Editing Chemotaxis

{{Short description|Movement of an organism or entity in response to a chemical stimulus}}
{{Use dmy dates|date=April 2022}}
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[[File:Chemotaxis1.jpg|thumb|Capillary tube assay for chemotaxis. Motile prokaryotes sense chemicals in their environment and change their motility accordingly. Absent chemicals, movement is completely random. When an attractant or repellent is present, runs become longer and tumbles become less frequent. The result is net movement towards or away from the chemical (i.e., up or down the chemical gradient). The net movement can be seen in the beaker, where the bacteria accumulate around the origin of the attractant, and away from the origin of the repellent.]]
'''Chemotaxis''' (from ''[[chemical substance|chemo-]]'' + ''[[taxis]]'') is the movement of an organism or entity in response to a chemical stimulus.<ref>{{cite EB1911|wstitle=Chemotaxis|volume=6|pages=77}}</ref> [[Somatic cell]]s, [[bacteria]], and other [[single-cell organism|single-cell]] or [[multicellular]] organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food (e.g., [[glucose]]) by swimming toward the highest concentration of food molecules, or to flee from [[poison]]s (e.g., [[phenol]]). In multicellular organisms, chemotaxis is critical to early development (e.g., movement of [[sperm]] towards the egg during [[fertilization]]) and development (e.g., migration of [[neuron]]s or [[lymphocyte]]s) as well as in normal function and health (e.g., migration of [[White blood cell|leukocytes]] during injury or infection).<ref>{{cite journal | vauthors = de Oliveira S, Rosowski EE, Huttenlocher A | title = Neutrophil migration in infection and wound repair: going forward in reverse | journal = Nature Reviews. Immunology | volume = 16 | issue = 6 | pages = 378–91 | date = May 2016 | pmid = 27231052 | pmc = 5367630 | doi = 10.1038/nri.2016.49 }}</ref> In addition, it has been recognized that mechanisms that allow chemotaxis in animals can be subverted during [[cancer]] [[metastasis]],<ref>{{cite journal | vauthors = Stuelten CH, Parent CA, Montell DJ | title = Cell motility in cancer invasion and metastasis: insights from simple model organisms | journal = Nature Reviews. Cancer | volume = 18 | issue = 5 | pages = 296–312 | date = May 2018 | pmid = 29546880 | pmc = 6790333 | doi = 10.1038/nrc.2018.15 }}</ref> and the aberrant change of the overall property of these networks, which control chemotaxis, can lead to [[carcinogenesis]].<ref>{{cite journal |vauthors = Zhan H, Bhattacharya S, Cai H, Iglesias PA, Huang CH, Devreotes PN | title = An excitable Ras/PI3K/ERK signaling network controls migration and oncogenic transformation in epithelial cells | journal = Developmental Cell | volume = 54 | issue = 5 | pages = 608–623 | date = September 2020 | pmid = 32877650 | doi = 10.1016/j.devcel.2020.08.001 | pmc = 7505206 }}</ref> The aberrant chemotaxis of leukocytes and lymphocytes also contribute to inflammatory diseases such as atherosclerosis, asthma, and arthritis.<ref>{{cite journal | vauthors = Li J, Ley K | title = Lymphocyte migration into atherosclerotic plaque | journal = Arteriosclerosis, Thrombosis, and Vascular Biology | volume = 35 | issue = 1 | pages = 40–9 | date = January 2015 | pmid = 25301842 | pmc = 4429868 | doi = 10.1161/ATVBAHA.114.303227 }}</ref><ref>{{cite journal | vauthors = Gelfand EW | title = Importance of the leukotriene B4-BLT1 and LTB4-BLT2 pathways in asthma | journal = Seminars in Immunology | volume = 33 | pages = 44–51 | date = October 2017 | pmid = 29042028 | pmc = 5679233 | doi = 10.1016/j.smim.2017.08.005 }}</ref><ref>{{cite journal | vauthors = Planagumà A, Domènech T, Pont M, Calama E, García-González V, López R, Aulí M, López M, Fonquerna S, Ramos I, de Alba J, Nueda A, Prats N, Segarra V, Miralpeix M, Lehner MD | title = Combined anti CXC receptors 1 and 2 therapy is a promising anti-inflammatory treatment for respiratory diseases by reducing neutrophil migration and activation | journal = Pulmonary Pharmacology & Therapeutics | volume = 34 | pages = 37–45 | date = October 2015 | pmid = 26271598 | doi = 10.1016/j.pupt.2015.08.002 }}</ref><ref>{{cite journal | vauthors = Rana AK, Li Y, Dang Q, Yang F | title = Monocytes in rheumatoid arthritis: Circulating precursors of macrophages and osteoclasts and, their heterogeneity and plasticity role in RA pathogenesis | journal = International Immunopharmacology | volume = 65 | pages = 348–359 | date = December 2018 | pmid = 30366278 | doi = 10.1016/j.intimp.2018.10.016 | s2cid = 53116963 }}</ref> Sub-cellular components, such as the polarity patch generated by mating yeast, may also display chemotactic behavior.<ref name="Ghose">{{cite journal | vauthors = Ghose D, Jacobs K, Ramirez S, Elston T, Lew D | title = Chemotactic movement of a polarity site enables yeast cells to find their mates | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 118 | issue = 22 | pages = e2025445118 | date = June 2021 | pmid = 34050026 | pmc = 8179161 | doi = 10.1073/pnas.2025445118 | doi-access = free | bibcode = 2021PNAS..11825445G }}</ref>

''Positive'' chemotaxis occurs if the movement is toward a higher concentration of the chemical in question; ''negative'' chemotaxis if the movement is in the opposite direction. Chemically prompted [[kinesis (biology)|kinesis]] (randomly directed or nondirectional) can be called [[chemokinesis]].

== History of chemotaxis research ==

Although migration of cells was detected from the early days of the development of microscopy by [[Leeuwenhoek]], a Caltech lecture regarding chemotaxis propounds that 'erudite description of chemotaxis was only first made by [[Theodor Wilhelm Engelmann|T. W. Engelmann]] (1881) and [[Wilhelm Pfeffer|W. F. Pfeffer]] (1884) in bacteria, and [[Herbert Spencer Jennings|H. S. Jennings]] (1906) in [[ciliate]]s'.<ref>{{cite web | vauthors = Phillips R | title = How Cells Decide Where to Go: The Case of Bacterial Chemotaxis | work = Chemotaxis Lecture | date = 2007 | url = http://www.rpgroup.caltech.edu/courses/aph161/2007/lectures/ChemotaxisLecture.pdf | archive-url = https://web.archive.org/web/20100619184346/http://www.rpgroup.caltech.edu/courses/aph161/2007/lectures/ChemotaxisLecture.pdf |archive-date = 19 June 2010 | access-date = 15 April 2017 }}</ref> The Nobel Prize laureate [[Élie Metchnikoff|I. Metchnikoff]] also contributed to the study of the field during 1882 to 1886, with investigations of the process as an initial step of [[phagocytosis]].<ref>{{cite encyclopedia | title = Élie Metchnikoff | url = https://www.britannica.com/biography/Elie-Metchnikoff | date = 12 May 2024 | encyclopedia = Encyclopædia Britannica | publisher = Encyclopædia Britannica, Inc. }}</ref> The significance of chemotaxis in biology and clinical pathology was widely accepted in the 1930s, and the most fundamental definitions underlying the phenomenon were drafted by this time.{{By whom|date=April 2017}} The most important aspects in quality control of [[chemotaxis assay]]s were described by [[Henry Harris (scientist)|H. Harris]] in the 1950s.<ref>{{cite web | vauthors = Roberts B, Chung E, Yu SH, Li SZ | collaboration = Mathematical Method of Bioengineering Group Presentation | work = Integrated Systems Neuroengineering | publisher = University of California - San Diego| title = Keller-Segel Models for Chemotaxis | date = 2012 | url = http://www.isn.ucsd.edu/courses/Beng221/problems/2012/BENG221_Project%20-%20Roberts%20Chung%20Yu%20Li.pdf | archive-url = https://web.archive.org/web/20170829175316/http://www.isn.ucsd.edu/courses/beng221/problems/2012/BENG221_Project%20-%20Roberts%20Chung%20Yu%20Li.pdf | archive-date=29 August 2017 | access-date = 1 April 2017 }}</ref> In the 1960s and 1970s, the revolution of modern cell biology and biochemistry provided a series of novel techniques that became available to investigate the migratory responder cells and subcellular fractions responsible for chemotactic activity.<ref name="pmid4873021">{{cite journal | vauthors = Snyderman R, Gewurz H, Mergenhagen SE | title = Interactions of the complement system with endotoxic lipopolysaccharide. Generation of a factor chemotactic for polymorphonuclear leukocytes | journal = The Journal of Experimental Medicine | volume = 128 | issue = 2 | pages = 259–75 | date = August 1968 | pmid = 4873021 | pmc = 2138524 | doi = 10.1084/jem.128.2.259 }}</ref> The availability of this technology led to the discovery of C5a, a major chemotactic factor involved in acute inflammation. The pioneering works of [[Julius Adler (biochemist)|J. Adler]] modernized Pfeffer's capillary assay and represented a significant turning point in understanding the whole process of intracellular signal transduction of bacteria.<ref>{{cite journal | vauthors = Adler J, Tso WW | s2cid = 7221477 | title = "Decision"-making in bacteria: chemotactic response of Escherichia coli to conflicting stimuli | journal = Science | volume = 184 | issue = 4143 | pages = 1292–4 | date = June 1974 | pmid = 4598187 | doi = 10.1126/science.184.4143.1292 | bibcode = 1974Sci...184.1292A }}</ref><ref>{{cite book | vauthors = Berg H |editor-first1=Howard C. |editor-last1=Berg |s2cid=35733036 |date=2004 |title=E. coli in Motion |url=https://archive.org/details/springer_10.1007-b97370 |publisher=Springer |page=[https://archive.org/details/springer_10.1007-b97370/page/n26 15], 19–29 |isbn=0-387-00888-8 |doi=10.1007/b97370 |series=Biological and Medical Physics, Biomedical Engineering }}</ref>

