Pilus
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2008-05-27T21:44:16Z
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robot Adding: [[sv:Pilus]]
[[image:Bacterial_Conjugation_en.png|right|thumb|350px|Schematic drawing of bacterial conjugation. '''1-''' Donor cell produces pilus. '''2-''' Pilus attaches to recipient cell, brings the two cells together. '''3-''' The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell. '''4-''' Both cells recircularize their plasmids, synthesize second strands, and reproduce pili; both cells are now viable donors.]]
A '''pilus''' ([[Latin]] for 'hair'; plural : ''pili'') is a hairlike appendage found on the surface of many [[bacterium|bacteria]]. The terms pilus and '''fimbria''' (Latin for 'thread' or 'fiber'; plural: ''fimbriae'') are often used interchangeably, although some researchers reserve the term pilus for the sexual appendage required for [[bacterial conjugation]]. All pili are primarily composed of oligomeric pilin proteins.
== Pili ==
Pili connect the bacterium to another of its species, or to another bacterium of a different species, and build a bridge between the cytoplasms of either cell. That enables the transfer of [[plasmid]]s between the bacteria. An exchanged plasmid can add new functions to a bacterium, e.g., an [[antibiotic resistance]].
Up to ten of these structures can exist on the bacteria. Some bacterial [[viruses]] or [[bacteriophage]]s attach to [[receptor (biochemistry)|receptor]]s on sex pili at the start of their [[reproductive]] cycle.
Despite its name, the sex pilus is not used for [[sexual reproduction]], and cannot be equated with a [[penis]], although such comparisons are often used to ease understanding.
A pilus is typically 6 to 7 [[nanometer|nm]] in diameter. During [[bacterial conjugation]], a sex pilus emerging from one bacterium ensnares the recipient bacterium, draws it in, and eventually triggers the formation of a [[mating bridge]], which is direct contact merging the cytoplasms of two bacteria via a controlled pore. This pore allows for the transfer of bacterial [[DNA]] from the bacteria with the pilus (donor) to the recipient bacteria. Through this mechanism of [[genetic engineering|genetic]] [[Transformation (genetics)|transformation]], advantageous genetic [[Trait (biological)|trait]]s can be disseminated amongst a population of bacteria. Not all bacteria have the ability to create sex pili, however sex pili can form between bacteria of different species.
Some pili, designated type IV pili, generate motile forces. The external termini of the pili adhere to solid substrate, either the surface to which the bacteria are attached or to other bacteria, and subsequent pilus contraction pulls the bacteria forward, not unlike a grappling hook. As type IV pilus-mediated movement is typically jerky, it is called twitching motility, as distinct from other forms of bacterial motility, such as are mediated by [[flagella]]. However, in [[Myxococcus xanthus|''Myxococcus xanthus'']], this movement is quite fluid.
== Fimbriae ==
Attachment of bacteria to host surfaces is required for colonization during infection or to initiate formation of a [[biofilm]]. A '''[[fimbria (bacteriology)|fimbria]]''' is a short pilus that is used to attach the bacterium to a surface. Fimbriae are either located at the poles of a cell, or are evenly spread over its entire surface. [[Mutant]] bacteria that lack fimbriae cannot adhere to their usual target surfaces and, thus, cannot cause [[disease]]s.
Some fimbriae can contain [[lectins]]. The lectins are necessary to adhere to target cells because they can recognize [[oligosaccharide]] units on the surface of these target cells. Other fimbriae bind to components of the [[extracellular matrix]].
Fimbriae are found in both [[Gram-negative]] and [[Gram-positive]] bacteria. In Gram positive bacteria, the pilin subunits are covalently linked.
==See also==
*[[Flagellum]]
[[Category:Organelles]]
[[Category:Bacteria]]
[[Category:Microbiology]]
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