Structure and function of pili of pathogenic Neisseria species.

Pili are hairlike filamentous appendages which extend several micrometers from the bacterial surface and have long had an important role in the pathogenesis of gonococcal infections. Pioneering studies by Kellogg et al. demonstrated that a loss of virulence was observed when gonococci were subjected to repeated laboratory subculture and that this change was associated with a change in colony morphology of the bacteria when grown on solid media (21). Four characteristically different colonial forms, designated types T1, T2, T3, and T4, could be observed; primary isolates produced predominantly small, domed, highlighted colonies (T1 and T2), whereas the laboratory subculture resulted in an increasing proportion of large, flat colonial forms (T3 and T4). Each type could, however, be stably maintained by careful colony selection during subculture, and T1-T2 colonial forms retained their virulence for human volunteers (20). Subsequent electron-microscopic studies revealed that the T1-T2 colonial forms produced pili, whereas the T3-T4 colonial forms did not (54). This association between piliation and virulence prompted a considerable body of work on the structure, function, immunochemistry, and genetics of gonococcal pili and, subsequently, related studies of meningococci.