== Bacterial chemotaxis—general characteristics ==<!-- This section is linked from [[Random walk]] -->
[[File:ChtxCCW CW (Fixed).png|right|350 px|<div style="text-align: center;border:none">Correlation of swimming behaviour and flagellar rotation</div>]]
Some [[bacteria]], such as ''[[Escherichia coli|E. coli]]'', have several [[flagellum|flagella]] per cell (4–10 typically). These can rotate in two ways:
# Counter-clockwise rotation aligns the flagella into a single rotating bundle, causing the bacterium to swim in a straight line; and
# Clockwise rotation breaks the flagella bundle apart such that each flagellum points in a different direction, causing the bacterium to tumble in place.<ref>{{cite journal | vauthors = Yuan J, Fahrner KA, Turner L, Berg HC | title = Asymmetry in the clockwise and counterclockwise rotation of the bacterial flagellar motor | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 107 | issue = 29 | pages = 12846–9 | date = July 2010 | pmid = 20615986 | pmc = 2919929 | doi = 10.1073/pnas.1007333107 | bibcode = 2010PNAS..10712846Y | doi-access = free }}</ref>
The directions of rotation are given for an observer outside the cell looking down the flagella toward the cell.<ref>{{Cite web|url=https://www.physik.uni-muenchen.de/lehre/vorlesungen/wise_16_17/Biophysics_of_Systems/013_BakterChemotaxis_DB_WS16.pdf |archive-url=https://web.archive.org/web/20170506024623/http://www.physik.uni-muenchen.de/lehre/vorlesungen/wise_16_17/Biophysics_of_Systems/013_BakterChemotaxis_DB_WS16.pdf |archive-date=2017-05-06 |url-status=live|title=Bacterial Chemotaxis}}</ref>

=== Behavior ===
The overall movement of a bacterium is the result of alternating tumble and swim phases, called [[run-and-tumble motion]].<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| vauthors = Berg HC |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 = 2509–2512 | date = September 1972 | pmid = 4560688 | pmc = 426976 | doi = 10.1073/pnas.69.9.2509 | doi-access = free | bibcode = 1972PNAS...69.2509M }}</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.<ref>{{cite journal | vauthors = Nedeljković M, Sastre DE, Sundberg EJ | title = Bacterial Flagellar Filament: A Supramolecular Multifunctional Nanostructure | journal = International Journal of Molecular Sciences | volume = 22 | issue = 14 | pages = 7521 | date = July 2021 | pmid = 34299141 | pmc = 8306008 | doi = 10.3390/ijms22147521 | doi-access = free }}</ref> Vertebrates seem to have taken advantage of this fact by possessing an immune receptor ([[Toll-like receptor|TLR5]]) designed to recognize this conserved protein.<ref>{{cite journal | vauthors = Zhong M, Yan H, Li Y | title = Flagellin: a unique microbe-associated molecular pattern and a multi-faceted immunomodulator | journal = Cellular & Molecular Immunology | volume = 14 | issue = 10 | pages = 862–864 | date = October 2017 | pmid = 28845044 | pmc = 5649114 | doi = 10.1038/cmi.2017.78 }}</ref>

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 | vauthors = Berg HC | 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>{{page needed|date=March 2017}}

===Signal transduction===
[[Image:ChtxAspRec.png|right|350 px|<div style="text-align: center;border:none">Domain structure of chemotaxis receptor for Asp</div>]]
Chemical gradients are sensed through multiple [[transmembrane receptor]]s, called methyl-accepting chemotaxis proteins (MCPs), which vary in the molecules that they detect.<ref name=":1"/> Thousands of MCP receptors are known to be encoded across the bacterial kingdom.<ref>{{cite journal | vauthors = Galperin MY | title = A census of membrane-bound and intracellular signal transduction proteins in bacteria: bacterial IQ, extroverts and introverts | journal = BMC Microbiology | volume = 5 | pages = 35 | date = June 2005 | pmid = 15955239 | pmc = 1183210 | doi = 10.1186/1471-2180-5-35 | doi-access = free }}</ref> These receptors may bind attractants or repellents directly or indirectly through interaction with proteins of [[periplasmatic space]].<ref>{{Cite book| vauthors = Auletta G |title=Cognitive Biology: Dealing with Information from Bacteria to Minds|publisher=Oxford University Press|year=2011|isbn=978-0-19-960848-5|location=United States|pages=266}}</ref> The signals from these receptors are transmitted across the [[plasma membrane]] into the [[cytosol]], where ''[[Che proteins]]'' are activated.<ref name="ReferenceA">{{cite journal | vauthors = Falke JJ, Bass RB, Butler SL, Chervitz SA, Danielson MA | title = The two-component signaling pathway of bacterial chemotaxis: a molecular view of signal transduction by receptors, kinases, and adaptation enzymes | journal = Annual Review of Cell and Developmental Biology | volume = 13 | pages = 457–512 | date = 1997 | pmid = 9442881 | pmc = 2899694 | doi = 10.1146/annurev.cellbio.13.1.457 }}</ref> The Che proteins alter the tumbling frequency, and alter the receptors.<ref name="ReferenceA"/>

====Flagellum regulation====
The proteins CheW and CheA bind to the receptor. The absence of receptor activation results in [[phosphorylation|autophosphorylation]] in the [[histidine kinase]], CheA, at a single highly conserved histidine residue.<ref>{{cite web|url=https://www.youtube.com/watch?v=h4lv7cBYVug| archive-url=https://web.archive.org/web/20150711181802/https://www.youtube.com/watch?v=h4lv7cBYVug| archive-date=2015-07-11 | url-status=dead|title=Chemotaxis|last=ToxCafe|date=2 June 2011|access-date=March 23, 2017|via=YouTube}}</ref>{{better source needed|date=March 2017}} CheA, in turn, transfers phosphoryl groups to conserved aspartate residues in the response regulators CheB and CheY; CheA is a histidine kinase and it does not actively transfer the phosphoryl group, rather, the response regulator [[Protein-glutamate methylesterase|CheB]] takes the phosphoryl group from CheA.{{citation needed|date=March 2017}} This mechanism of signal transduction is called a [[two-component regulatory system|two-component system]], and it is a common form of signal transduction in bacteria.{{citation needed|date=March 2017}} CheY induces tumbling by interacting with the flagellar switch protein FliM, inducing a change from counter-clockwise to clockwise rotation of the flagellum. Change in the rotation state of a single flagellum can disrupt the entire flagella bundle and cause a tumble.{{citation needed|date=March 2017}}

