Pilus

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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 recipent 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. The terms pilus and fibria (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.

[edit] Sex pili

Sex 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 plasmids 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 bacteriophages attach to receptors on sex pili at the start of their reproductive cycle.

Despite the name "sex pilus", this has nothing to do with sexual reproduction or mating, nor is it the bacterial equivalent of a penis; such misnomers are used quite frequently in describing the process, and while possibly useful in understanding underlying concepts they are misleading nonetheless.

A pilus is typically 9 to 10 nm in diameter. The pilus 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.

It is an extension of the cytoplasm and used for conjugation with another cell of the same species.

Pili generate motile force via interactions with the bacteria cytoskeleton MreB which is homologous to eukaryotic actin. The process is akin to the myosin power stroke. The external termini of the pili adhere to solid substrate, and subsequent pili contraction pulls the bacteria forward, not unlike a grappling hook.

[edit] Fimbriae

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.