last2 = Oldewurtel|first2 = Enno R.|last3 = Kouzel|first3 = Nadzeya|last4 = Volkmann|first4 = Thorsten|last5 = Henseler|first5 = Katja|last6 = Maier|first6 = Berenike|journal = Biophysical Journal|volume = 110|issue = 3|pages = 469a|bibcode = 2016BpJ...110..469D|doi-access = free}}</ref> surface sensing,<ref>{{cite journal |doi = 10.1126/science.aan5706|title = Obstruction of pilus retraction stimulates bacterial surface sensing|year = 2017|last1 = Ellison|first1 = Courtney K.|last2 = Kan|first2 = Jingbo|last3 = Dillard|first3 = Rebecca S.|last4 = Kysela|first4 = David T.|last5 = Ducret|first5 = Adrien|last6 = Berne|first6 = Cecile|last7 = Hampton|first7 = Cheri M.|last8 = Ke|first8 = Zunlong|last9 = Wright|first9 = Elizabeth R.|last10 = Biais|first10 = Nicolas|last11 = Dalia|first11 = Ankur B.|last12 = Brun|first12 = Yves V.|journal = Science|volume = 358|issue = 6362|pages = 535–538|pmid = 29074778|pmc = 5805138|bibcode = 2017Sci...358..535E}}</ref><ref>{{cite journal |doi = 10.1073/pnas.1502025112|title = Type IV pili mechanochemically regulate virulence factors in ''Pseudomonas'' aeruginosa|year = 2015|last1 = Persat|first1 = Alexandre|last2 = Inclan|first2 = Yuki F.|last3 = Engel|first3 = Joanne N.|last4 = Stone|first4 = Howard A.|last5 = Gitai|first5 = Zemer|journal = Proceedings of the National Academy of Sciences|volume = 112|issue = 24|pages = 7563–7568|pmid = 26041805|pmc = 4475988|bibcode = 2015PNAS..112.7563P|doi-access = free}}</ref> and DNA uptake.<ref>{{cite journal |doi = 10.1038/s41564-018-0174-y|title = Retraction of DNA-bound type IV competence pili initiates DNA uptake during natural transformation in Vibrio cholerae|year = 2018|last1 = Ellison|first1 = Courtney K.|last2 = Dalia|first2 = Triana N.|last3 = Vidal Ceballos|first3 = Alfredo|last4 = Wang|first4 = Joseph Che-Yen|last5 = Biais|first5 = Nicolas|last6 = Brun|first6 = Yves V.|last7 = Dalia|first7 = Ankur B.|journal = Nature Microbiology|volume = 3|issue = 7|pages = 773–780|pmid = 29891864|pmc = 6582970}}</ref><ref name=Jayathilake2019 />

{{clear}}

===Gliding===
[[File:Bacterial gliding.jpg|thumb|upright=1.25| {{center|'''Bacterial gliding{{hsp}}<ref name=BastosArrieta2018 />'''<br />Focal adhesion model<br /><small>Adapted from: Islam and Mignot (2015){{hsp}}<ref name=Islam2015 /></small>}}]]
{{main|Gliding motility}}

[[Gliding motility]] is a type of translocation that is independent of propulsive structures such as [[flagellum|flagella]] or [[pilus|pili]].<ref name=":0"/> Gliding allows microorganisms to travel along the surface of low aqueous films. The mechanisms of this motility are only partially known. The speed of gliding varies between organisms, and the reversal of direction is seemingly regulated by some sort of internal clock.<ref name=":1"/> For example the [[apicomplexan]]s are able to travel at fast rates between 1–10 μm/s. In contrast ''[[Myxococcus xanthus]]'', a slime bacterium, can glide at a rate of 5 μm/min.<ref>{{Cite journal|last1=Sibley|first1=L.David|last2=Håkansson|first2=Sebastian|last3=Carruthers|first3=Vern B|date=1998-01-01|title=Gliding motility: An efficient mechanism for cell penetration|journal=Current Biology|volume=8|issue=1|pages=R12–R14|doi=10.1016/S0960-9822(98)70008-9|pmid=9427622|s2cid=17555804|doi-access=free}}</ref><ref name=":1b">{{cite journal|last1=Sibley|first1=LDI|date=Oct 2010|title=How apicomplexan parasites move in and out of cells|journal=Curr Opin Biotechnol|volume=21|issue=5|pages=592–8|doi=10.1016/j.copbio.2010.05.009|pmc=2947570|pmid=20580218}}<!--|access-date=May 2015--></ref> In [[myxobacteria]] individual bacteria move together to form waves of cells that then differentiate to form fruiting bodies containing spores.<ref name=Kaiser>{{cite journal | vauthors = Kaiser D | title = Signaling in myxobacteria | journal = [[Annual Review of Microbiology]] | volume = 58 | pages = 75–98 | year = 2004 | pmid = 15487930 | doi = 10.1146/annurev.micro.58.030603.123620 }}</ref> Myxobacteria move only when on solid surfaces, unlike say ''E. coli'', which is motile in liquid or solid media.<ref name="pmid21910630">{{cite journal |vauthors=Nan B, Zusman DR |title=Uncovering the mystery of gliding motility in the myxobacteria |journal=[[Annual Review of Genetics]] |volume=45 |pages=21–39 |date=2011 |pmid=21910630 |pmc=3397683 |doi=10.1146/annurev-genet-110410-132547 }}</ref>

===Non-motile===
{{further|Non-motile bacteria}}

[[Non-motile bacteria|Non-motile species]] lack the ability and structures that would allow them to propel themselves, under their own power, through their environment. When non-motile bacteria are cultured in a stab tube, they only grow along the stab line. If the bacteria are mobile, the line will appear diffuse and extend into the medium.<ref>{{cite web|url=http://faculty.ccbcmd.edu/courses/bio141/labmanua/lab7/nonmotil.html|title=BIOL 230 Lab Manual: Nonmotile Bacteria in Motility Medium|website=faculty.ccbcmd.edu|access-date=8 June 2021|archive-date=15 April 2017|archive-url=https://web.archive.org/web/20170415004813/http://faculty.ccbcmd.edu/courses/bio141/labmanua/lab7/nonmotil.html|url-status=dead}} [[File:CC-BY icon.svg|50px]] Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>

==Bacterial taxis: Directed motion==
Bacteria are said to exhibit [[taxis]] if they move in a manner directed toward or away from some stimulus in their environment. This behaviour allows bacteria to reposition themselves in relation to the stimulus. Different types of taxis can be distinguished according to the nature of the stimulus controlling the directed movement, such as [[chemotaxis]] (chemical gradients like glucose), [[aerotaxis]] (oxygen), [[phototaxis]] (light), [[thermotaxis]] (heat), and [[magnetotaxis]] (magnetic fields).<ref name=BastosArrieta2018 />