====Receptor regulation====
[[Image:Chtxbactsign1.png|right|450 px|<div style="text-align: center;border:none">Signalling pathways of E.coli</div>]]
CheB, when activated by CheA, acts as a [[demethylase|methylesterase]], removing methyl groups from [[glutamate]] residues on the [[cytosol]]ic side of the receptor; it works antagonistically with CheR, a methyl[[transferase]], which adds methyl residues to the same glutamate residues.<ref name=":1" /> If the level of an attractant remains high, the level of phosphorylation of CheA (and, therefore, CheY and CheB) will remain low, the cell will swim smoothly, and the level of methylation of the MCPs will increase (because CheB-P is not present to demethylate).<ref name=":1">{{cite journal | vauthors = Wadhams GH, Armitage JP | title = Making sense of it all: bacterial chemotaxis | journal = Nature Reviews. Molecular Cell Biology | volume = 5 | issue = 12 | pages = 1024–1037 | date = December 2004 | pmid = 15573139 | doi = 10.1038/nrm1524 | s2cid = 205493118 }}</ref> The MCPs no longer respond to the attractant when they are fully methylated; therefore, even though the level of attractant might remain high, the level of CheA-P (and CheB-P) increases and the cell begins to tumble.<ref name=":1"/> The MCPs can be demethylated by CheB-P, and, when this happens, the receptors can once again respond to attractants.<ref name=":1" /> The situation is the opposite with regard to repellents: fully methylated MCPs respond best to repellents, while least-methylated MCPs respond worst to repellents.{{citation needed|date=March 2017}} This regulation allows the bacterium to 'remember' chemical concentrations from the recent past, a few seconds, and compare them to those it is currently experiencing, thus 'know' whether it is traveling up or down a gradient.
<ref>{{Cite book| vauthors = Shu C, Chen PC, Fung YC |title=An Introductory Text to Bioengineering (Advanced Series in Biomechanics - Vol. 4)|publisher=World Scientific Publishing Co. Pte. Ltd.|year=2008|isbn=9789812707932|location=Singapore|pages=418}}</ref> that bacteria have to chemical gradients, other mechanisms are involved in increasing the absolute value of the sensitivity on a given background. Well-established examples are the ultra-sensitive response of the motor to the CheY-P signal, and the clustering of chemoreceptors.<ref>{{cite journal | vauthors = Cluzel P, Surette M, Leibler S | s2cid = 5334523 | title = An ultrasensitive bacterial motor revealed by monitoring signaling proteins in single cells | journal = Science | volume = 287 | issue = 5458 | pages = 1652–5 | date = March 2000 | pmid = 10698740 | doi = 10.1126/science.287.5458.1652 | bibcode = 2000Sci...287.1652C }}</ref><ref>{{cite journal | vauthors = Sourjik V | title = Receptor clustering and signal processing in E. coli chemotaxis | journal = Trends in Microbiology | volume = 12 | issue = 12 | pages = 569–76 | date = December 2004 | pmid = 15539117 | doi = 10.1016/j.tim.2004.10.003 | citeseerx = 10.1.1.318.4824 }}</ref>

== Chemoattractants and chemorepellents ==
Chemoattractants and chemorepellents are [[inorganic]] or [[Organic compound|organic]] substances possessing chemotaxis-inducer effect in motile cells. These chemotactic [[ligand]]s create chemical concentration gradients that organisms, prokaryotic and eukaryotic, move toward or away from, respectively.<ref name=":0">{{cite journal | vauthors = Xu F, Bierman R, Healy F, Nguyen H |title=A multi-scale model of Escherichia coli chemotaxis from intracellular signaling pathway to motility and nutrient uptake in nutrient gradient and isotropic fluid environments|doi = 10.1016/j.camwa.2015.12.019 |journal=Computers & Mathematics with Applications|volume=71| issue = 11 |pages=2466–2478 | year=2016| doi-access = free }}</ref>
[[File:Chtx-AttrRep-en.png|right|250x250px|float]]
Effects of chemoattractants are elicited via chemoreceptors such as [[methyl-accepting chemotaxis protein]]s (MCP).<ref name="twenty">{{cite journal | vauthors = Szurmant H, Ordal GW | title = Diversity in chemotaxis mechanisms among the bacteria and archaea | journal = Microbiology and Molecular Biology Reviews | volume = 68 | issue = 2 | pages = 301–19 | date = June 2004 | pmid = 15187186 | pmc = 419924 | doi = 10.1128/MMBR.68.2.301-319.2004 }}</ref> MCPs in E.coli include Tar, Tsr, Trg and Tap.<ref name="four">{{cite journal | vauthors = Yamamoto K, Macnab RM, Imae Y | title = Repellent response functions of the Trg and Tap chemoreceptors of Escherichia coli | journal = Journal of Bacteriology | volume = 172 | issue = 1 | pages = 383–8 | date = January 1990 | pmid = 2403544 | pmc = 208443 | doi = 10.1128/jb.172.1.383-388.1990 }}</ref> Chemoattracttants to Trg include [[ribose]] and [[galactose]] with [[phenol]] as a chemorepellent. Tap and Tsr recognize [[dipeptide]]s and [[serine]] as chemoattractants, respectively.<ref name="four"/>

Chemoattractants or chemorepellents bind MCPs at its extracellular domain; an intracellular signaling domain relays the changes in concentration of these chemotactic ligands to downstream proteins like that of CheA which then relays this signal to flagellar motors via phosphorylated CheY (CheY-P).<ref name="twenty" /> CheY-P can then control flagellar rotation influencing the direction of cell motility.<ref name="twenty"/>

For ''E.coli'', ''[[S. meliloti]]'', and ''[[Rhodobacter sphaeroides|R. spheroides]],'' the binding of chemoattractants to MCPs inhibit CheA and therefore CheY-P activity, resulting in smooth runs, but for ''[[Bacillus subtilis|B. substilis]]'', CheA activity increases.<ref name="twenty"/> Methylation events in ''E.coli'' cause MCPs to have lower affinity to chemoattractants which causes increased activity of CheA and CheY-P resulting in tumbles.<ref name="twenty" /> In this way cells are able to adapt to the immediate chemoattractant concentration and detect further changes to modulate cell motility.<ref name="twenty"/>

Chemoattractants in eukaryotes are well characterized for immune cells. [[Formyl]] [[peptide]]s, such as [[N-Formylmethionine-leucyl-phenylalanine|fMLF]], attract [[White blood cell|leukocytes]] such as [[neutrophil]]s and [[macrophage]]s, causing movement toward infection sites.<ref name="shiffmann">{{cite journal | vauthors = Schiffmann E, Corcoran BA, Wahl SM | title = N-formylmethionyl peptides as chemoattractants for leucocytes | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 72 | issue = 3 | pages = 1059–62 | date = March 1975 | pmid = 1093163 | pmc = 432465 | doi = 10.1073/pnas.72.3.1059 | bibcode = 1975PNAS...72.1059S | doi-access = free }}</ref> Non-acylated methioninyl peptides do not act as chemoattractants to neutrophils and macrophages.<ref name="shiffmann" /> Leukocytes also move toward chemoattractants C5a, a [[Complement system|complement]] component, and [[pathogen]]-specific ligands on bacteria.<ref name="shiffmann" />

Mechanisms concerning chemorepellents are less known than chemoattractants. Although chemorepellents work to confer an avoidance response in organisms, [[Tetrahymena|''Tetrahymena thermophila'']]  adapt to a chemorepellent, [[Netrin-1 peptide]], within 10 minutes of exposure; however, exposure to chemorepellents such as [[Guanosine triphosphate|GTP]], [[PACAP-38]], and [[nociceptin]] show no such adaptations.<ref>{{cite journal | vauthors = Kuruvilla H, Schmidt B, Song S, Bhajjan M, Merical M, Alley C, Griffin C, Yoder D, Hein J, Kohl D, Puffenberger C, Petroff D, Newcomer E, Good K, Heston G, Hurtubise A | title = Netrin-1 Peptide Is a Chemorepellent in Tetrahymena thermophila | journal = International Journal of Peptides | volume = 2016 | pages = 7142868 | date = 2016 | pmid = 27123011 | pmc = 4830718 | doi = 10.1155/2016/7142868 | doi-access = free }}</ref> GTP and [[Adenosine triphosphate|ATP]] are chemorepellents in micro-molar concentrations to both ''Tetrahymena'' and ''[[Paramecium]]''. These organisms avoid these molecules by producing avoiding reactions to re-orient themselves away from the gradient.<ref>{{cite journal | vauthors = Hennessey TM | title = Responses of the ciliates Tetrahymena and Paramecium to external ATP and GTP | language = en | journal = Purinergic Signalling | volume = 1 | issue = 2 | pages = 101–10 | date = June 2005 | pmid = 18404496 | pmc = 2096533 | doi = 10.1007/s11302-005-6213-1 }}</ref>

