Editing Flagellum (section)

===Bacterial flagella===

====Structure and composition====
The bacterial flagellum is made up of [[protein]] subunits of [[flagellin]].<ref name="Alberts1"/> Its shape is a 20-[[nanometer]]-thick hollow tube. It is [[helix|helical]] and has a sharp bend just outside the outer membrane; this "hook" allows the axis of the helix to point directly away from the cell. A shaft runs between the hook and the [[basal body]], passing through protein rings in the cell's membrane that act as bearings. [[Gram-positive]] organisms have two of these basal body rings, one in the [[peptidoglycan]] layer and one in the [[plasma membrane]]. [[Gram-negative]] organisms have four such rings: the [[L ring]] associates with the [[lipopolysaccharides]], the [[P ring]] associates with [[peptidoglycan]] layer, the M ring is embedded in the [[plasma membrane]], and the S ring is directly attached to the [[cytoplasm]]. The filament ends with a capping protein.<ref name=MacNab1>{{cite journal |last1=Macnab |first1=Robert M. |title=How Bacteria Assemble Flagella |journal=Annual Review of Microbiology |date=October 2003 |volume=57 |issue=1 |pages=77–100 |doi=10.1146/annurev.micro.57.030502.090832 |pmid=12730325}}</ref><ref name=dioszeghy1>{{cite journal |last1=Diószeghy |first1=Zoltán |last2=Závodszky |first2=Péter |last3=Namba |first3=Keiichi |last4=Vonderviszt |first4=Ferenc |title=Stabilization of flagellar filaments by HAP2 capping |journal=FEBS Letters |date=18 June 2004 |volume=568 |issue=1–3 |pages=105–109 |doi=10.1016/j.febslet.2004.05.029 |pmid=15196929 |s2cid=33886010 |bibcode=2004FEBSL.568..105D}}</ref>

The flagellar filament is the long, helical screw that propels the bacterium when rotated by the motor, through the hook. In most bacteria that have been studied, including the gram-negative ''[[Escherichia coli]], [[Salmonella typhimurium]], [[Caulobacter crescentus]]'', and ''[[Vibrio alginolyticus]]'', the filament is made up of 11 protofilaments approximately parallel to the filament axis. Each protofilament is a series of tandem protein chains. However, ''[[Campylobacter jejuni]]'' has seven protofilaments.<ref name="SevenProtofilamentsGalkin">{{cite journal |last1=Galkin |first1=Vitold E. |last2=Yu |first2=Xiong |last3=Bielnicki |first3=Jakub |last4=Heuser |first4=John |last5=Ewing |first5=Cheryl P. |last6=Guerry |first6=Patricia |last7=Egelman |first7=Edward H. |title=Divergence of Quaternary Structures Among Bacterial Flagellar Filaments |journal=Science |date=18 April 2008 |volume=320 |issue=5874 |pages=382–385 |doi=10.1126/science.1155307 |pmid=18420936 |bibcode=2008Sci...320..382G |s2cid=7702002}}</ref>

The basal body has several traits in common with some types of [[secretion#In gram-negative bacteria|secretory pores]], such as the hollow, rod-like "plug" in their centers extending out through the plasma membrane. The similarities between bacterial flagella and bacterial secretory system structures and proteins provide scientific evidence supporting the theory that bacterial flagella evolved from the [[type three secretion system|type-three secretion system]] (TTSS).

The atomic structure of both bacterial flagella as well as the TTSS [[injectisome]] have been elucidated in great detail, especially with the development of [[Cryogenic electron microscopy|cryo-electron microscopy]]. The best understood parts are the parts between the inner and outer [[Cell membrane|membrane]], that is, the scaffolding rings of the inner membrane (IM), the scaffolding pairs of the outer membrane (OM), and the rod/needle (injectisome) or rod/hook (flagellum) sections.<ref>{{Cite journal |last1=Worrall |first1=Liam J. |last2=Majewski |first2=Dorothy D. |last3=Strynadka |first3=Natalie C.J. |date=2023-09-15 |title=Structural Insights into Type III Secretion Systems of the Bacterial Flagellum and Injectisome |journal=Annual Review of Microbiology |language=en |volume=77 |issue=1 |pages=669–698 |doi=10.1146/annurev-micro-032521-025503 |pmid=37713458 |s2cid=261963968 |issn=0066-4227|doi-access=free }}</ref>