===Chemotaxis===
The overall movement of a bacterium can be the result of alternating tumble and swim phases.<ref>{{cite journal | vauthors = Berg HC, Brown DA | s2cid = 1909173 | title = Chemotaxis in Escherichia coli analysed by Three-dimensional Tracking | journal = Nature | volume = 239 | issue = 5374 | pages = 500–504 | date = October 1972 | doi = 10.1038/239500a0 | pmid = 4563019 | bibcode = 1972Natur.239..500B }}</ref> As a result, the trajectory of a bacterium swimming in a uniform environment will form a [[random walk]] with relatively straight swims interrupted by random tumbles that reorient the bacterium.<ref>{{cite journal | vauthors = Sourjik V, Wingreen NS | title = Responding to chemical gradients: bacterial chemotaxis | journal = Current Opinion in Cell Biology | volume = 24| issue = 2| pages = 262–268 | date = April 2012 | doi = 10.1016/j.ceb.2011.11.008 | pmid = 22169400 | pmc = 3320702 }}</ref> Bacteria such as ''[[Escherichia coli|E. coli]]'' are unable to choose the direction in which they swim, and are unable to swim in a straight line for more than a few seconds due to rotational [[diffusion]]; in other words, bacteria "forget" the direction in which they are going. By repeatedly evaluating their course, and adjusting if they are moving in the wrong direction, bacteria can direct their random walk motion toward favorable locations.<ref>{{cite book|last=Berg|first=Howard C. |title=Random walks in biology|year=1993|pages=83–94|publisher=Princeton Univ. Press|location=Princeton, NJ|isbn=978-0-691-00064-0|edition=Expanded, rev.}}</ref>

In the presence of a chemical [[gradient]] bacteria will chemotax, or direct their overall motion based on the gradient. If the bacterium senses that it is moving in the correct direction (toward attractant/away from repellent), it will keep swimming in a straight line for a longer time before tumbling; however, if it is moving in the wrong direction, it will tumble sooner. Bacteria like ''[[Escherichia coli|E. coli]]'' use temporal sensing to decide whether their situation is improving or not, and in this way, find the location with the highest concentration of attractant, detecting even small differences in concentration.<ref>{{cite journal |vauthors= Sourjik V, Wingreen N|date=April 2012|title=Responding to Chemical Gradients: Bacterial Chemotaxis |journal= Current Opinion in Cell Biology|volume=24 |issue= 2|pages=262–8|doi=10.1016/j.ceb.2011.11.008|pmid= 22169400|pmc=3320702}}</ref>

This biased random walk is a result of simply choosing between two methods of random movement; namely tumbling and straight swimming.<ref>{{cite journal | vauthors = Macnab RM, Koshland DE | title = The gradient-sensing mechanism in bacterial chemotaxis | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 69 | issue = 9 | pages = 2509–12 | date = September 1972 | pmid = 4560688 | pmc = 426976 | doi = 10.1073/pnas.69.9.2509 | bibcode = 1972PNAS...69.2509M | doi-access = free }}</ref> The helical nature of the individual flagellar filament is critical for this movement to occur. The protein structure that makes up the flagellar filament, [[flagellin]], is conserved among all flagellated bacteria. Vertebrates seem to have taken advantage of this fact by possessing an immune receptor ([[Toll-like receptor|TLR5]]) designed to recognize this conserved protein.

As in many instances in biology, there are bacteria that do not follow this rule. Many bacteria, such as ''Vibrio'', are monoflagellated and have a single flagellum at one pole of the cell. Their method of chemotaxis is different. Others possess a single flagellum that is kept inside the cell wall. These bacteria move by spinning the whole cell, which is shaped like a corkscrew.<ref>{{cite book |first=Howard C. |last=Berg | name-list-style = vanc | title=''E. coli'' in motion | year=2003 | isbn = 978-0-387-00888-2 |publisher=Springer | location=New York, NY }}{{page needed|date=March 2017}}</ref>

The ability of [[marine microbes]] to navigate toward chemical hotspots can determine their nutrient uptake and has the potential to affect the [[Marine biogeochemical cycles|cycling of elements]] in the ocean. The link between bacterial navigation and [[nutrient cycling]] highlights the need to understand how chemotaxis functions in the context of marine microenvironments. Chemotaxis hinges on the stochastic binding/unbinding of molecules with surface receptors, the transduction of this information through an [[intracellular signaling cascade]], and the activation and control of flagellar motors. The intrinsic randomness of these processes is a central challenge that cells must deal with in order to navigate, particularly under dilute conditions where noise and signal are similar in magnitude. Such conditions are ubiquitous in the ocean, where nutrient concentrations are often extremely low and subject to rapid variation in space (e.g., particulate matter, nutrient plumes) and time (e.g., diffusing sources, fluid mixing).<ref name=Brumley2020>{{cite journal |author1=Douglas R. Brumley |author2=Francesco Carrara |author3=Andrew M. Hein |author4=George I. Hagstrom |author5=Simon A. Levin |author6=Roman Stocker |title=Cutting Through the Noise: Bacterial Chemotaxis in Marine Microenvironments |journal=Frontiers in Marine Science |date=24 July 2020 |volume=7 |doi = 10.3389/fmars.2020.00527|hdl=11343/274205 |hdl-access=free |doi-access=free }} [[File:CC-BY icon.svg|50px]] Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>