==Eukaryotic chemotaxis==
[[Image:Chtxbaceukkl1.png|right|350 px|<div style="text-align: center;border:none">Difference of gradient sensing in prokaryotes and eukaryotes</div>]]
The mechanism of chemotaxis that [[eukaryotic]] cells employ is quite different from that in the bacteria ''E. coli''; however, sensing of chemical gradients is still a crucial step in the process.<ref>{{cite book | vauthors = Köhidai L |chapter=Chemotaxis as an Expression of Communication of Tetrahymena|title=Biocommunication of Ciliates| veditors = Witzany G, Nowacki M |date=2016|pages=65&ndash;82|doi=10.1007/978-3-319-32211-7_5|isbn=978-3-319-32211-7}}</ref>{{better source needed|date=March 2017}} Due to their small size and other biophysical constraints, ''E. coli'' cannot directly detect a concentration gradient.<ref>{{cite journal | vauthors = Berg HC, Purcell EM | title = Physics of chemoreception | journal = Biophysical Journal | volume = 20 | issue = 2 | pages = 193–219 | date = November 1977 | pmid = 911982 | pmc = 1473391 | doi = 10.1016/s0006-3495(77)85544-6 | bibcode = 1977BpJ....20..193B }}</ref> Instead, they employ temporal gradient sensing, where they move over larger distances several times their own width and measure the rate at which perceived chemical concentration changes.<ref name="Levine2013">{{cite journal | vauthors = Levine H, Rappel WJ | title = The physics of eukaryotic chemotaxis | journal = Physics Today | volume = 66 | issue = 2 | pages = 24–30 | date = February 2013 | pmid = 24363460 | pmc = 3867297 | doi = 10.1063/PT.3.1884 | bibcode = 2013PhT....66b..24L }}</ref><ref name="Vlad2009">{{cite journal | vauthors = Vladimirov N, Sourjik V | title = Chemotaxis: how bacteria use memory | journal = Biological Chemistry | volume = 390 | issue = 11 | pages = 1097–1104 | date = November 2009 | pmid = 19747082 | doi = 10.1515/BC.2009.130 | s2cid = 207440927 }}</ref>

Eukaryotic cells are much larger than prokaryotes and have receptors embedded uniformly throughout the [[cell membrane]].<ref name="Levine2013" /> Eukaryotic chemotaxis involves detecting a concentration gradient spatially by comparing the asymmetric activation of these receptors at the different ends of the cell.<ref name="Levine2013" /> Activation of these receptors results in migration towards chemoattractants, or away from chemorepellants.<ref name="Levine2013" /> In mating yeast, which are non-motile, patches of polarity proteins on the cell cortex can relocate in a chemotactic fashion up pheromone gradients.<ref>{{cite journal | vauthors = Ghose D, Lew D | title = Mechanistic insights into actin-driven polarity site movement in yeast | journal = Molecular Biology of the Cell | volume = 31 | issue = 10 | pages = 1085–1102 | date = May 2020 | pmid = 32186970 | pmc = 7346724 | doi = 10.1091/mbc.e20-01-0040 }}</ref><ref name="Ghose"/>

It has also been shown that both prokaryotic and eukaryotic cells are capable of chemotactic memory.<ref name="Vlad2009" /><ref name="Skoge2014">{{cite journal | vauthors = Skoge M, Yue H, Erickstad M, Bae A, Levine H, Groisman A, Loomis WF, Rappel WJ | title = Cellular memory in eukaryotic chemotaxis | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 111 | issue = 40 | pages = 14448–53 | date = October 2014 | pmid = 25249632 | pmc = 4210025 | doi = 10.1073/pnas.1412197111 | bibcode = 2014PNAS..11114448S | doi-access = free }}</ref> In prokaryotes, this mechanism involves the [[methylation]] of receptors called [[methyl-accepting chemotaxis protein]]s (MCPs).<ref name="Vlad2009" /> This results in their desensitization and allows prokaryotes to "remember" and adapt to a chemical gradient.<ref name="Vlad2009" /> In contrast, chemotactic memory in eukaryotes can be explained by the Local Excitation Global Inhibition (LEGI) model.<ref name="Skoge2014"/><ref name="Kutscher2004">{{cite journal | vauthors = Kutscher B, Devreotes P, Iglesias PA | title = Local excitation, global inhibition mechanism for gradient sensing: an interactive applet | journal = Science's STKE | date = Feb 2004 | volume = 2004 | issue = 219 | pages = pl3 | pmid = 14872096 | doi = 10.1126/stke.2192004pl3 | s2cid = 4660870 }}</ref> LEGI involves the balance between a fast excitation and delayed inhibition which controls downstream signaling such as [[Ras subfamily|Ras]] activation and [[Phosphatidylinositol (3,4,5)-trisphosphate|PIP3]] production.<ref>{{cite journal | vauthors = Xiong Y, Huang CH, Iglesias PA, Devreotes PN | title = Cells navigate with a local-excitation, global-inhibition-biased excitable network | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 107 | issue = 40 | pages = 17079–86 | date = October 2010 | pmid = 20864631 | pmc = 2951443 | doi = 10.1073/pnas.1011271107 | bibcode = 2010PNAS..10717079X | doi-access = free }}</ref>

Levels of receptors, intracellular signalling pathways and the effector mechanisms all represent diverse, eukaryotic-type components. In eukaryotic unicellular cells, amoeboid movement and cilium or the eukaryotic flagellum are the main effectors (e.g., [[Amoeba (genus)|Amoeba]] or [[Tetrahymena]]).<ref>{{cite journal | vauthors = Bagorda A, Parent CA | title = Eukaryotic chemotaxis at a glance | journal = Journal of Cell Science | volume = 121 | issue = Pt 16 | pages = 2621–4 | date = August 2008 | pmid = 18685153 | doi = 10.1242/jcs.018077 | pmc = 7213762 | citeseerx = 10.1.1.515.32 }}</ref><ref>{{cite journal | vauthors = Köhidai L | title = Chemotaxis: the proper physiological response to evaluate phylogeny of signal molecules | journal = Acta Biologica Hungarica | volume = 50 | issue = 4 | pages = 375–94 | year = 1999 | doi = 10.1007/BF03543060 | pmid = 10735174 | s2cid = 248703226 }}</ref> Some eukaryotic cells of [[higher vertebrate]] origin, such as [[immune system|immune cells]] also move to where they need to be. Besides immune competent cells ([[granulocyte]], [[monocyte]], [[lymphocyte]]) a large group of cells—considered previously to be fixed into tissues—are also motile in special physiological (e.g., [[mast cell]], [[fibroblast]], [[endothelial cells]]) or pathological conditions (e.g., [[metastases]]).<ref>{{cite journal | vauthors = Kedrin D, van Rheenen J, Hernandez L, Condeelis J, Segall JE | s2cid = 31704677 | title = Cell motility and cytoskeletal regulation in invasion and metastasis | journal = Journal of Mammary Gland Biology and Neoplasia | volume = 12 | issue = 2–3 | pages = 143–52 | date = September 2007 | pmid = 17557195 | doi = 10.1007/s10911-007-9046-4 }}</ref> Chemotaxis has high significance in the early phases of [[embryogenesis]] as development of [[germ layers]] is guided by gradients of signal molecules.<ref>{{cite journal | vauthors = Solnica-Krezel L, Sepich DS | s2cid = 11331182 | title = Gastrulation: making and shaping germ layers | journal = Annual Review of Cell and Developmental Biology | volume = 28 | pages = 687–717 | date = 2012 | pmid = 22804578 | doi = 10.1146/annurev-cellbio-092910-154043 }}</ref><ref>{{cite journal | vauthors = Shellard A, Mayor R | title = Chemotaxis during neural crest migration | journal = Seminars in Cell & Developmental Biology | volume = 55 | pages = 111–8 | date = July 2016 | pmid = 26820523 | doi = 10.1016/j.semcdb.2016.01.031 }}</ref>