====Motor====
{{Further |Rotating locomotion in living systems}}
[[File:Flagellar Motor Assembly.jpg|alt=Bacterial flagellar motor assembly|thumb|440x440px|Bacterial flagellar motor assembly: Shown here is the C-ring at the base with FliG in red, FliM in yellow, and FliN in shades of purple; the MS-ring in blue; the MotAB in brown; the LP-ring in pink; and the rod in gray.<ref name="Singh2024">{{Cite journal |last1=Singh |first1=Prashant K. |last2=Sharma |first2=Pankaj |last3=Afanzar |first3=Oshri |last4=Goldfarb |first4=Margo H. |last5=Maklashina |first5=Elena |last6=Eisenbach |first6=Michael |last7=Cecchini |first7=Gary |last8=Iverson |first8=T. M. |date=2024-04-17 |title=CryoEM structures reveal how the bacterial flagellum rotates and switches direction |journal=Nature Microbiology |volume=9 |issue=5 |language=en |pages=1271–1281 |doi=10.1038/s41564-024-01674-1 |issn=2058-5276|doi-access=free |pmid=38632342 |pmc=11087270 }}</ref>]]
The bacterial flagellum is driven by a rotary engine ([[MotA|Mot complex]]) made up of protein, located at the flagellum's anchor point on the inner cell membrane. The engine is powered by [[proton-motive force]], i.e., by the flow of protons (hydrogen ions) across the bacterial cell membrane due to a [[concentration gradient]] set up by the cell's metabolism (''[[Vibrio]]'' species have two kinds of flagella, lateral and polar, and some are driven by a sodium [[ion transporter|ion pump]] rather than a [[proton pump]]<ref name="Atsumi2">{{cite journal |last1=Atsumi |first1=Tatsuo |last2=McCartert |first2=Linda |last3=Imae |first3=Yasuo |title=Polar and lateral flagellar motors of marine Vibrio are driven by different ion-motive forces |journal=Nature |date=January 1992 |volume=355 |issue=6356 |pages=182–184 |doi=10.1038/355182a0 |pmid=1309599 |bibcode=1992Natur.355..182A |s2cid=4315167 }}</ref>). The rotor transports protons across the membrane, and is turned in the process. The rotor alone can operate at 6,000 to 100,000 [[revolutions per minute|rpm]],<ref>{{Citation |last1=Kojima |first1=Seiji |title=The Bacterial Flagellar Motor: Structure and Function of a Complex Molecular Machine |date=2004 |series=International Review of Cytology |volume=233 |pages=93–134 |url=https://linkinghub.elsevier.com/retrieve/pii/S0074769604330032 |access-date=2024-04-23 |publisher=Elsevier |language=en |doi=10.1016/s0074-7696(04)33003-2 |isbn=978-0-12-364637-8 |last2=Blair |first2=David F|pmid=15037363 }}</ref> but with the flagellar filament attached usually only reaches 200 to 1000 rpm. The direction of rotation can be changed by the [[flagellar motor switch]] almost instantaneously, caused by a slight change in the position of a protein, [[FliG]], in the rotor.<ref>{{cite journal |last1=Dean |first1=Tim |url=http://lifescientist.com.au/content/molecular-biology/news/inside-nature-s-most-efficient-motor-the-flagellar-1216235209 |title=Inside nature's most efficient motor: the flagellar |journal=Australian Life Scientist |date=2 August 2010 }}</ref> The torque is transferred from the MotAB to the torque helix on FliG's D5 domain and with the increase in the requirement of the torque or speed more MotAB are employed.<ref name="Singh2024"/> Because the flagellar motor has no on-off switch, the protein epsE is used as a mechanical clutch to disengage the motor from the rotor, thus stopping the flagellum and allowing the bacterium to remain in one place.<ref>{{cite journal|last1=Whitfield|first1=John |title=Bacterial engines have their own clutch|url=http://www.nature.com/news/2008/080619/full/news.2008.903.html|journal=Nature News|access-date=17 May 2017|language=en|doi=10.1038/news.2008.903|date=19 June 2008|pages=news.2008.903 }}</ref>{{microbial and microbot movement|biological}}