The fine-scale interactions between [[marine bacteria]] and both [[Dissolved organic matter|dissolved]] and [[particulate organic matter]] underpin marine biogeochemistry, thereby supporting productivity and influencing carbon storage and sequestration in the planet's oceans.<ref>{{cite journal |doi = 10.1126/science.280.5364.694|title = OCEANOGRAPHY: Microbial Control of Oceanic Carbon Flux: The Plot Thickens|year = 1998|last1 = Azam|first1 = F.|journal = Science|volume = 280|issue = 5364|pages = 694–696|bibcode = 1998Sci...280..694.|s2cid = 83762501}}</ref> It has been historically very difficult to characterize marine environments on the microscales that are most relevant to individual bacteria. Rather, research efforts have typically sampled much larger volumes of water and made comparisons from one sampling site to another.<ref>{{cite journal |doi = 10.1371/journal.pbio.1001177|title = A Holistic Approach to Marine Eco-Systems Biology|year = 2011|last1 = Karsenti|first1 = Eric|last2 = Acinas|first2 = Silvia G.|last3 = Bork|first3 = Peer|last4 = Bowler|first4 = Chris|last5 = De Vargas|first5 = Colomban|last6 = Raes|first6 = Jeroen|last7 = Sullivan|first7 = Matthew|last8 = Arendt|first8 = Detlev|last9 = Benzoni|first9 = Francesca|last10 = Claverie|first10 = Jean-Michel|last11 = Follows|first11 = Mick|last12 = Gorsky|first12 = Gaby|last13 = Hingamp|first13 = Pascal|last14 = Iudicone|first14 = Daniele|last15 = Jaillon|first15 = Olivier|last16 = Kandels-Lewis|first16 = Stefanie|last17 = Krzic|first17 = Uros|last18 = Not|first18 = Fabrice|last19 = Ogata|first19 = Hiroyuki|last20 = Pesant|first20 = Stéphane|last21 = Reynaud|first21 = Emmanuel Georges|last22 = Sardet|first22 = Christian|last23 = Sieracki|first23 = Michael E.|last24 = Speich|first24 = Sabrina|last25 = Velayoudon|first25 = Didier|last26 = Weissenbach|first26 = Jean|last27 = Wincker|first27 = Patrick|journal = PLOS Biology|volume = 9|issue = 10|pages = e1001177|pmid = 22028628|pmc = 3196472 | doi-access=free }}</ref><ref>{{cite journal |doi = 10.1126/science.aac5605|title = Tara Oceans studies plankton at planetary scale|year = 2015|last1 = Bork|first1 = P.|last2 = Bowler|first2 = C.|last3 = De Vargas|first3 = C.|last4 = Gorsky|first4 = G.|last5 = Karsenti|first5 = E.|last6 = Wincker|first6 = P.|journal = Science|volume = 348|issue = 6237|page = 873|pmid = 25999501|bibcode = 2015Sci...348..873B|s2cid = 36627341|doi-access = free}}</ref> However, at the length scales relevant to individual microbes, the ocean is an intricate and dynamic landscape of nutrient patches, at times too small to be mixed by turbulence.<ref>{{cite book |doi = 10.1515/9780691190310|title = A Mechanistic Approach to Plankton Ecology|year = 2009|last1 = Kiørboe|first1 = Thomas|isbn = 9780691190310}}</ref><ref name=Stocker2012>{{cite journal |doi = 10.1126/science.1208929|title = Marine Microbes See a Sea of Gradients|year = 2012|last1 = Stocker|first1 = R.|journal = Science|volume = 338|issue = 6107|pages = 628–633|pmid = 23118182|bibcode = 2012Sci...338..628S|s2cid = 7919921}}</ref> The capacity for microbes to actively navigate these structured environments using chemotaxis can strongly influence their nutrient uptake. Although some work has examined time-dependent chemical profiles,<ref>{{cite journal |doi = 10.1103/PhysRevLett.108.128101|title = Frequency-Dependent ''Escherichia'' coli ''Chemotaxis'' Behavior|year = 2012|last1 = Zhu|first1 = Xuejun|last2 = Si|first2 = Guangwei|last3 = Deng|first3 = Nianpei|last4 = Ouyang|first4 = Qi|last5 = Wu|first5 = Tailin|last6 = He|first6 = Zhuoran|last7 = Jiang|first7 = Lili|last8 = Luo|first8 = Chunxiong|last9 = Tu|first9 = Yuhai|journal = Physical Review Letters|volume = 108|issue = 12|page = 128101|pmid = 22540625|pmc = 3412125|bibcode = 2012PhRvL.108l8101Z}}</ref> past investigations of chemotaxis using ''E. coli'' and other model organisms have routinely examined steady chemical gradients strong enough to elicit a discernible chemotactic response.<ref>{{cite journal |doi = 10.1073/pnas.1113706109|title = Stochastic coordination of multiple actuators reduces latency and improves chemotactic response in bacteria|year = 2012|last1 = Sneddon|first1 = M. W.|last2 = Pontius|first2 = W.|last3 = Emonet|first3 = T.|journal = Proceedings of the National Academy of Sciences|volume = 109|issue = 3|pages = 805–810|pmid = 22203971|pmc = 3271881|bibcode = 2012PNAS..109..805S|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1038/s41467-019-09521-2|title = Bacterial chemotaxis in a microfluidic T-maze reveals strong phenotypic heterogeneity in chemotactic sensitivity|year = 2019|last1 = Salek|first1 = M. Mehdi|last2 = Carrara|first2 = Francesco|last3 = Fernandez|first3 = Vicente|last4 = Guasto|first4 = Jeffrey S.|last5 = Stocker|first5 = Roman|journal = Nature Communications|volume = 10|issue = 1|page = 1877|pmid = 31015402|pmc = 6478840|bibcode = 2019NatCo..10.1877S}}</ref> However, the typical chemical gradients wild marine bacteria encounter are often very weak, ephemeral in nature, and with low background concentrations.<ref name=Stocker2012 /> Shallow gradients are relevant for marine bacteria because, in general, gradients become weaker as one moves away from the source. Yet, detecting such gradients at distance has tremendous value, because they point toward nutrient sources. Shallow gradients are important precisely because they can be used to navigate to regions in the vicinity of sources where gradients become steep, concentrations are high, and bacteria can acquire resources at a high rate.<ref name=Brumley2020 />

{{clear}}

===Phototaxis===
[[Phototaxis]] is a kind of [[taxis]], or locomotory movement, that occurs when a whole [[organism]] moves towards or away from a [[stimulus (physiology)|stimulus]] of [[light]].<ref name=Martin1983>{{citation|editor=Martin, E.A.|year=1983|title=Macmillan Dictionary of Life Sciences|edition= 2nd|page=362|place=London|publisher=Macmillan Press|isbn=978-0-333-34867-3}}</ref> This is advantageous for [[phototroph]]ic organisms as they can orient themselves most efficiently to receive light for [[photosynthesis]]. Phototaxis is called positive if the movement is in the direction of increasing light intensity and negative if the direction is opposite.<ref name=Menzel1979>{{citation|author=Menzel, Randolf|year=1979|editor=H. Autrum|chapter=Spectral Sensitivity and Color Vision in Invertebrates |title=Comparative Physiology and Evolution of Vision in Invertebrates- A: Invertebrate Photoreceptors|location=New York|series=Handbook of Sensory Physiology|volume=VII/6A|pages=503–580. See section D: Wavelength–Specific Behavior and Color Vision|publisher=Springer-Verlag|isbn=978-3-540-08837-0}}</ref>

Two types of positive phototaxis are observed in [[prokaryote]]s. The first is called "scotophobotaxis" (from the word "[[scotophobia]]"), which is observed only under a microscope. This occurs when a bacterium swims by chance out of the area illuminated by the microscope. Entering darkness signals the cell to reverse [[Flagellum|flagella]] rotation direction and reenter the light. The second type of phototaxis is true phototaxis, which is a directed movement up a gradient to an increasing amount of light. This is analogous to positive [[chemotaxis]] except that the attractant is light rather than a chemical.

