pilus

(fimbria) A projection from the surface of a bacterium. It is finer than a flagellum and also differs in having a hollow core. Pili are often found in large numbers and are thought to help attach bacteria to other cells rather than serve a locomotive function. Pilus formation may be initiated by a plasmid, in which case it serves as a conjugation tube for the transmission of a copy of the plasmid to a suitable recipient cell. Plasmids with this ability are termed sex factors or F factors.