The production and rotation of a flagellum can take up to 10% of an  ''Escherichia coli'' cell's energy budget and has been described as an "energy-guzzling machine".<ref name="t665">{{cite journal |last1=Bhattacharyya |first1=Souvik |last2=Lopez |first2=Shelby |last3=Singh |first3=Abhyudai |last4=Harshey |first4=Rasika M. |year=2024 |title=Flagellar motility is mutagenic |journal=Proceedings of the National Academy of Sciences |volume=121 |issue=41 |pages=e2412541121 |doi=10.1073/pnas.2412541121 |issn=0027-8424 |doi-access=free|pmid=39352926 |pmc=11474059 |bibcode=2024PNAS..12112541B }}</ref> Its operation generates [[reactive oxygen species]] that elevate mutation rates.<ref name="t665" /> 

The cylindrical shape of flagella is suited to locomotion of microscopic organisms; these organisms operate at a low [[Reynolds number]], where the viscosity of the surrounding water is much more important than its mass or inertia.<ref name="isbn0-674-03116-4">{{cite book | last1 = Dusenbery |first1=DB |title= Living at Micro Scale: The Unexpected Physics of Being Small |publisher= Harvard University Press |location= Cambridge |year= 2009 |isbn= 978-0-674-03116-6 |chapter= Chapter 13}}</ref>

The rotational speed of flagella varies in response to the intensity of the proton-motive force, thereby permitting certain forms of speed control, and also permitting some types of bacteria to attain remarkable speeds in proportion to their size; some achieve roughly 60 cell lengths per second. At such a speed, a bacterium would take about 245 days to cover 1&nbsp;km; although that may seem slow, the perspective changes when the concept of scale is introduced. In comparison to macroscopic life forms, it is very fast indeed when expressed in terms of number of body lengths per second. A cheetah, for example, only achieves about 25 body lengths per second.<ref name="cbn">{{cite journal |title=Motions of the running Cheetah and Horse |first=Milton |last=Hildebrand |date=November 1959 |journal=[[Journal of Mammalogy]] |volume=44 |issue=4 |pages=481–495 |doi=10.2307/1376265 |jstor=1376265}} Although according to {{cite book |first1=Luke |last1=Hunter |first2=Dave |last2=Hamman |title=Cheetah |publisher=Struik Publishers |year=2003 |pages=37–38 |quote=the cheetah's fastest recorded speed was {{convert|110|km/h|abbr=on}}}}</ref>

Through use of their flagella, bacteria are able to move rapidly towards attractants and away from repellents, by means of a [[biased random walk (biochemistry)|biased random walk]], with [[Run-and-tumble motion|runs and tumbles]] brought about by rotating its flagellum [[counterclockwise]] and [[clockwise]], respectively. The two directions of rotation are not identical (with respect to flagellum movement) and are selected by a molecular switch.<ref>{{cite journal |last1=Meadows |first1=Robin |title=How Bacteria Shift Gears |journal=PLOS Biology |date=10 May 2011 |volume=9 |issue=5 |pages=e1001061 |doi=10.1371/journal.pbio.1001061 | pmid = 21572986 | pmc = 3091840 | doi-access = free }}</ref> Clockwise rotation is called the ''traction mode'' with the body following the flagella. Counterclockwise rotation is called the ''thruster mode'' with the flagella lagging behind the body.<ref name="Sun">{{cite journal |last1=Sun |first1=Qifang |last2=Yuan |first2=Chengzhi |last3=Zhou |first3=Sainan |last4=Lu |first4=Jing |last5=Zeng |first5=Meiyan |last6=Cai |first6=Xiong |last7=Song |first7=Houpan |title=Helicobacter pylori infection: a dynamic process from diagnosis to treatment |journal=Frontiers in Cellular and Infection Microbiology |date=19 October 2023 |volume=13 |doi=10.3389/fcimb.2023.1257817 |pmid=37928189 |pmc=10621068 |doi-access=free}}</ref>

====Assembly====
During flagellar assembly, components of the flagellum pass through the hollow cores of the basal body and the nascent filament. During assembly, protein components are added at the flagellar tip rather than at the base.<ref name="MinaminoReview1">{{cite journal |last1=Minamino |first1=Tohru |last2=Imada |first2=Katsumi |last3=Namba |first3=Keiichi |title=Mechanisms of type III protein export for bacterial flagellar assembly |journal=Molecular BioSystems |date=2008 |volume=4 |issue=11 |pages=1105–1115 |doi=10.1039/b808065h |pmid = 18931786}}</ref> ''In vitro'', flagellar filaments assemble spontaneously in a solution containing purified flagellin as the sole protein.<ref name="Asakura1">{{cite journal |last1=Asakura |first1=Sho |last2=Eguchi |first2=Goro |last3=Iino |first3=Tetsuo |title=Reconstitution of bacterial flagella in vitro |journal=Journal of Molecular Biology |date=October 1964 |volume=10 |issue=1 |pages=42–IN9 |doi=10.1016/S0022-2836(64)80026-7 | pmid = 14222895}}</ref>