Phototactic responses are observed in a number of bacteria and archae, such as ''[[Serratia marcescens]]''. [[Photoreceptor protein]]s are light-sensitive proteins involved in the sensing and response to light in a variety of organisms. Some examples are [[bacteriorhodopsin]] and bacteriophytochromes in some bacteria. See also: [[phytochrome]] and [[phototropism]].

Most prokaryotes (bacteria and archaea) are unable to sense the direction of light, because at such a small scale it is very difficult to make a detector that can distinguish a single light direction. Still, prokaryotes can measure light intensity and move in a light-intensity gradient. Some gliding filamentous prokaryotes can even sense light direction and make directed turns, but their phototactic movement is very slow. Some bacteria and archaea are phototactic.<ref>{{cite journal |doi = 10.1016/0014-5793(94)80183-5|title = Phototactic behaviour of the archaebacterial Natronobacterium pharaonis|year = 1994|last1 = Scharf|first1 = Birgit|last2 = Wolff|first2 = Elmar K.|journal = FEBS Letters|volume = 340|issue = 1–2|pages = 114–116|pmid = 8119392|s2cid = 20435383|doi-access = }}</ref><ref>{{cite journal |doi = 10.1023/A:1024974111818|year = 2003|last1 = Armitage|first1 = Judith P.|last2 = Hellingwerf|first2 = Klaas J.|title = Light-induced behavioral responses (;phototaxis') in prokaryotes|journal = Photosynthesis Research|volume = 76|issue = 1–3|pages = 145–155|pmid = 16228574|s2cid = 9325066}}</ref><ref name="Jékely2009" />

In most cases the mechanism of phototaxis is a biased random walk, analogous to bacterial chemotaxis. [[Halophilic]] archaea, such as ''[[Halobacterium salinarum]]'', use sensory [[rhodopsin]]s (SRs) for phototaxis.<ref>{{cite journal |doi = 10.1126/science.1062977|title = Crystal Structure of Sensory Rhodopsin II at 2.4 Angstroms: Insights into Color Tuning and Transducer Interaction|year = 2001|last1 = Luecke|first1 = H.|last2 = Schobert|first2 = B.|last3 = Lanyi|first3 = J. K.|last4 = Spudich|first4 = E. N.|last5 = Spudich|first5 = J. L.|journal = Science|volume = 293|issue = 5534|pages = 1499–1503|pmid = 11452084|pmc = 4996266|bibcode = 2001Sci...293.1499L}}</ref><ref>{{cite journal |doi = 10.1016/j.tim.2006.09.005|title = The multitalented microbial sensory rhodopsins|year = 2006|last1 = Spudich|first1 = John L.|journal = Trends in Microbiology|volume = 14|issue = 11|pages = 480–487|pmid = 17005405}}</ref> Rhodopsins are [[7 transmembrane]] proteins that bind [[retinal]] as a [[chromophore]]. Light triggers the [[isomerization]] of retinal,<ref>{{cite journal |doi = 10.1016/S0006-3495(90)82600-X|title = All-trans/13-cis isomerization of retinal is required for phototaxis signaling by sensory rhodopsins in Halobacterium halobium|year = 1990|last1 = Yan|first1 = B.|last2 = Takahashi|first2 = T.|last3 = Johnson|first3 = R.|last4 = Derguini|first4 = F.|last5 = Nakanishi|first5 = K.|last6 = Spudich|first6 = J.L.|journal = Biophysical Journal|volume = 57|issue = 4|pages = 807–814|pmid = 2344465|pmc = 1280781|bibcode = 1990BpJ....57..807Y}}</ref> which leads to phototransductory signalling via a two-component phosphotransfer relay system. ''Halobacterium salinarum'' has two SRs, [[SRI (gene)|SRI]] and SRII, which signal via the transducer proteins HtrI and HtrII (halobacterial transducers for SRs I and II), respectively.<ref>{{cite journal |doi = 10.1038/nature01109|title = Molecular basis of transmembrane signalling by sensory rhodopsin II–transducer complex|year = 2002|last1 = Gordeliy|first1 = Valentin I.|last2 = Labahn|first2 = Jörg|last3 = Moukhametzianov|first3 = Rouslan|last4 = Efremov|first4 = Rouslan|last5 = Granzin|first5 = Joachim|last6 = Schlesinger|first6 = Ramona|last7 = Büldt|first7 = Georg|last8 = Savopol|first8 = Tudor|last9 = Scheidig|first9 = Axel J.|last10 = Klare|first10 = Johann P.|last11 = Engelhard|first11 = Martin|journal = Nature|volume = 419|issue = 6906|pages = 484–487|pmid = 12368857|bibcode = 2002Natur.419..484G|s2cid = 4425659}}</ref><ref>{{cite journal |doi = 10.1111/j.1751-1097.2008.00314.x|title = Signal Transfer in Haloarchaeal Sensory Rhodopsin Transducer Complexes|year = 2008|last1 = Sasaki|first1 = Jun|last2 = Spudich|first2 = John L.|journal = Photochemistry and Photobiology|volume = 84|issue = 4|pages = 863–868|pmid = 18346091|s2cid = 2811584|doi-access = free}}</ref> The downstream signalling in phototactic archaebacteria involves CheA, a [[histidine kinase]], which phosphorylates the response regulator, CheY.<ref>{{cite journal |doi = 10.1002/j.1460-2075.1995.tb07045.x|title = Chemotaxis and phototaxis require a CheA histidine kinase in the archaeon Halobacterium salinarium|year = 1995|last1 = Rudolph|first1 = J.|last2 = Oesterhelt|first2 = D.|journal = The EMBO Journal|volume = 14|issue = 4|pages = 667–673|pmid = 7882970|pmc = 398130}}</ref> Phosphorylated CheY induces swimming reversals. The two SRs in Halobacterium have different functions. SRI acts as an attractant receptor for orange light and, through a two-photon reaction, a repellent receptor for near-UV light, while SRII is a repellent receptor for blue light. Depending on which receptor is expressed, if a cell swims up or down a steep light gradient, the probability of flagellar switch will be low. If light intensity is constant or changes in the wrong direction, a switch in the direction of flagellar rotation will reorient the cell in a new, random direction.<ref>{{cite journal |doi = 10.1128/jb.169.10.4750-4758.1987|title = Kinetically resolved states of the Halobacterium halobium flagellar motor switch and modulation of the switch by sensory rhodopsin I|year = 1987|last1 = McCain|first1 = D. A.|last2 = Amici|first2 = L. A.|last3 = Spudich|first3 = J. L.|journal = Journal of Bacteriology|volume = 169|issue = 10|pages = 4750–4758|pmid = 3654583|pmc = 213850}}</ref> As the length of the tracks is longer when the cell follows a light gradient, cells will eventually get closer to or further away from the light source. This strategy does not allow orientation along the light vector and only works if a steep light gradient is present (i.e. not in open water).<ref name="Jékely2009">{{cite journal |doi = 10.1098/rstb.2009.0072|title = Evolution of phototaxis|year = 2009|last1 = Jékely|first1 = Gáspár|journal = Philosophical Transactions of the Royal Society B: Biological Sciences|volume = 364|issue = 1531|pages = 2795–2808|pmid = 19720645|pmc = 2781859}} [[File:CC-BY icon.svg|50px]] Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>