===Detection of a gradient of chemoattractant===
The specific molecule/s that allow a eukaryotic cells detect a gradient of chemoattractant ligands (that is, a sort of the molecular compass that detects the direction of a chemoattractant) seems to change depending on the cell and chemoattractant receptor involved or even the concentration of the chemoattractant. However, these molecules apparently are activated independently of the motility of the cell. That is, even an immnobilized cell is still able to detect the direction of a chemoattractant.<ref>{{cite journal | vauthors = Rodríguez-Fernández JL, Criado-García O | title = A meta-analysis indicates that the regulation of cell motility is a non-intrinsic function of chemoattractant receptors that is governed independently of directional sensing | journal = Front. Immunol. | volume = 13 | issue = eCollection 2022 | pages = 10011086 | date = October 2022 | pmid = 36341452}}</ref> There appear to be mechanisms by which an external chemotactic gradient is sensed and turned into an intracellular Ras and [[PIP3]] gradients, which results in a gradient and the activation of a signaling pathway, culminating in the [[polymerisation]] of [[actin]] filaments. The growing distal end of actin filaments develops connections with the internal surface of the plasma membrane via different sets of peptides and results in the formation of anterior [[pseudopods]] and posterior [[Uropod (immunology)|uropods]].<ref>{{cite journal | vauthors = Pal DS, Banerjee T, Lin Y, de Trogoff F, Borleis J, Iglesias PA, Devreotes PN | title = Actuation of single downstream nodes in growth factor network steers immune cell migration | journal = Developmental Cell | volume = 58 | issue = 13 | pages = 1170–1188.e7 | date = July 2023 | pmid = 37220748 | pmc = 10524337 | doi = 10.1016/j.devcel.2023.04.019 }}</ref><ref>{{cite journal | vauthors = Lin Y, Pal DS, Banerjee P, Banerjee T, Qin G, Deng Y, Borleis J, Iglesias PA, Devreotes PN | title = Ras suppression potentiates rear actomyosin contractility-driven cell polarization and migration | journal = Nature Cell Biology | pages = 1–15 | date = July 2024 | pmid = 38951708 | doi = 10.1038/s41556-024-01453-4 | pmc = 11364469 }}</ref>
[[Cilium|Cilia]] of eukaryotic cells can also produce chemotaxis; in this case, it is mainly a Ca<sup>2+</sup>-dependent induction of the [[microtubule|microtubular]] system of the [[basal body]] and the beat of the 9&nbsp;+&nbsp;2 microtubules within cilia. The orchestrated beating of hundreds of cilia is synchronized by a submembranous system built between basal bodies.
The details of the signaling pathways are still not totally clear.

====Chemotaxis-related migratory responses====
[[Image:Chtxphenomen1.png|right|400 px|<div style="text-align: center;border:none">Chemotaxis related migratory responses</div>]]
Chemotaxis refers to the directional migration of cells in response to chemical gradients; several variations of chemical-induced migration exist as listed below. 
* ''[[Chemokinesis]]'' refers to an increase in cellular motility in response to chemicals in the surrounding environment. Unlike chemotaxis, the migration stimulated by chemokinesis lacks directionality, and instead increases environmental scanning behaviors.<ref>{{cite journal | vauthors = Becker EL | title = Stimulated neutrophil locomotion: chemokinesis and chemotaxis | journal = Archives of Pathology & Laboratory Medicine | volume = 101 | issue = 10 | pages = 509–13 | date = October 1977 | pmid = 199132 }}</ref> 
* In ''[[haptotaxis]]'' the [[gradient]] of the chemoattractant is expressed or bound on a surface, in contrast to the classical model of chemotaxis, in which the gradient develops in a soluble fluid.<ref name="pmid6030602">{{cite journal | vauthors = Carter SB | s2cid = 4212997 | title = Haptotaxis and the mechanism of cell motility | journal = Nature | volume = 213 | issue = 5073 | pages = 256–60 | date = January 1967 | pmid = 6030602 | doi = 10.1038/213256a0 | bibcode = 1967Natur.213..256C }}</ref> The most common biologically active haptotactic surface is the [[extracellular matrix]] (ECM); the presence of bound [[ligands]] is responsible for induction of transendothelial migration and [[angiogenesis]].
* ''[[Necrotaxis]]'' embodies a special type of chemotaxis when the chemoattractant molecules are released from [[necrosis|necrotic]] or [[apoptosis|apoptotic]] cells. Depending on the chemical character of released substances, necrotaxis can accumulate or repel cells, which underlines the pathophysiological significance of this phenomenon.

===Receptors===
In general, eukaryotic cells sense the presence of chemotactic stimuli through the use of 7-transmembrane (or serpentine) heterotrimeric [[G protein|G-protein]]-coupled receptors, a class representing a significant portion of the [[genome]].<ref>{{cite journal | vauthors = Kim JY, Haastert PV, Devreotes PN | title = Social senses: G-protein-coupled receptor signaling pathways in Dictyostelium discoideum | journal = Chemistry & Biology | volume = 3 | issue = 4 | pages = 239–243 | date = April 1996 | pmid = 8807851 | doi = 10.1016/s1074-5521(96)90103-9 | doi-access = free }}</ref> Some members of this gene superfamily are used in eyesight ([[rhodopsins]]) as well as in [[olfaction]] (smelling).<ref>{{cite journal | vauthors = Montell C | title = Visual transduction in Drosophila | journal = Annual Review of Cell and Developmental Biology | volume = 15 | issue = 1 | pages = 231–268 | date = November 1999 | pmid = 10611962 | doi = 10.1146/annurev.cellbio.15.1.231 | s2cid = 14193715 }}</ref><ref>{{Cite book | vauthors = Antunes G, Simoes de Souza FM | title = G Protein-Coupled Receptors - Signaling, Trafficking and Regulation | chapter = Olfactory receptor signaling | series = Methods in Cell Biology | volume = 132 | pages = 127–45 | date = 2016 | pmid = 26928542 | doi = 10.1016/bs.mcb.2015.11.003  | isbn = 9780128035955 }}</ref> The main classes of chemotaxis receptors are triggered by:
* Formyl peptides - [[formyl peptide receptor]]s (FPR), 
* [[Chemokines]] - [[chemokine receptor]]s (CCR or CXCR), and 
* [[Leukotrienes]] - [[leukotriene receptor]]s (BLT).<ref>{{cite journal | vauthors = Thomas MA, Kleist AB, Volkman BF | title = Decoding the chemotactic signal | journal = Journal of Leukocyte Biology | volume = 104 | issue = 2 | pages = 359–374 | date = August 2018 | pmid = 29873835 | pmc = 6099250 | doi = 10.1002/JLB.1MR0218-044 }}</ref>
However, induction of a wide set of membrane receptors (e.g., [[cyclic nucleotide]]s, [[amino acids]], [[insulin]], vasoactive peptides) also elicit migration of the cell.<ref>{{cite journal | vauthors = van Haastert PJ, De Wit RJ, Konijn TM | title = Antagonists of chemoattractants reveal separate receptors for cAMP, folic acid and pterin in Dictyostelium | journal = Experimental Cell Research | volume = 140 | issue = 2 | pages = 453–6 | date = August 1982 | pmid = 7117406 | doi = 10.1016/0014-4827(82)90139-2 | s2cid = 27784085 | url = https://pure.rug.nl/ws/files/14539904/1982ExpCellResvHaastert.pdf }}</ref>

====Chemotactic selection====
[[Image:chtxsel.png|right|300 px|<div style="text-align: center;border:none">Chemotactic selection</div>]]
While some chemotaxis receptors are expressed in the surface membrane with long-term characteristics, as they are determined genetically, others have short-term dynamics, as they are assembled ''ad hoc'' in the presence of the ligand.<ref>{{cite book | vauthors = Witzany G, Nowacki M |title=Biocommunication of Ciliates |date=2016 |publisher=Springer |isbn=978-3-319-32211-7 }}{{page needed|date=July 2020}}</ref> The diverse features of the chemotaxis receptors and ligands allows for the possibility of selecting chemotactic responder cells with a simple chemotaxis assay By [[chemotactic selection]], we can determine whether a still-uncharacterized molecule acts via the long- or the short-term receptor pathway.<ref>{{cite book | vauthors = Köhidai L |chapter=Chemotaxis as an Expression of Communication of Tetrahymena |pages=65–82 |chapter-url=https://books.google.com/books?id=KnFBDAAAQBAJ&pg=PA65 |doi=10.1007/978-3-319-32211-7_5 | veditors = Witzany G, Nowacki M |title=Biocommunication of Ciliates |date=2016 |publisher=Springer |isbn=978-3-319-32211-7 }}</ref> The term ''chemotactic selection'' is also used to designate a technique that separates eukaryotic or prokaryotic cells according to their chemotactic responsiveness to selector ligands.<ref>{{cite journal | vauthors = Köhidai L, Csaba G | s2cid = 33755476 | title = Chemotaxis and chemotactic selection induced with cytokines (IL-8, RANTES and TNF-alpha) in the unicellular Tetrahymena pyriformis | journal = Cytokine | volume = 10 | issue = 7 | pages = 481–6 | date = July 1998 | pmid = 9702410 | doi = 10.1006/cyto.1997.0328 }}</ref>{{primary source inline|date=March 2017}}{{primary source inline|date=March 2017}}