====Evolution====
{{Main|Evolution of flagella}}

At least 10 protein components of the bacterial flagellum share homologous proteins with the [[type three secretion system]] (T3SS) found in many gram-negative bacteria,<ref name="ReferenceA">{{cite journal |last1=Pallen |first1=Mark J. |last2=Matzke |first2=Nicholas J. |title=From The Origin of Species to the origin of bacterial flagella |journal=Nature Reviews Microbiology |date=October 2006 |volume=4 |issue=10 |pages=784–790 |doi=10.1038/nrmicro1493 | pmid = 16953248 | s2cid = 24057949 }}</ref> hence one likely evolved from the other. Because the T3SS has a similar number of components as a flagellar apparatus (about 25 proteins), which one evolved first is difficult to determine. However, the flagellar system appears to involve more proteins overall, including various regulators and chaperones, hence it has been argued that flagella evolved from a T3SS. However, it has also been suggested<ref name="Saier">{{cite journal |last1=Saier |first1=M |title=Evolution of bacterial type III protein secretion systems |journal=Trends in Microbiology |date=March 2004 |volume=12 |issue=3 |pages=113–115 |doi=10.1016/j.tim.2004.01.003 |pmid=15001186}}</ref> that the flagellum may have evolved first or the two structures evolved in parallel. Early single-cell organisms' need for [[motility]] (mobility) support that the more mobile flagella would be selected by evolution first,<ref name="Saier"/> but the T3SS evolving from the flagellum can be seen as 'reductive evolution', and receives no topological support from the [[phylogenetic]] trees.<ref name="Gophna2003">{{cite journal |last1=Gophna |first1=Uri |last2=Ron |first2=Eliora Z. |last3=Graur |first3=Dan |title=Bacterial type III secretion systems are ancient and evolved by multiple horizontal-transfer events |journal=Gene |date=July 2003 |volume=312 |pages=151–163 |doi=10.1016/S0378-1119(03)00612-7 | pmid = 12909351}}</ref> The hypothesis that the two structures evolved separately from a common ancestor accounts for the protein similarities between the two structures, as well as their functional diversity.<ref>{{cite journal |last1=McCann |first1=Honour C. |last2=Guttman |first2=David S. |title=Evolution of the type III secretion system and its effectors in plant–microbe interactions |journal=New Phytologist |date=January 2008 |volume=177 |issue=1 |pages=33–47 |doi=10.1111/J.1469-8137.2007.02293.X | pmid = 18078471 |doi-access=free|bibcode=2008NewPh.177...33M }}</ref>