Some cyanobacteria (e.g. ''[[Anabaena]]'', ''[[Synechocystis]]'') can slowly orient along a light vector. This orientation occurs in filaments or colonies, but only on surfaces and not in suspension.<ref>{{cite journal|last1=Nultsch|first1=Wilhelm|last2=Schuchart|first2=Hartwig|last3=Höhl|first3=Marga|year=1979|title=Investigations on the phototactic orientation of Anabaena variabilis|journal=Archives of Microbiology|volume=122|pages=85–91|doi=10.1007/BF00408050|s2cid=12242837}}</ref><ref>{{cite journal |doi = 10.1111/j.1751-1097.1999.tb01954.x|title = Photomovement of the Gliding Cyanobacterium Synechocystis sp. PCC 6803|year = 1999|last1 = Choi|first1 = Jong-Soon|last2 = Chung|first2 = Young-Ho|last3 = Moon|first3 = Yoon-Jung|last4 = Kim|first4 = Changhoon|last5 = Watanabe|first5 = Masakatsu|last6 = Song|first6 = Pill-Soon|last7 = Joe|first7 = Cheol-O|last8 = Bogorad|first8 = Lawrence|last9 = Park|first9 = Young Mok|journal = Photochemistry and Photobiology|volume = 70|issue = 1|pages = 95–102|pmid = 10420848|s2cid = 25364218}}</ref> The filamentous cyanobacterium ''Synechocystis'' is capable of both positive and negative two-dimensional phototactic orientation. The positive response is probably mediated by a bacteriophytochrome photoreceptor, TaxD1. This protein has two chromophore-binding GAF domains, which bind [[biliverdin]] chromophore,<ref>{{cite journal |doi = 10.1038/414776a|title = Bacteriophytochromes are photochromic histidine kinases using a biliverdin chromophore|year = 2001|last1 = Bhoo|first1 = Seong-Hee|last2 = Davis|first2 = Seth J.|last3 = Walker|first3 = Joseph|last4 = Karniol|first4 = Baruch|last5 = Vierstra|first5 = Richard D.|journal = Nature|volume = 414|issue = 6865|pages = 776–779|pmid = 11742406|bibcode = 2001Natur.414..776B|s2cid = 4424642}}</ref> and a [[C-terminal domain]] typical for bacterial taxis receptors ([[Methyl-accepting chemotaxis protein|MCP]] signal domain). TaxD1 also has two [[N-terminus|N-terminal]] transmembrane segments that anchor the protein to the membrane.<ref>Zhulin, I.B. (2000) "A novel phototaxis receptor hidden in the cyanobacterial genome". ''Journal of molecular microbiology and biotechnology'', '''2'''(4): 491–494.</ref><ref>{{cite journal |doi = 10.1111/j.1365-2958.2004.04160.x|title = Light matters: Phototaxis and signal transduction in unicellular cyanobacteria|year = 2004|last1 = Bhaya|first1 = Devaki|journal = Molecular Microbiology|volume = 53|issue = 3|pages = 745–754|pmid = 15255889|s2cid = 9549058|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1039/b402320j|title = Phototactic motility in the unicellular cyanobacterium Synechocystis sp. PCC 6803|year = 2004|last1 = Yoshihara|first1 = Shizue|last2 = Ikeuchi|first2 = Masahiko|journal = Photochemical & Photobiological Sciences|volume = 3|issue = 6|pages = 512–518|pmid = 15170479| s2cid=27566851 |doi-access = free}}</ref> The photoreceptor and signalling domains are cytoplasmic and signal via a CheA/CheY-type signal transduction system to regulate motility by type IV pili.<ref>{{cite journal |doi = 10.1093/pcp/pce010|title = Novel Putative Photoreceptor and Regulatory Genes Required for the Positive Phototactic Movement of the Unicellular Motile Cyanobacterium Synechocystis sp. PCC 6803|year = 2000|last1 = Yoshihara|first1 = Shizue|last2 = Suzuki|first2 = Fumiko|last3 = Fujita|first3 = Hironori|last4 = Geng|first4 = Xiao Xing|last5 = Ikeuchi|first5 = Masahiko|journal = Plant and Cell Physiology|volume = 41|issue = 12|pages = 1299–1304|pmid = 11134414|doi-access = free}}</ref> TaxD1 is localized at the poles of the rod-shaped cells of ''[[Synechococcus elongatus]]'', similarly to MCP containing chemosensory receptors in bacteria and archaea.<ref>{{cite journal |doi = 10.1128/JB.182.22.6499-6502.2000|title = Evolutionary Conservation of Methyl-Accepting Chemotaxis Protein Location in Bacteria and Archaea|year = 2000|last1 = Gestwicki|first1 = Jason E.|last2 = Lamanna|first2 = Allison C.|last3 = Harshey|first3 = Rasika M.|last4 = McCarter|first4 = Linda L.|last5 = Kiessling|first5 = Laura L.|last6 = Adler|first6 = Julius|journal = Journal of Bacteriology|volume = 182|issue = 22|pages = 6499–6502|pmid = 11053396|pmc = 94798}}</ref> How the steering of the filaments is achieved is not known. The slow steering of these cyanobacterial filaments is the only light-direction sensing behaviour prokaryotes could evolve owing to the difficulty in detecting light direction at this small scale.<ref name="Jékely2009" />