===Chemotactic ligands===
[[Image:chtxChemokineStruct.png|right|350 px|<div style="text-align: center;border:none">Structure of chemokine classes</div>]]
[[Image:chtxChemkinStr2.png|left|300 px|<div style="text-align: center;border:none">Three dimensional structure of chemokines</div>]]
The number of molecules capable of eliciting chemotactic responses is relatively high, and we can distinguish primary and secondary chemotactic molecules.{{citation needed|date=March 2017}} The main groups of the primary ligands are as follows:
* ''Formyl peptides'' are di-, tri-, tetrapeptides of bacterial origin, formylated on the N-terminus of the peptide.{{citation needed|date=March 2017}}<ref>{{cite journal | vauthors = Zigmond SH | title = Ability of polymorphonuclear leukocytes to orient in gradients of chemotactic factors | journal = The Journal of Cell Biology | volume = 75 | issue = 2 Pt 1 | pages = 606–16 | date = November 1977 | pmid = 264125 | pmc = 2109936 | doi = 10.1083/jcb.75.2.606 }}</ref> They are released from bacteria in vivo or after decomposition of the cell, a typical member of this group is the N-formylmethionyl-leucyl-phenylalanine (abbreviated fMLF or fMLP).{{citation needed|date=March 2017}} Bacterial fMLF is a key component of inflammation has characteristic chemoattractant effects in neutrophil granulocytes and monocytes.{{citation needed|date=March 2017}} The chemotactic factor ligands and receptors related to formyl peptides are summarized in the related article, [[Formyl peptide receptors]].
* ''Complement 3a ([[C3 (complement)|C3a]]) and complement 5a ([[Complement component 5a|C5a]])'' are intermediate products of the complement cascade.{{citation needed|date=March 2017}} Their synthesis is joined to the three alternative pathways (classical, lectin-dependent, and alternative) of complement activation by a convertase enzyme.{{citation needed|date=March 2017}} The main target cells of these derivatives are neutrophil granulocytes and monocytes as well.{{citation needed|date=March 2017}}
* ''[[Chemokines]]'' belong to a special class of [[cytokines]]; not only do their groups (C, CC, CXC, CX<sub>3</sub>C chemokines) represent structurally related molecules with a special arrangement of disulfide bridges but also their target cell specificity is diverse.{{citation needed|date=March 2017}} CC chemokines act on monocytes (e.g., [[RANTES]]), and CXC chemokines are neutrophil granulocyte-specific (e.g., [[Interleukin 8|IL-8]]).{{citation needed|date=March 2017}} Investigations of the three-dimensional structures of chemokines provided evidence that a characteristic composition of beta-sheets and an alpha helix provides expression of sequences required for interaction with the chemokine receptors.{{citation needed|date=March 2017}} Formation of dimers and their increased biological activity was demonstrated by crystallography of several chemokines, e.g. IL-8.{{citation needed|date=March 2017}}
*Metabolites of [[polyunsaturated fatty acid]]s
** ''[[Leukotrienes]]'' are [[eicosanoid]] lipid mediators made by the metabolism of [[arachidonic acid]] by [[ALOX5]] (also termed 5-lipoxygenase). Their most prominent member with chemotactic factor activity is [[leukotriene B4]], which elicits adhesion, chemotaxis, and aggregation of leukocytes. The chemoattractant action of LTB4 is induced via either of two [[G protein–coupled receptor]]s, BLT1 and [[BLT2]], which are highly expressed in cells involved in [[inflammation]] and [[allergy]].<ref name="pmid25449650">{{cite journal | vauthors = Powell WS, Rokach J | title = Biosynthesis, biological effects, and receptors of hydroxyeicosatetraenoic acids (HETEs) and oxoeicosatetraenoic acids (oxo-ETEs) derived from arachidonic acid | journal = Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids | volume = 1851 | issue = 4 | pages = 340–55 | date = April 2015 | pmid = 25449650 | pmc = 5710736 | doi = 10.1016/j.bbalip.2014.10.008 }}</ref>
** The family of [[5-Hydroxyicosatetraenoic acid]] eicosanoids are arachidonic acid metabolites also formed by ALOX5. Three members of the family form naturally and have prominent chemotactic activity. These, listed in order of decreasing potency, are: ''[[5-oxo-eicosatetraenoic acid]]'', ''5-oxo-15-hydroxy-eicosatetraenoic acid'', and ''[[5-Hydroxyeicosatetraenoic acid]]''. This family of agonists stimulates chemotactic responses in human [[eosinophils]], [[neutrophils]], and [[monocytes]] by binding to the [[Oxoeicosanoid receptor 1]], which like the receptors for leukotriene B4, is a G protein-coupled receptor.<ref name="pmid25449650"/> Aside from the skin, [[neutrophils]] are the body's first line of defense against [[bacterial]] infections. After leaving nearby blood vessels, these cells recognize chemicals produced by bacteria in a cut or scratch and migrate "toward the smell". 
**''5-hydroxyeicosatrienoic acid'' and ''5-oxoeicosatrienoic acid'' are metabolites of [[Mead acid]] (5''Z'',8''Z'',11''Z''-eicosatrirenoid acid); they stimulate leukocyte chemotaxis through the oxoeicosanoid receptor 1<ref name="pmid24056189">{{cite journal | vauthors = Powell WS, Rokach J | title = The eosinophil chemoattractant 5-oxo-ETE and the OXE receptor | journal = Progress in Lipid Research | volume = 52 | issue = 4 | pages = 651–65 | date = October 2013 | pmid = 24056189 | pmc = 5710732 | doi = 10.1016/j.plipres.2013.09.001 }}</ref> with 5-oxoeicosatrienoic acid being as potent as its arachidonic acid-derived analog, 5-oxo-eicosatetraenoic acid, in stimulating human blood [[eosinophil]] and [[neutrophil]] chemotaxis.<ref name="pmid25449650"/>
**''[[12-Hydroxyeicosatetraenoic acid]]'' is an eicosanoid metabolite of arachidonic acid made by [[ALOX12]] which stimulates leukocyte chemotaxis through the leukotriene B4 receptor, BLT2.<ref name="pmid25449650"/>
**''[[Prostaglandin D2]]'' is an eicosanoid metabolite of arachidononic acid made by [[cyclooxygenase 1]] or [[cyclooxygenase 2]] that stimulates chemotaxis through the [[Prostaglandin DP2 receptor]]. It elicits chemotactic responses in eosinophils, basophils, and [[T helper cell]]s of the Th2 subtype.<ref name="pmid17767353">{{cite journal | vauthors = Matsuoka T, Narumiya S | title = Prostaglandin receptor signaling in disease | journal = TheScientificWorldJournal | volume = 7 | pages = 1329–47 | date = September 2007 | pmid = 17767353 | pmc = 5901339 | doi = 10.1100/tsw.2007.182 | doi-access = free }}</ref>{{primary source inline|date=March 2017}}{{primary source inline|date=March 2017}}
**''[[12-Hydroxyheptadecatrienoic acid]]'' is a non-eicosanoid metabolite of arachidonic acid made by cyclooxygenase 1 or cyclooxygenase 2 that stimulates leukocyte chemotaxis though the leukotriene B4 receptor, BLT2.<ref name="pmid25480980">{{cite journal | vauthors = Yokomizo T | title = Two distinct leukotriene B4 receptors, BLT1 and BLT2 | journal = Journal of Biochemistry | volume = 157 | issue = 2 | pages = 65–71 | date = February 2015 | pmid = 25480980 | doi = 10.1093/jb/mvu078 | doi-access =  }}</ref>{{primary source inline|date=March 2017}}{{primary source inline|date=March 2017}}
**''15-oxo-eicosatetraenoic acid'' is an eicosanoid metabolite of arachidonic acid made my [[ALOX15]]; it has weak chemotactic activity for human monocytes (sees [[15-Hydroxyeicosatetraenoic acid#15-oxo-ETE]]).<ref>{{cite journal | vauthors = Sozzani S, Zhou D, Locati M, Bernasconi S, Luini W, Mantovani A, O'Flaherty JT | title = Stimulating properties of 5-oxo-eicosanoids for human monocytes: synergism with monocyte chemotactic protein-1 and -3 | journal = Journal of Immunology | volume = 157 | issue = 10 | pages = 4664–71 | date = November 1996 | doi = 10.4049/jimmunol.157.10.4664 | pmid = 8906847 | s2cid = 23499393 | doi-access = free }}</ref>{{primary source inline|date=March 2017}}{{primary source inline|date=March 2017}} The receptor or other mechanism by which this metabolite stimulates chemotaxis has not been elucidated.

