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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 bacteria.[1][2] 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 appendage required for bacterial conjugation. All pili are primarily composed of oligomeric pilin proteins.



Pili connect a bacterium to another of its species, or to another bacterium of a different species, and build a bridge between the interior of the cells. This enables the transfer of plasmids between the bacteria. An exchanged plasmid can code for new functions, e.g., antibiotic resistance. The pilus is made out of the protein pilin.

Dozens of these structures can exist on the bacteria. Some bacterial viruses or bacteriophages attach to receptors on sex pili at the start of their reproductive cycle.

Pili are antigenic. They are also fragile and constantly replaced, sometimes with pili of different composition, resulting in altered antigenicity. Specific host responses to old pili structure are not effective on the new structure. Recombination genes of pili code for variable (V) and constant (C) regions of the pili (similar to Immunoglobulin diversity).

Sex pili

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 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 establishes 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 transformation, advantageous genetic traits 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.

The fertility factor is required to produce sex pili.

IV pili

Some pili, designated type IV pili, generate motile forces.[3] 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,some bacteria, for example Myxococcus xanthus, exhibit gliding motility. Bacterial type IV pilins are similar in structure to the component flagellins of Archaeal flagella.[4]


Attachment of bacteria to host surfaces is required for colonization during infection or to initiate formation of a biofilm. A 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 diseases.

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


  1. ^ pilus at Dorland's Medical Dictionary
  2. ^ Jarrell, K (editor) (2009). Pili and Flagella: Current Research and Future Trends. Caister Academic Press. ISBN 978-1-904455-48-6. 
  3. ^ Mattick JS (2002). "Type IV pili and twitching motility". Annu. Rev. Microbiol. 56: 289–314. doi:10.1146/annurev.micro.56.012302.160938. PMID 12142488. 
  4. ^ Jarrell et al (2009). "Archaeal Flagella and Pili". Pili and Flagella: Current Research and Future Trends. Caister Academic Press. ISBN 978-1-904455-48-6. 

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