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 | caption2  = Magnetosome chain with octahedral habits modelled lower right <ref>Pósfai, M., Lefèvre, C., Trubitsyn, D., Bazylinski, D.A. and Frankel, R. (2013) "Phylogenetic significance of composition and crystal morphology of magnetosome minerals". ''Frontiers in microbiology'', '''4''': 344. {{doi|10.3389/fmicb.2013.00344}}. [[File:CC-BY icon.svg|50px]] Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/3.0/ Creative Commons Attribution 3.0 International License].</ref>
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===Magnetotaxis===
[[Magnetotactic bacteria]] orient themselves along the [[magnetic field|magnetic field lines]] of [[Earth's magnetic field]].<ref>{{Cite journal|last1=Lin |first1=Wei |last2=Zhang |first2=Wensi |last3=Zhao |first3=Xiang |last4=Roberts |first4=Andrew |last5=Paterson  |first5=Greig |last6=Bazylinski |first6=Dennis |last7=Pan |first7=Yongxin |date=March 2018|title=Genomic expansion of magnetotactic bacteria reveals an early common origin of magnetotaxis with lineage-specific evolution|journal=The ISME Journal|volume=12 |issue=6 |pages=1508–1519 |doi=10.1038/s41396-018-0098-9 |pmid=29581530 |pmc=5955933 }}</ref> This alignment is believed to aid these organisms in reaching regions of optimal oxygen concentration.<ref name=Dusenbery2009>{{cite book |last1=Dusenbery |first1=David B. |title=Living at micro scale : the unexpected physics of being small |date=2009 |publisher=Harvard University Press |location=Cambridge, Mass. |pages= 100–101|isbn=978-0-674-03116-6}}</ref> To perform this task, these bacteria have [[Biomineralisation|biomineralised]] [[organelle]]s called [[magnetosome]]s that contain magnetic [[crystal]]s. The biological phenomenon of microorganisms tending to move in response to the environment's magnetic characteristics is known as [[magnetotaxis]].  However, this term is misleading in that every other application of the term [[taxis]] involves a stimulus-response mechanism.  In contrast to the [[magnetoreception]] of animals, the bacteria contain fixed magnets that force the bacteria into alignment—even dead cells are dragged into alignment, just like a compass needle.<ref name=Dusenbery2009 />

===Escape response===
[[File:Halobacterium salinarum NRC-1.png|thumb|upright=0.6| {{center|''[[Halobacterium salinarum]]'' NRC-1<br /><small> Size bar {{=}} 270 nm</small>}}]]

An [[escape response]] is a form of negative taxis. Stimuli that have the potential to harm or kill demand rapid detection. This is fundamentally distinct from navigation or exploration, in terms of the timescales available for response. Most motile species harbour a form of phobic or emergency response distinct from their steady state locomotion.<ref name=Wan2021 /> [[Escape reaction]]s are not strictly oriented—but commonly involve backward movement, sometimes with a negatively [[geotaxis|geotactic]] component.<ref>{{cite journal |doi = 10.3354/meps214067|title = Escape response of planktonic protists to fluid mechanical signals|year = 2001|last1 = Jakobsen|first1 = HH|journal = Marine Ecology Progress Series|volume = 214|pages = 67–78|bibcode = 2001MEPS..214...67J|doi-access = free}}</ref><ref name=Wan2021>{{cite journal |doi = 10.1098/rstb.2019.0758|arxiv=2007.13388 |title = Origins of eukaryotic excitability|year = 2021|last1 = Wan|first1 = Kirsty Y.|last2 = Jékely|first2 = Gáspár|journal = Philosophical Transactions of the Royal Society B: Biological Sciences|volume = 376|issue = 1820|pmid = 33487111|pmc = 7935092}} [[File:CC-BY icon.svg|50px]] Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref> In bacteria and archaea, [[action potential]]-like phenomena have been observed in [[biofilm]]s{{hsp}}<ref>{{cite journal |doi = 10.1016/j.cels.2020.04.002|title = Encoding Membrane-Potential-Based Memory within a Microbial Community|year = 2020|last1 = Yang|first1 = Chih-Yu|last2 = Bialecka-Fornal|first2 = Maja|last3 = Weatherwax|first3 = Colleen|last4 = Larkin|first4 = Joseph W.|last5 = Prindle|first5 = Arthur|last6 = Liu|first6 = Jintao|last7 = Garcia-Ojalvo|first7 = Jordi|last8 = Süel|first8 = Gürol M.|journal = Cell Systems|volume = 10|issue = 5|pages = 417–423.e3|pmid = 32343961|pmc = 7286314}}</ref> and also single cells such as [[cable bacteria]].<ref name=Wan2021 /> The archaeon ''[[Halobacterium salinarium]]'' shows a photophobic response characterized by a 180° reversal of its swimming direction induced by a reversal in the direction of flagellar rotation. At least some aspects of this response are likely mediated by changes in membrane potential by [[bacteriorhodopsin]], a light-driven [[proton pump]].<ref>{{cite journal |doi = 10.1128/JB.178.11.3008-3014.1996|title = Delta psi-mediated signalling in the bacteriorhodopsin-dependent photoresponse|year = 1996|last1 = Grishanin|first1 = R. N.|last2 = Bibikov|first2 = S. I.|last3 = Altschuler|first3 = I. M.|last4 = Kaulen|first4 = A. D.|last5 = Kazimirchuk|first5 = S. B.|last6 = Armitage|first6 = J. P.|last7 = Skulachev|first7 = V. P.|journal = Journal of Bacteriology|volume = 178|issue = 11|pages = 3008–3014|pmid = 8655473|pmc = 178045}}</ref> Action potential-like phenomena in prokaryotes are dissimilar from classical eukaryotic action potentials. The former are less reproducible, slower and exhibit a broader distribution in pulse amplitude and duration.<ref>{{cite journal |doi = 10.1073/pnas.1703084114|title = Voltage-gated calcium flux mediates Escherichia coli mechanosensation|year = 2017|last1 = Bruni|first1 = Giancarlo N.|last2 = Weekley|first2 = R. Andrew|last3 = Dodd|first3 = Benjamin J. T.|last4 = Kralj|first4 = Joel M.|journal = Proceedings of the National Academy of Sciences|volume = 114|issue = 35|pages = 9445–9450|pmid = 28808010|pmc = 5584419| bibcode=2017PNAS..114.9445B |doi-access = free}}</ref><ref name=Wan2021 />

===Other taxes===
* Aerotaxis is the response of an organism to variation in oxygen concentration, and is mainly found in aerobic bacteria.<ref name=Martin1983 />
* Energy taxis is the orientation of bacteria towards conditions of optimal metabolic activity by sensing the internal energetic conditions of cell. Therefore, in contrast to chemotaxis (taxis towards or away from a specific extracellular compound), energy taxis responds on an intracellular stimulus (e.g. [[proton motive force]], activity of [[NADH dehydrogenase|NDH- 1]]) and requires metabolic activity.<ref>Schweinitzer T, Josenhans C. [https://doi.org/10.1007%2Fs00203-010-0575-7 Bacterial energy taxis: a global strategy?] Arch Microbiol. 2010 Jul;192(7):507-20.</ref>

