Fimbriae are short, hair-like appendages on many bacteria that facilitate adhesion to surfaces, host cells and other microbes.
Structure and function
Fimbriae are thin proteinaceous filaments that extend from the cell envelope of many Gram-negative and some Gram-positive bacteria. They are composed primarily of pilin subunits arranged helically around a core and are typically only a few nanometres in diameter but can be several micrometres long. Unlike flagella, fimbriae are not involved in motility; instead they act as adhesins that enable bacteria to attach to abiotic surfaces, host tissues and other cells. Each type of fimbria recognizes specific receptors, such as carbohydrates or proteins on epithelial cells, and binding is often mediated by a minor tip protein. Many bacteria produce multiple types of fimbriae whose expression is regulated according to environmental cues. This regulation allows microbes to switch between free-living and attached lifestyles and to evade immune responses through phase variation. Because fimbriae anchor bacteria to host tissues, they are important virulence factors in many infections.
Pathogenic examples and roles
Type 1 fimbriae of Escherichia coli bind to mannose residues on uroepithelial cells, facilitating urinary tract colonization; these interactions can be inhibited by soluble mannose analogues. P pili of uropathogenic E. coli recognize globoside receptors in the kidney, contributing to pyelonephritis. Neisseria gonorrhoeae uses pili to attach to mucosal surfaces and to resist neutrophil killing, with antigenic variation helping the pathogen evade antibodies. Bordetella pertussis expresses fimbrial hemagglutinins that help colonize the respiratory tract. Sex pili, structurally related but longer than typical fimbriae, mediate conjugation by forming a bridge between donor and recipient cells, allowing plasmid transfer. Beyond pathogenesis, fimbriae play a key role in biofilm formation on medical devices, tooth surfaces and aquatic environments by promoting initial adhesion and intercellular aggregation.
Fimbriae underscore the importance of surface structures in micrcell-celllogy and disease. By mediating adhesion and cell–cell interactions, they help bacteria colonize diverse niches and exchange genetic material.
Related Terms: Pili, Adhesins, Biofilm, Capsule, Cell surface structures