Editing Pilus (section)

== Types by function ==
A few names are given to different types of pili by their function. The classification does not always overlap with the structural or evolutionary-based types, as [[convergent evolution]] occurs.<ref name="pmid24133488"/> 

===Conjugative pili===
Conjugative pili allow for the transfer of [[DNA]] between bacteria, in the process of [[bacterial conjugation]]. They are sometimes called "sex pili", in analogy to [[sexual reproduction]], because they allow for the exchange of genes via the formation of "mating pairs". Perhaps the most well-studied is the F-pilus of ''[[Escherichia coli]]'', encoded by the [[Fertility factor (bacteria)|F sex factor]].
[[File:Bacterial conjugation.png|thumb|''[[Escherichia coli]]'' undergoing [[Bacterial conjugation|conjugation]]. Bacteria produce long extracellular appendages called sex pili, which connect two neighbouring cells and serve as a physical conduit for transfer of DNA. Adapted from <ref>{{cite news |title=Gut bacteria use super-polymers to dodge antibiotics {{!}} Imperial News {{!}} Imperial College London |url=https://www.imperial.ac.uk/news/244513/gut-bacteria-super-polymers-dodge-antibiotics/ |work=Imperial News |language=en}}</ref>
]] 
A sex pilus is typically 6 to 7 [[nanometer|nm]] in diameter. During conjugation, a pilus emerging from the donor bacterium ensnares the recipient bacterium, draws it in close, and eventually triggers the formation of a [[mating bridge]], which establishes direct contact and the formation of a controlled pore that allows transfer of DNA from the donor to the recipient. Typically, the DNA transferred consists of the genes required to make and transfer pili (often encoded on a [[plasmid]]), and so is a kind of [[selfish DNA]]; however, other pieces of DNA are often co-transferred and this can result in dissemination of genetic traits throughout a bacterial population, such as [[antibiotic resistance]]. The connection established by the F-pilus is extremely mechanically and thermochemically resistant thanks to the robust properties of the F-pilus, which ensures successful gene transfer in a variety of environments. <ref>{{cite journal |last1=Patkowski |first1=Jonasz B. |last2=Dahlberg |first2=Tobias |last3=Amin |first3=Himani |last4=Gahlot |first4=Dharmender K. |last5=Vijayrajratnam |first5=Sukhithasri |last6=Vogel |first6=Joseph P. |last7=Francis |first7=Matthew S. |last8=Baker |first8=Joseph L. |last9=Andersson |first9=Magnus |last10=Costa |first10=Tiago R. D. |title=The F-pilus biomechanical adaptability accelerates conjugative dissemination of antimicrobial resistance and biofilm formation |journal=Nature Communications |date=5 April 2023 |volume=14 |issue=1 |pages=1879 |doi=10.1038/s41467-023-37600-y |pmid=37019921 |pmc=10076315}}</ref> Not all bacteria can make conjugative pili, but conjugation can occur between bacteria of different species.<ref>{{Cite journal |last1=Petitjean |first1=Marie |last2=Condamine |first2=Bénédicte |last3=Burdet |first3=Charles |last4=Denamur |first4=Erick |last5=Ruppé |first5=Etienne |date=2021 |title=Phylum barrier and Escherichia coli intra-species phylogeny drive the acquisition of antibiotic-resistance genes |journal=Microbial Genomics |volume=7 |issue=8 |pages=000489 |doi=10.1099/mgen.0.000489 |doi-access=free |issn=2057-5858 |pmc=8549366 |pmid=34435947}}</ref><ref>{{Cite journal |last1=Gschwind |first1=Rémi |last2=Petitjean |first2=Marie |last3=Fournier |first3=Claudine |last4=Lao |first4=Julie |last5=Clermont |first5=Olivier |last6=Nordmann |first6=Patrice |last7=Mellmann |first7=Alexander |last8=Denamur |first8=Erick |last9=Poirel |first9=Laurent |last10=Ruppé |first10=Etienne |date=2024-04-03 |editor-last=Uhlemann |editor-first=Anne-Catrin |title=Inter-phylum circulation of a beta-lactamase-encoding gene: a rare but observable event |journal=Antimicrobial Agents and Chemotherapy |language=en |volume=68 |issue=4 |pages=e0145923 |doi=10.1128/aac.01459-23 |issn=0066-4804 |pmc=10989005 |pmid=38441061}}</ref>
[[File:Beltran et al Fig5.png|thumb|Proposed conjugation mechanisms between donor and recipient cells in archaea (left) and bacteria (right). The schematic shows how ssDNA substrates are generated by the HerA-NurA machinery in the donor archaeal cells and by the plasmid-encoded relaxosome in bacteria. The figure is reproduced from <ref name=":2" />]]
Hyperthermophilic [[archaea]] encode pili structurally similar to the bacterial conjugative pili.<ref name=":2">{{Cite journal |last1=Beltran |first1=Leticia C. |last2=Cvirkaite-Krupovic |first2=Virginija |last3=Miller |first3=Jessalyn |last4=Wang |first4=Fengbin |last5=Kreutzberger |first5=Mark A. B. |last6=Patkowski |first6=Jonasz B. |last7=Costa |first7=Tiago R. D. |last8=Schouten |first8=Stefan |last9=Levental |first9=Ilya |last10=Conticello |first10=Vincent P. |last11=Egelman |first11=Edward H. |last12=Krupovic |first12=Mart |date=2023-02-07 |title=Archaeal DNA-import apparatus is homologous to bacterial conjugation machinery |journal=Nature Communications |volume=14 |issue=1 |pages=666 |doi=10.1038/s41467-023-36349-8 |issn=2041-1723 |pmc=9905601 |pmid=36750723|bibcode=2023NatCo..14..666B }}</ref> However, unlike in bacteria, where conjugation apparatus typically mediates the transfer of mobile genetic elements, such as plasmids or transposons, the conjugative machinery of hyperthermophilic archaea, called Ced (Crenarchaeal system for exchange of DNA)<ref>{{Cite journal |last1=van Wolferen |first1=Marleen |last2=Wagner |first2=Alexander |last3=van der Does |first3=Chris |last4=Albers |first4=Sonja-Verena |author-link4=Sonja-Verena Albers |date=2016-03-01 |title=The archaeal Ced system imports DNA |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=113 |issue=9 |pages=2496–2501 |bibcode=2016PNAS..113.2496V |doi=10.1073/pnas.1513740113 |issn=1091-6490 |pmc=4780597 |pmid=26884154 |doi-access=free}}</ref> and Ted (Thermoproteales system for exchange of DNA),<ref name=":2" /> appears to be responsible for the transfer of cellular DNA between members of the same species. It has been suggested that in these archaea the conjugation machinery has been fully domesticated for promoting DNA repair through homologous recombination rather than spread of mobile genetic elements.<ref name=":2" />

