Pilus

A pilus (Latin for 'hair'; plural : pili) is a hairlike appendage found on the surface of many 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 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 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. 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.

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, 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 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.