==Mathematical modelling==
[[File:Simulation of bacterial in the presence of a chemoattractant.jpg|thumb|upright=1.7| {{center|Simulation by the bacteria network simulator BNSim{{hsp}}<ref>[https://radum.ece.utexas.edu/bnsim-bacteria-network-simulator/ BNSim] University of Texas. Accessed 10 June 2021.</ref><ref>{{cite journal |doi = 10.1109/JSAC.2013.SUP2.12130019|title = Efficient Modeling and Simulation of Bacteria-Based Nanonetworks with BNSim|year = 2013|last1 = Wei|first1 = Guopeng|last2 = Bogdan|first2 = Paul|last3 = Marculescu|first3 = Radu|journal = IEEE Journal on Selected Areas in Communications|volume = 31|issue = 12|pages = 868–878|s2cid = 1888353}}</ref>
<br />of the swimming behaviour of ''[[Serratia marcescens]]''<br />(a) without, and (b) with, the presence of a [[chemoattractant]]{{hsp}}<ref name=Koorehdavoudi2017>{{cite journal |doi = 10.1098/rspa.2017.0154|title = Multi-fractal characterization of bacterial swimming dynamics: a case study on real and simulated Serratia marcescens|year = 2017|last1 = Koorehdavoudi|first1 = Hana|last2 = Bogdan|first2 = Paul|last3 = Wei|first3 = Guopeng|last4 = Marculescu|first4 = Radu|last5 = Zhuang|first5 = Jiang|last6 = Carlsen|first6 = Rika Wright|last7 = Sitti|first7 = Metin|journal = Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences|volume = 473|issue = 2203|pmid = 28804259|pmc = 5549567|bibcode = 2017RSPSA.47370154K}} [[File:CC-BY icon.svg|50px]] Modified text was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>}}]]

The mathematical models used to describe the bacterial swimming dynamics can be classified into two categories. The first category is based on a microscopic (i.e. cell-level) view of bacterial swimming through a set of equations where each equation describes the state of a single agent.<ref>{{cite journal |doi = 10.1371/journal.pone.0018623|title = On the Origin and Characteristics of Noise-Induced Lévy Walks of e. Coli|year = 2011|last1 = Matthäus|first1 = Franziska|last2 = Mommer|first2 = Mario S.|last3 = Curk|first3 = Tine|last4 = Dobnikar|first4 = Jure|journal = PLOS ONE|volume = 6|issue = 4|pages = e18623|pmid = 21494629|pmc = 3072995|bibcode = 2011PLoSO...618623M|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1007/BF00532948|title = Some stochastic processes which arise from a model of the motion of a bacterium|year = 1974|last1 = Stroock|first1 = Daniel W.|journal = Zeitschrift für Wahrscheinlichkeitstheorie und Verwandte Gebiete|volume = 28|issue = 4|pages = 305–315|s2cid = 123428558|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1529/biophysj.105.061150|title = Cell Motility as Persistent Random Motion: Theories from Experiments|year = 2005|last1 = Selmeczi|first1 = David|last2 = Mosler|first2 = Stephan|last3 = Hagedorn|first3 = Peter H.|last4 = Larsen|first4 = Niels B.|last5 = Flyvbjerg|first5 = Henrik|journal = Biophysical Journal|volume = 89|issue = 2|pages = 912–931|pmid = 15951372|pmc = 1366641|bibcode = 2005BpJ....89..912S}}</ref><ref>{{cite journal |doi = 10.1098/rsif.2008.0014|title = Random walk models in biology|year = 2008|last1 = Codling|first1 = Edward A.|last2 = Plank|first2 = Michael J.|last3 = Benhamou|first3 = Simon|journal = Journal of the Royal Society Interface|volume = 5|issue = 25|pages = 813–834|pmid = 18426776|pmc = 2504494}}</ref><ref>{{cite journal |doi = 10.1371/journal.pone.0074878|title = Chemotactic Sensing towards Ambient and Secreted Attractant Drives Collective Behaviour of e. Coli|year = 2013|last1 = Curk|first1 = Tine|last2 = Marenduzzo|first2 = Davide|last3 = Dobnikar|first3 = Jure|journal = PLOS ONE|volume = 8|issue = 10|pages = e74878|pmid = 24098352|pmc = 3789734|bibcode = 2013PLoSO...874878C|doi-access = free}}</ref> The second category provides a macroscopic (i.e. population-level) view via continuum-based partial differential equations that capture the dynamics of population density over space and time, without considering the intracellular characteristics directly.<ref>{{cite journal |doi = 10.1038/srep27602|title = A Statistical Physics Characterization of the Complex Systems Dynamics: Quantifying Complexity from Spatio-Temporal Interactions|year = 2016|last1 = Koorehdavoudi|first1 = Hana|last2 = Bogdan|first2 = Paul|journal = Scientific Reports|volume = 6|page = 27602|pmid = 27297496|pmc = 4906350|bibcode = 2016NatSR...627602K}}</ref><ref>{{cite journal |doi = 10.1038/nature12673|title = Emergence of macroscopic directed motion in populations of motile colloids|year = 2013|last1 = Bricard|first1 = Antoine|last2 = Caussin|first2 = Jean-Baptiste|last3 = Desreumaux|first3 = Nicolas|last4 = Dauchot|first4 = Olivier|last5 = Bartolo|first5 = Denis|journal = Nature|volume = 503|issue = 7474|pages = 95–98|pmid = 24201282|arxiv = 1311.2017|bibcode = 2013Natur.503...95B|s2cid = 1174081}}</ref><ref>{{cite book|last1=Othmer |first1=Hans G.|last2= Xue |first2=Chuan |series=Lecture Notes in Mathematics |date=2013|volume=2071 |chapter=The Mathematical Analysis of Biological Aggregation and Dispersal: Progress, Problems and Perspectives|title=Dispersal, individual movement and spatial ecology: a mathematical perspective|editor-last1=Lewis|editor-first1= Mark A.|editor-last2= Maini|editor-first2=Philip K.|editor-last3= Petrovskii|editor-first3=Sergei V.|publisher=Springer|isbn=978-3-642-35497-7|doi=10.1007/978-3-642-35497-7_4|pages=79–127}}</ref><ref>{{cite journal |doi = 10.3934/krm.2012.5.817|title = From individual to collective behaviour of coupled velocity jump processes: A locust example|year = 2012|last1 = Erban|first1 = Radek|last2 = Haskovec|first2 = Jan|journal = Kinetic & Related Models|volume = 5|issue = 4|pages = 817–842|s2cid = 18153478|arxiv = 1104.2584}}</ref><ref>{{cite journal |doi = 10.1137/070711505|title = Multiscale Models of Taxis-Driven Patterning in Bacterial Populations|year = 2009|last1 = Xue|first1 = Chuan|last2 = Othmer|first2 = Hans G.|journal = SIAM Journal on Applied Mathematics|volume = 70|issue = 1|pages = 133–167|pmid = 19784399|pmc = 2752049}}</ref><ref>{{cite journal |doi = 10.1007/s00285-008-0201-3|title = A user's guide to PDE models for chemotaxis|year = 2009|last1 = Hillen|first1 = T.|last2 = Painter|first2 = K. J.|journal = Journal of Mathematical Biology|volume = 58|issue = 1–2|pages = 183–217|pmid = 18626644|s2cid = 249201}}</ref><ref>{{cite journal |doi = 10.1088/0951-7715/19/12/010|title = The fractional Keller–Segel model|year = 2006|last1 = Escudero|first1 = Carlos|journal = Nonlinearity|volume = 19|issue = 12|pages = 2909–2918|arxiv = math/0611496|bibcode = 2006Nonli..19.2909E|s2cid = 17774620}}</ref><ref>{{cite journal |doi = 10.1387/ijdb.052048ap|title = Spatial patterns formed by chemotactic bacteria Escherichia coli|year = 2006|last1 = Polezhaev|first1 = Andrey A.|last2 = Pashkov|first2 = Ruslan A.|last3 = Lobanov|first3 = Alexey I.|last4 = Petrov|first4 = Igor B.|journal = The International Journal of Developmental Biology|volume = 50|issue = 2–3|pages = 309–314|pmid = 16479498|doi-access = free}}</ref><ref>{{cite journal |doi = 10.1007/s11538-009-9467-x|title = From Microscopic to Macroscopic Descriptions of Cell Migration on Growing Domains|year = 2010|last1 = Baker|first1 = Ruth E.|last2 = Yates|first2 = Christian A.|last3 = Erban|first3 = Radek|journal = Bulletin of Mathematical Biology|volume = 72|issue = 3|pages = 719–762|pmid = 19862577|s2cid = 17797787|url = https://ora.ox.ac.uk/objects/uuid:09316945-d40d-4018-bf53-f502b012014d}}</ref><ref name=Koorehdavoudi2017 />