====Flagella and the intelligent design debate====
{{Main|Intelligent design|Irreducible complexity}}
Some authors have argued that flagella cannot have evolved, assuming that they can only function properly when all proteins are in place. In other words, the flagellar apparatus is "[[irreducible complexity|irreducibly complex]]".<ref>{{cite book |last1=Behe |first1=Michael J. |title=The edge of evolution: the search for the limits of Darwinism |date=2007 |publisher=Free Press |location=New York, NY |isbn=978-0-7432-9620-5}}</ref> However, many proteins can be deleted or mutated and the flagellum still works, though sometimes at reduced efficiency.<ref>{{cite journal |last1=Rajagopala |first1=Seesandra V |last2=Titz |first2=Björn |last3=Goll |first3=Johannes |last4=Parrish |first4=Jodi R |last5=Wohlbold |first5=Katrin |last6=McKevitt |first6=Matthew T |last7=Palzkill |first7=Timothy |last8=Mori |first8=Hirotada |last9=Finley |first9=Russell L |last10=Uetz |first10=Peter |title=The protein network of bacterial motility |journal=Molecular Systems Biology |date=January 2007 |volume=3 |issue=1 |page=128 |doi=10.1038/msb4100166 | pmid = 17667950 | pmc = 1943423}}</ref> Moreover, with many proteins unique to some number across species, diversity of bacterial flagella composition was higher than expected.<ref>{{cite journal |last1=Titz |first1=Björn |last2=Rajagopala |first2=Seesandra V. |last3=Ester |first3=Claudia |last4=Häuser |first4=Roman |last5=Uetz |first5=Peter |title=Novel Conserved Assembly Factor of the Bacterial Flagellum |journal=Journal of Bacteriology |date=November 2006 |volume=188 |issue=21 |pages=7700–7706 |doi=10.1128/JB.00820-06 | pmid = 16936039 | pmc = 1636259}}</ref> Hence, the flagellar apparatus is clearly very flexible in evolutionary terms and perfectly able to lose or gain protein components. For instance, a number of mutations have been found that ''increase'' the motility of ''E. coli''.<ref>{{cite journal |last1=Kakkanat |first1=Asha |last2=Phan |first2=Minh-Duy |last3=Lo |first3=Alvin W. |last4=Beatson |first4=Scott A. |last5=Schembri |first5=Mark A. |title=Novel genes associated with enhanced motility of Escherichia coli ST131 |journal=PLOS ONE |date=10 May 2017 |volume=12 |issue=5 |pages=e0176290 |doi=10.1371/journal.pone.0176290 | pmid = 28489862 | bibcode = 2017PLoSO..1276290K | pmc=5425062| doi-access = free }}</ref> Additional evidence for the evolution of bacterial flagella includes the existence of vestigial flagella, intermediate forms of flagella and patterns of similarities among flagellar protein sequences, including the observation that almost all of the core flagellar proteins have known homologies with non-flagellar proteins.<ref name="ReferenceA"/> Furthermore, several processes have been identified as playing important roles in flagellar evolution, including self-assembly of simple repeating subunits, gene duplication with subsequent divergence, recruitment of elements from other systems ('molecular bricolage') and recombination.<ref>{{cite journal |last1=Pallen |first1=M.J. |last2=Gophna |first2=U. |title=Bacterial Flagella and Type III Secretion: Case Studies in the Evolution of Complexity |journal=Genome Dynamics |date=2007 |volume=3 |pages=30–47 |doi=10.1159/000107602 | pmid = 18753783 | isbn = 978-3-8055-8340-4 }}</ref>

====Flagellar arrangements====
Different species of bacteria have different numbers and arrangements of flagella,<ref name="ndvsu">{{cite web |title=Bacterial flagella |url=http://www.ndvsu.org/images/StudyMaterials/Micro/Bacterial-Flagella.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.ndvsu.org/images/StudyMaterials/Micro/Bacterial-Flagella.pdf |archive-date=2022-10-09 |url-status=live |access-date=29 December 2021}}</ref><ref name="Ruan">{{cite journal |last1=Ruan |first1=Juanfang |last2=Kato |first2=Takayuki |last3=Santini |first3=Claire-Lise |last4=Miyata |first4=Tomoko |last5=Kawamoto |first5=Akihiro |last6=Zhang |first6=Wei-Jia |last7=Bernadac |first7=Alain |last8=Wu |first8=Long-Fei |last9=Namba |first9=Keiichi |title=Architecture of a flagellar apparatus in the fast-swimming magnetotactic bacterium MO-1 |journal=Proceedings of the National Academy of Sciences |date=11 December 2012 |volume=109 |issue=50 |pages=20643–20648 |doi=10.1073/pnas.1215274109 |pmid=23184985 |pmc=3528567 |bibcode=2012PNAS..10920643R |doi-access=free }}</ref> named using the term ''tricho'', from the Greek ''trichos'' meaning ''hair''.<ref name="rxlist">{{cite web |title=tricho- prefix |url=https://www.rxlist.com/tricho-_prefix/definition.htm |access-date=26 March 2022}}</ref>
*'''Monotrichous''' bacteria such as ''[[Vibrio cholerae]]'' have a single '''polar flagellum'''.<ref name="Echazarreta">{{cite journal |last1=Echazarreta |first1=MA |last2=Klose |first2=KE |title=''Vibrio'' Flagellar Synthesis. |journal=Frontiers in Cellular and Infection Microbiology |date=2019 |volume=9 |pages=131 |doi=10.3389/fcimb.2019.00131 |pmid=31119103|pmc=6504787 |doi-access=free }}</ref>
*'''Amphitrichous''' bacteria have a single flagellum on each of two opposite ends (e.g., ''[[Campylobacter jejuni]]'' or ''[[Alcaligenes faecalis]]'')&mdash;both flagella rotate but coordinate to produce coherent thrust.
*'''Lophotrichous''' bacteria (''lopho'' Greek combining term meaning ''crest'' or ''tuft'')<ref name="Collins">{{cite web |title=Lopho |url=https://www.collinsdictionary.com/dictionary/english/lopho |access-date=26 March 2022}}</ref> have multiple flagella located at the same spot on the bacterial surface such as ''[[Helicobacter pylori]]'', which act in concert to drive the bacteria in a single direction. In many cases, the bases of multiple flagella are surrounded by a specialized region of the cell membrane, called the [[polar organelle]].{{Citation needed|date=January 2009}}
*'''Peritrichous''' bacteria have flagella projecting in all directions (e.g., ''E. coli'').
Counterclockwise rotation of a monotrichous polar flagellum pushes the cell forward with the flagellum trailing behind, much like a corkscrew moving inside cork. Water on the microscopic scale is highly [[viscous]], unlike usual [[water]].