==== Chemotactic range fitting ====
[[File:chtxCRF2.png|right|thumb|alt=Chemotactic range fitting|[[Chemotactic range fitting]]]]
Chemotactic responses elicited by [[ligand (biochemistry)|ligand]]-[[receptor (biochemistry)|receptor]] interactions vary with the concentration of the ligand. Investigations of ligand families (e.g. [[amino acids]] or [[oligopeptide]]s) demonstrates that [[chemoattractant]] activity occurs over a wide range, while [[chemorepellent]] activities have narrow ranges.<ref>{{cite journal | author=Kohidai L, Lang O and Csaba G | title=Chemotactic-range-fitting of amino acids and its correlations to physicochemical parameters in Tetrahymena pyriformis - Evolutionary consequences | journal= Cellular and Molecular Biology | year=2003 | volume=49 | pages=OL487–95 | pmid=14995080}}</ref>

==Clinical significance==
A changed migratory potential of cells has relatively high importance in the development of several clinical symptoms and syndromes.
Altered chemotactic activity of extracellular (e.g., [[Escherichia coli]]) or intracellular (e.g., [[Listeria monocytogenes]]) pathogens itself represents a significant clinical target. Modification of endogenous chemotactic ability of these microorganisms by pharmaceutical agents can decrease or inhibit the ratio of infections or spreading of infectious diseases.
Apart from infections, there are some other diseases wherein impaired chemotaxis is the primary etiological factor, as in [[Chédiak–Higashi syndrome]], where giant intracellular vesicles inhibit normal migration of cells.

{| class="wikitable" style="margin: 1em auto 1em auto"
|+ '''Chemotaxis in diseases'''{{citation needed|date=March 2017}}
! Type of disease || Chemotaxis increased || Chemotaxis decreased
|-
| Infections || [[Inflammation]]s || [[AIDS]], [[Brucellosis]]
|-
| Chemotaxis results in the disease || — ||[[Chédiak–Higashi syndrome]], [[Kartagener syndrome]]
|-
| Chemotaxis is affected ||[[Atherosclerosis]], [[arthritis]], [[periodontitis]], [[psoriasis]], [[reperfusion injury]], [[metastatic tumor]]s||[[Multiple sclerosis]], [[Hodgkin disease]], [[male infertility]]
|-
| Intoxications ||[[Asbestos]], [[benzpyrene]] ||[[Mercury (element)|Hg]] and [[Chromium|Cr]] salts, [[ozone]]
|-
|}

==Mathematical models==
Several mathematical models of chemotaxis were developed depending on the type of
* Migration (e.g., basic differences of bacterial swimming, movement of unicellular eukaryotes with [[cilia]]/[[flagellum]] and [[ameboid|amoeboid]] migration)
* Physico-chemical characteristics of the chemicals (e.g., [[diffusion]]) working as ligands
* Biological characteristics of the ligands (attractant, neutral, and repellent molecules)
* Assay systems applied to evaluate chemotaxis (see incubation times, development, and stability of concentration gradients)
* Other environmental effects possessing direct or indirect influence on the migration (lighting, temperature, magnetic fields, etc.)

Although interactions of the factors listed above make the behavior of the solutions of mathematical models of chemotaxis rather complex, it is possible to describe the basic phenomenon of chemotaxis-driven motion in a straightforward way. Indeed, let us denote with <math> \varphi</math> the spatially non-uniform concentration of the chemo-attractant and <math>\nabla \varphi</math> as its gradient. Then the chemotactic cellular flow (also called current) <math> {\bf J} </math> that is generated by the chemotaxis is linked to the above gradient by the law:<ref name=":2">{{Cite book | vauthors = Murray JD |url=https://www.ucl.ac.uk/~rmjbale/3307/Reading_Chemotaxis1.pdf |archive-url=https://web.archive.org/web/20220506024008/https://www.ucl.ac.uk/~rmjbale/3307/Reading_Chemotaxis1.pdf |archive-date=2022-05-06 |url-status=live |title=Mathematical Biology I: An Introduction |publisher=Springer |year=2002 |isbn=978-0-387-95223-9 |edition=3rd |series=Interdisciplinary Applied Mathematics |volume=17 |location=New York |pages=395–417 |language= |doi=10.1007/b98868}}</ref>{{Equation box 1|cellpadding|border|indent=:|equation=<math> {\bf J} = C \chi(\varphi) \nabla\varphi </math>|border colour=#0073CF|background colour=#F5FFFA}}where <math> C </math> is the spatial density of the cells and <math> \chi </math> is the so-called 'Chemotactic coefficient' - <math>\chi</math> is often not constant, but a decreasing function of the chemo-attractant. For some quantity <math>\rho</math> that is subject to total flux <math>{\bf J}</math> and generation/destruction term <math>S</math>, it is possible to formulate a [[continuity equation]]:

:<math> {\partial \rho\over{\partial t}} + \nabla \cdot {\bf J} = S </math>

where <math>\nabla \cdot ()</math> is the [[divergence]]. This general equation applies to both the cell density and the chemo-attractant. Therefore, incorporating a diffusion flux into the total flux term, the interactions between these quantities are governed by a set of coupled [[Reaction–diffusion system|reaction-diffusion]] [[partial differential equation]]s describing the change in <math>C</math> and <math>\varphi</math>:<ref name=":2" />{{Equation box 1|cellpadding|border|indent=:|equation=<math> \begin{aligned}
{\partial C\over{\partial t}} &= f(C) + \nabla\cdot \left[D_{C}\nabla C - C\chi(\varphi)\nabla\varphi \right ] \\
{\partial \varphi\over{\partial t}} &= g(\varphi,C) + \nabla \cdot (D_{\varphi}\nabla\varphi)
\end{aligned} </math>|border colour=#0073CF|background colour=#F5FFFA}}where <math>f(C)</math> describes the growth in cell density, <math>g(\varphi,C)</math> is the kinetics/source term for the chemo-attractant, and the diffusion coefficients for cell density and the chemo-attractant are respectively <math>D_{C}</math> and <math>D_{\varphi}</math>.

[[Spatial ecology]] of soil microorganisms is a function of their chemotactic sensitivities towards substrate and fellow organisms.<ref name="Gharasoo2014">{{cite journal|doi=10.1016/j.soilbio.2013.11.019|title=How the chemotactic characteristics of bacteria can determine their population patterns|journal=Soil Biology and Biochemistry |volume=69 |pages=346–358 |year=2014 | vauthors = Gharasoo M, Centler F, Fetzer I, Thullner M |bibcode=2014SBiBi..69..346G }}</ref>{{primary source inline|date=March 2017}}{{primary source inline|date=March 2017}} The chemotactic behavior of the bacteria was proven to lead to non-trivial population patterns even in the absence of environmental heterogeneities. The presence of structural pore scale heterogeneities has an extra impact on the emerging bacterial patterns.

