sex pili (s.f. pilus)

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.