[[Spirochete]]s, in contrast, have flagella called [[endoflagella]] arising from opposite poles of the cell, and are located within the [[periplasmic space]] as shown by breaking the outer-membrane and also by [[electron cryotomography]] microscopy.<ref>{{cite journal |last1=Kudryashev |first1=Mikhail |last2=Cyrklaff |first2=Marek |last3=Baumeister |first3=Wolfgang |last4=Simon |first4=Markus M. |last5=Wallich |first5=Reinhard |last6=Frischknecht |first6=Friedrich |title=Comparative cryo-electron tomography of pathogenic Lyme disease spirochetes |journal=Molecular Microbiology |date=March 2009 |volume=71 |issue=6 |pages=1415–1434 |doi=10.1111/j.1365-2958.2009.06613.x | pmid = 19210619 | s2cid = 19650892 | doi-access = free }}</ref> The rotation of the filaments relative to the cell body causes the entire bacterium to move forward in a corkscrew-like motion, even through material viscous enough to prevent the passage of normally flagellated bacteria.

In certain large forms of ''[[Selenomonad|Selenomonas]]'', more than 30 individual flagella are organized outside the cell body, helically twining about each other to form a thick structure (easily visible with the light microscope) called a "[[wikt:fascicle|fascicle]]".

In some ''Vibrio'' spp. (particularly ''[[Vibrio parahaemolyticus]]''<ref name="Kim1">{{cite journal |last1=Kim |first1=Yun-Kyeong |last2=McCarter |first2=Linda L. |title=Analysis of the Polar Flagellar Gene System of Vibrio parahaemolyticus |journal=Journal of Bacteriology |date=July 2000 |volume=182 |issue=13 |pages=3693–3704 |doi=10.1128/JB.182.13.3693-3704.2000 | pmid = 10850984 | pmc = 94540}}</ref>) and related [[bacteria]] such as ''[[Aeromonas]]'', two flagellar systems co-exist, using different sets of genes and different ion gradients for energy. The polar flagella are constitutively expressed and provide motility in bulk fluid, while the lateral flagella are expressed when the polar flagella meet too much resistance to turn.<ref name="Atsumi1">{{cite journal |last1=Atsumi |first1=T |last2=Maekawa |first2=Y |last3=Yamada |first3=T |last4=Kawagishi |first4=I |last5=Imae |first5=Y |last6=Homma |first6=M |title=Effect of viscosity on swimming by the lateral and polar flagella of Vibrio alginolyticus |journal=Journal of Bacteriology |date=August 1996 |volume=178 |issue=16 |pages=5024–5026 |doi=10.1128/jb.178.16.5024-5026.1996 |pmid=8759871 |pmc=178290}}</ref><ref name="McCarter2">{{cite journal |last1=McCarter |first1=Linda L. |title=Dual Flagellar Systems Enable Motility under Different Circumstances |journal=Microbial Physiology |date=2004 |volume=7 |issue=1–2 |pages=18–29 |doi=10.1159/000077866 |pmid = 15170400 | s2cid = 21963003 }}</ref><ref name="Merino1">{{cite journal |last1=Merino |first1=Susana |last2=Shaw |first2=Jonathan G. |last3=TomÃ¡s |first3=Juan M. |title=Bacterial lateral flagella: an inducible flagella system |journal=FEMS Microbiology Letters |date=October 2006 |volume=263 |issue=2 |pages=127–135 |doi=10.1111/j.1574-6968.2006.00403.x |pmid=16978346 |doi-access=free }}</ref><ref name="Belas1">{{cite journal |last1=Belas |first1=R |last2=Simon |first2=M |last3=Silverman |first3=M |title=Regulation of lateral flagella gene transcription in Vibrio parahaemolyticus |journal=Journal of Bacteriology |date=July 1986 |volume=167 |issue=1 |pages=210–218 |doi=10.1128/jb.167.1.210-218.1986 |pmid = 3013835 | pmc=212863}}</ref><ref name="Canals1">{{cite journal |last1=Canals |first1=Rocío |last2=Altarriba |first2=Maria |last3=Vilches |first3=Silvia |last4=Horsburgh |first4=Gavin |last5=Shaw |first5=Jonathan G. |last6=Tomás |first6=Juan M. |last7=Merino |first7=Susana |title=Analysis of the Lateral Flagellar Gene System of Aeromonas hydrophila AH-3 |journal=Journal of Bacteriology |date=February 2006 |volume=188 |issue=3 |pages=852–862 |doi=10.1128/JB.188.3.852-862.2006 |pmid = 16428388 | pmc = 1347325}}</ref><ref name="Canals2">{{cite journal |last1=Canals |first1=Rocío |last2=Ramirez |first2=Silvia |last3=Vilches |first3=Silvia |last4=Horsburgh |first4=Gavin |last5=Shaw |first5=Jonathan G. |last6=Tomás |first6=Juan M. |last7=Merino |first7=Susana |title=Polar Flagellum Biogenesis in Aeromonas hydrophila |journal=Journal of Bacteriology |date=15 January 2006 |volume=188 |issue=2 |pages=542–555 |doi=10.1128/JB.188.2.542-555.2006 |pmid = 16385045 | pmc = 1347287}}</ref> These provide swarming motility on surfaces or in viscous fluids.

===== Bundling =====
Bundling is an event that can happen in multi-flagellated cells, bundling the flagella together and causing them to rotate in a coordinated manner.

Flagella are left-handed helices, and when rotated counter-clockwise by their rotors, they can bundle and rotate together. When the rotors reverse direction, thus rotating clockwise, the flagellum unwinds from the bundle. This may cause the cell to stop its forward motion and instead start twitching in place, referred to as [[Run-and-tumble motion|tumbling]]. Tumbling results in a stochastic reorientation of the cell, causing it to change the direction of its forward swimming.

It is not known which stimuli drive the switch between bundling and tumbling, but the motor is highly adaptive to different signals. In the model describing [[chemotaxis]] ("movement on purpose") the clockwise rotation of a flagellum is suppressed by chemical compounds favorable to the cell (e.g. food). When moving in a favorable direction, the concentration of such chemical attractants increases and therefore tumbles are continually suppressed, allowing forward motion; likewise, when the cell's direction of motion is unfavorable (e.g., away from a chemical attractant), tumbles are no longer suppressed and occur much more often, with the chance that the cell will be thus reoriented in the correct direction.

Even if all flagella would rotate clockwise, however, they often cannot form a bundle due to geometrical and hydrodynamic reasons.<ref name="pmid14671319">{{cite journal |last1=Kim |first1=MunJu |last2=Bird |first2=James C. |last3=Van Parys |first3=Annemarie J. |last4=Breuer |first4=Kenneth S. |last5=Powers |first5=Thomas R. |title=A macroscopic scale model of bacterial flagellar bundling |journal=Proceedings of the National Academy of Sciences |date=23 December 2003 |volume=100 |issue=26 |pages=15481–15485 |doi=10.1073/pnas.2633596100 |arxiv=cond-mat/0312562 |bibcode=2003PNAS..10015481K |pmc=307593 |pmid=14671319 |doi-access=free}}</ref><ref name="pmid264676">{{cite journal |last1=Macnab |first1=RM |date=January 1977 |title=Bacterial flagella rotating in bundles: a study in helical geometry |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=74 |issue=1 |pages=221–5 |bibcode=1977PNAS...74..221M |doi=10.1073/pnas.74.1.221 |pmc=393230 |pmid=264676 |doi-access=free}}</ref>