Among the present models, [[Mark Schnitzer|Schnitzer]] uses the [[Smoluchowski equation]] to describe the biased [[random walk]] of the bacteria during chemotaxis to search for food.<ref>{{cite journal |doi = 10.1103/PhysRevE.48.2553|title = Theory of continuum random walks and application to chemotaxis|year = 1993|last1 = Schnitzer|first1 = Mark J.|journal = Physical Review E|volume = 48|issue = 4|pages = 2553–2568|pmid = 9960890|bibcode = 1993PhRvE..48.2553S}}</ref> To focus on a detailed description of the motion taking place during one run interval of the bacteria, [[Pierre-Gilles de Gennes|de Gennes]] derives the average run length travelled by bacteria during one counterclockwise interval.<ref>{{cite journal |doi = 10.1007/s00249-004-0426-z|title = Chemotaxis: The role of internal delays|year = 2004|last1 = De Gennes|first1 = P.-G.|journal = European Biophysics Journal|volume = 33|issue = 8|pages = 691–693|pmid = 15257424|s2cid = 412945}}</ref> Along the same direction, to consider the environmental condition affecting the biased random walk of bacteria, Croze and his co-workers study experimentally and theoretically the effect of concentration of soft agar on chemotaxis of bacteria.<ref name="Migration of Chemotactic Bacteria i">{{cite journal |doi = 10.1016/j.bpj.2011.06.023|title = Migration of Chemotactic Bacteria in Soft Agar: Role of Gel Concentration|year = 2011|last1 = Croze|first1 = Ottavio A.|last2 = Ferguson|first2 = Gail P.|last3 = Cates|first3 = Michael E.|last4 = Poon|first4 = Wilson C.K.|journal = Biophysical Journal|volume = 101|issue = 3|pages = 525–534|pmid = 21806920|pmc = 3145277|arxiv = 1101.5063|bibcode = 2011BpJ...101..525C}}</ref><ref name=Koorehdavoudi2017 />

To study the effect of obstacles (another environmental condition) on the motion of bacteria, Chepizhko and his co-workers study the motion of [[self-propelled particle]]s in a heterogeneous two-dimensional environment and show that the [[mean square displacement]] of particles is dependent on the density of obstacles and the particle turning speed.<ref name="Migration of Chemotactic Bacteria i"/><ref>{{cite journal |doi = 10.1103/PhysRevLett.111.160604|title = Diffusion, Subdiffusion, and Trapping of Active Particles in Heterogeneous Media|year = 2013|last1 = Chepizhko|first1 = Oleksandr|last2 = Peruani|first2 = Fernando|journal = Physical Review Letters|volume = 111|issue = 16|page = 160604|pmid = 24182247|arxiv = 1310.0830|bibcode = 2013PhRvL.111p0604C|s2cid = 11143298}}</ref> Building on these models, [[Michael Cates|Cates]] highlights that bacterial dynamics does not always obey detailed balance, which means it is a biased [[diffusion]] process depending on the environmental conditions.<ref>{{cite journal |doi = 10.1088/0034-4885/75/4/042601|title = Diffusive transport without detailed balance in motile bacteria: Does microbiology need statistical physics?|year = 2012|last1 = Cates|first1 = M. E.|journal = Reports on Progress in Physics|volume = 75|issue = 4|page = 042601|pmid = 22790505|arxiv = 1208.3957|bibcode = 2012RPPh...75d2601C|s2cid = 3122097}}</ref> Moreover, Ariel and his co-workers focus on diffusion of bacteria and show that the bacteria perform [[super-diffusion]] during swarming on a surface.<ref>{{cite journal |doi = 10.1038/ncomms9396|title = Swarming bacteria migrate by Lévy Walk|year = 2015|last1 = Ariel|first1 = Gil|last2 = Rabani|first2 = Amit|last3 = Benisty|first3 = Sivan|last4 = Partridge|first4 = Jonathan D.|last5 = Harshey|first5 = Rasika M.|last6 = Be'Er|first6 = Avraham|journal = Nature Communications|volume = 6|page = 8396|pmid = 26403719|pmc = 4598630|bibcode = 2015NatCo...6.8396A}}</ref><ref name=Koorehdavoudi2017 />

{{clear}}

==See also==
* [[Cyanobacterial movement]]
* [[Protist locomotion]]

==References==
{{reflist}}

==External links==
* Review of the hydrodynamics of bacterial swimming: {{cite journal |doi = 10.1146/annurev-fluid-122414-034606|title = Bacterial Hydrodynamics|year = 2016|last1 = Lauga|first1 = Eric|journal = Annual Review of Fluid Mechanics|volume = 48|issue = 1|pages = 105–130|arxiv = 1509.02184|bibcode = 2016AnRFM..48..105L|s2cid = 13849152}}
* [http://www.textbookofbacteriology.net/ On-line text book on bacteriology (2015)]

{{Bacteria}}
{{microorganisms}}
{{Authority control}}

[[Category:Bacteria]]
[[Category:Bacteriology]]
[[Category:Microswimmers]]