=== Fimbriae ===
[[Image:E. coli fimbriae.png|thumb|''[[Escherichia coli]]''.]]
'''Fimbria''' ([[Latin]] for 'fringe', {{plural form}}: '''fimbriae''') is a term used for a short pilus, an [[Appendage#Types in prokaryotes|appendage]] that is used to attach the bacterium to a surface, sometimes also called an "attachment pilus"<ref>{{cite journal |last1=Proft |first1=T. |last2=Baker |first2=E. N. |title=Pili in Gram-negative and Gram-positive bacteria — structure, assembly and their role in disease |journal=Cellular and Molecular Life Sciences |date=February 2009 |volume=66 |issue=4 |pages=613–635 |doi=10.1007/s00018-008-8477-4|pmid=18953686 |s2cid=860681 |doi-access=free |pmc=11131518 }}</ref> or '''adhesive pilus'''. The term "fimbria" can refer to many different (structural) types of pilus. Indeed, many different types of pili have been used for adhesion, a case of [[convergent evolution]].<ref name="pmid24133488">{{cite journal |last1=Chagnot |first1=C |last2=Zorgani |first2=MA |last3=Astruc |first3=T |last4=Desvaux |first4=M |title=Proteinaceous determinants of surface colonization in bacteria: bacterial adhesion and biofilm formation from a protein secretion perspective. |doi-access=free |journal=Frontiers in Microbiology |date=14 October 2013 |volume=4 |pages=303 |doi=10.3389/fmicb.2013.00303 |pmid=24133488|pmc=3796261 }}</ref> The [[Gene Ontology]] system does not treat fimbriae as a distinct type of appendage, using the generic pilus (GO:0009289) type instead.

This appendage ranges from 3–10 nanometers in diameter and can be as much as several micrometers long. Fimbriae are used by bacteria to adhere to one another and to adhere to animal cells and some inanimate objects. A bacterium can have as many as 1,000 fimbriae. Fimbriae are only visible with the use of an [[electron microscope]]. They may be straight or flexible.

Fimbriae possess [[bacterial adhesin|adhesins]] which attach them to some sort of substratum so that the bacteria can withstand [[shear force]]s and obtain nutrients.  For example, ''[[E. coli]]'' uses them to attach to [[Mannose receptor|mannose receptors]]. 

Some [[aerobic organism|aerobic bacteria]] form a very thin layer at the surface of a [[growth medium#Types|broth culture]]. This layer, called a [[wiktionary:pellicle|pellicle]], consists of many aerobic bacteria that adhere to the surface by their fimbriae. Thus, fimbriae allow the aerobic bacteria to remain both on the broth, from which they take nutrients, and near the air.

Fimbriae are required for the formation of [[biofilm]], as they attach bacteria to host surfaces for colonization during infection. Fimbriae are either located at the poles of a cell or are evenly spread over its entire surface.

This term was also used in a lax sense to refer to all pili, by those who use "pilus" to specifically refer to sex pili.<ref>{{cite journal |last1=Ottow |first1=JC |title=Ecology, physiology, and genetics of fimbriae and pili. |journal=Annual Review of Microbiology |date=1975 |volume=29 |pages=79–108 |doi=10.1146/annurev.mi.29.100175.000455 |pmid=1180526}}</ref>