1. (in microbiology) A hairlike appendage projecting from the surface of a bacterium. Pili are found in both Gram-positive and Gram-negative bacteria and are typically 0.3–2.0 µm long; they fulfil a variety of functions, including adherence to surfaces, cell–cell interactions (e.g. between a pathogen and a host cell), biofilm formation, and a form of motility called ‘twitching’, whereby the pili alternately extend and retract. Multiple subunits of the protein pilin make up each pilus, and variation in the nature of the subunits can alter the antigenic nature of the bacterium. A single cell may have numerous short pili, which are also called fimbriae. Bacteria containing a sex factor can form one or several longer sex pili, which extend to contact another cell of the same species. The sex pilus then retracts to draw the two cells together so that DNA can be transferred in the process called conjugation.