Thin, hairlike appendages, 1 to 20 microns in length and often occurring in large numbers, present on the cells of Gram-negative bacteria, particularly enterobacteriaceae and neisseria. Unlike flagella, they do not possess motility, but being protein (pilin) in nature, they possess antigenic and hemagglutinating properties. They are of medical importance because some fimbriae mediate the attachment of bacteria to cells via adhesins (bacterial adhesins). Bacterial fimbriae refer to common pili, to be distinguished from the preferred use of "pili", which is confined to sex pili (sex pili).