==Measurement of chemotaxis==
{{main|Chemotaxis assay}}
A wide range of techniques is available to evaluate chemotactic activity of cells or the chemoattractant and chemorepellent character of ligands.
The basic requirements of the measurement are as follows:
* Concentration gradients can develop relatively quickly and persist for a long time in the system
* Chemotactic and chemokinetic activities are distinguished
* Migration of cells is free toward and away on the axis of the concentration gradient
* Detected responses are the results of active migration of cells

Despite the fact that an ideal [[chemotaxis assay]] is still not available, there are several protocols and pieces of equipment that offer good correspondence with the conditions described above. The most commonly used are summarised in the table below:
{|class="wikitable"
!Type of assay
!Agar-plate assays
!Two-chamber assays
!Others
|-
!Examples
||
* PP-chamber
||
* Boyden chamber
* Zigmond chamber
* Dunn chambers
* Multi-well chambers
* Capillary techniques
||
* T-maze technique
* Opalescence technique
* Orientation assays
|}

==Artificial chemotactic systems==
''Chemical robots'' that use artificial chemotaxis to navigate autonomously have been designed.<ref name="Mackenzie">{{cite journal | vauthors = Mackenzie D |title=How animals follow their nose |journal=Knowable Magazine |publisher=Annual Reviews |date=6 March 2023 |doi=10.1146/knowable-030623-4 |s2cid=257388244 |doi-access=free |url=https://knowablemagazine.org/article/living-world/2023/how-animals-follow-their-nose |access-date=13 March 2023 |language=en}}</ref><ref name="Reddy">{{cite journal | vauthors = Reddy G, Murthy VN, Vergassola M |title=Olfactory Sensing and Navigation in Turbulent Environments |journal=Annual Review of Condensed Matter Physics |date=10 March 2022 |volume=13 |issue=1 |pages=191–213 |doi=10.1146/annurev-conmatphys-031720-032754 |bibcode=2022ARCMP..13..191R |s2cid=243966350 |url=https://www.annualreviews.org/doi/10.1146/annurev-conmatphys-031720-032754 |language=en |issn=1947-5454}}</ref> Applications include targeted delivery of drugs in the body.<ref>{{cite journal| vauthors = Lagzi I |s2cid=84150518|title=Chemical Robotics—Chemotactic Drug Carriers|journal=Central European Journal of Medicine|year=2013|volume=8|issue=4|pages=377–382|doi=10.2478/s11536-012-0130-9|doi-access=free}}</ref> More recently, enzyme molecules have also shown positive chemotactic behavior in the gradient of their substrates.<ref>{{cite journal | vauthors = Sengupta S, Dey KK, Muddana HS, Tabouillot T, Ibele ME, Butler PJ, Sen A | title = Enzyme molecules as nanomotors | journal = Journal of the American Chemical Society | volume = 135 | issue = 4 | pages = 1406–1414 | date = January 2013 | pmid = 23308365 | doi = 10.1021/ja3091615 }}</ref> The thermodynamically favorable binding of enzymes to their specific substrates is recognized as the origin of enzymatic chemotaxis.<ref>{{cite journal | vauthors = Mohajerani F, Zhao X, Somasundar A, Velegol D, Sen A | title = A Theory of Enzyme Chemotaxis: From Experiments to Modeling | journal = Biochemistry | volume = 57 | issue = 43 | pages = 6256–6263 | date = October 2018 | pmid = 30251529 | doi = 10.1021/acs.biochem.8b00801 | arxiv = 1809.02530 | s2cid = 52816076 }}</ref> Additionally, enzymes in cascades have also shown substrate-driven chemotactic aggregation.<ref>{{cite journal | vauthors = Zhao X, Palacci H, Yadav V, Spiering MM, Gilson MK, Butler PJ, Hess H, Benkovic SJ, Sen A | title = Substrate-driven chemotactic assembly in an enzyme cascade | journal = Nature Chemistry | volume = 10 | issue = 3 | pages = 311–317 | date = March 2018 | pmid = 29461522 | doi = 10.1038/nchem.2905 | bibcode = 2018NatCh..10..311Z }}</ref>

Apart from active enzymes, non-reacting molecules also show chemotactic behavior. This has been demonstrated by using dye molecules that move directionally in gradients of polymer solution through favorable hydrophobic interactions.<ref>{{cite journal | vauthors = Guha R, Mohajerani F, Collins M, Ghosh S, Sen A, Velegol D | title = Chemotaxis of Molecular Dyes in Polymer Gradients in Solution | journal = Journal of the American Chemical Society | volume = 139 | issue = 44 | pages = 15588–15591 | date = November 2017 | pmid = 29064685 | doi = 10.1021/jacs.7b08783 }}</ref>

== See also ==
{{div col|colwidth=22em}}
* [[McCutcheon index]]
* [[Tropism]]
* [[Durotaxis]]
* [[Haptotaxis]]
* [[Mechanotaxis]]
* [[Plithotaxis]]
* [[Thin layers (oceanography)]]
{{div col end}}

== References ==
{{reflist|32em}}

== Further reading ==
{{Scholia|topic}}{{clear}}
{{Refbegin|32em}}
* {{cite book | vauthors = Alberts B, Johnson A, Lewis J, Walter P, Raff MC |chapter=Bacterial Chemotaxis Depends on a Two-Component Signaling Pathway Activated by Histidine-Kinase-associated Receptors|title=Molecular Biology of the Cell|date=2002|publisher=Taylor & Francis Group|isbn=978-0-8153-4069-0 | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK26822/#2878 |access-date=18 September 2017}}
* {{cite journal | vauthors = Bagorda A, Parent CA | title = Eukaryotic chemotaxis at a glance | journal = Journal of Cell Science | volume = 121 | issue = Pt 16 | pages = 2621–4 | date = August 2008 | pmid = 18685153 | doi = 10.1242/jcs.018077 | pmc = 7213762 | citeseerx = 10.1.1.515.32 }}
* {{cite book| vauthors = Berg HC |title=Random walks in biology|year=1993|publisher=Princeton Univ. Press|location=Princeton, NJ|isbn=978-0-691-00064-0|edition=Expanded, rev.}} 
* {{cite journal | vauthors = Berg HC | title = E. coli in motion| journal = Physics Today|volume=58|issue=2|pages=64–65|date=2003|publisher=Springer|location=New York|isbn=978-0-387-00888-2|bibcode=2005PhT....58b..64B|doi=10.1063/1.1897527}}
* {{cite book| vauthors = Dusenbery DB |title=Living at micro scale : the unexpected physics of being small|date=2009|publisher=Harvard University Press|location=Cambridge, Mass.|isbn=978-0-674-03116-6}}
* {{cite book| vauthors = Eisenbach M | veditors = Lengeler JW |title=Chemotaxis|year=2004|publisher=Imperial College Press|location=London|isbn=978-1-86094-413-0}}
* {{Cite book| vauthors = Eisenbach M |title=Encyclopedia of Life Sciences |chapter=Bacterial Chemotaxis |date=December 2011|doi=10.1002/9780470015902.a0001251.pub3|isbn=978-0470016176}}
* {{cite journal | vauthors = Hazelbauer GL | title = Bacterial chemotaxis: the early years of molecular studies | journal = Annual Review of Microbiology | volume = 66 | issue = 1 | pages = 285–303 | date = 13 October 2012 | pmid = 22994495 | pmc = 3989901 | doi = 10.1146/annurev-micro-092611-150120 }}
* {{cite book| vauthors = Jin T, Hereld D |title=Chemotaxis: Methods and Protocols|date=2016|publisher=Humana Press|isbn=978-1-4939-3480-5}}
* {{Cite book | vauthors = Miller LD, Russell MH, Alexandre G | chapter = Diversity in bacterial chemotactic responses and niche adaptation | title = Advances in Applied Microbiology | volume = 66 | pages = 53–75 | year = 2009 | pmid = 19203648 | doi = 10.1016/S0065-2164(08)00803-4 | isbn = 9780123747884 }}
* {{cite journal | vauthors = Rao CV, Kirby JR, Arkin AP | title = Design and diversity in bacterial chemotaxis: a comparative study in Escherichia coli and Bacillus subtilis | journal = PLOS Biology | volume = 2 | issue = 2 | pages = E49 | date = February 2004 | pmid = 14966542 | pmc = 340952 | doi = 10.1371/journal.pbio.0020049 | doi-access = free }}
* {{cite journal | vauthors = Williams AH | title = Chemotaxis on the move - active learning teaching tool | journal = Journal of Microbiology & Biology Education | volume = 11 | issue = 2 | pages = 177–8 | date = 20 December 2010 | pmid = 23653726 | pmc = 3577161 | doi = 10.1128/jmbe.v11i2.216 }}
{{Refend}}

== External links ==
{{Wikibooks}}
{{commons category|Chemotaxis}}
* [http://chemotaxis.semmelweis.hu Chemotaxis]
* [http://chemotaxis.tiddlyspot.com/ Neutrophil Chemotaxis]
* [https://web.archive.org/web/20141211173306/http://www.cellmigration.org/index.shtml Cell Migration Gateway]
* [http://www.mathworks.com/matlabcentral/fileexchange/9732-taxi-movement-of-bacteria-to-the-food Downloadable Matlab chemotaxis simulator]
* [http://wormweb.org/bacteriachemo Bacterial Chemotaxis Interactive Simulator (web-app)]

{{Taxes (Biology)}}

[[Category:Motile cells]]
[[Category:Perception]]
[[Category:Taxes (biology)]]
[[Category:Transmembrane receptors]]
[[Category:Transport phenomena]]