pili

Pili are composed of pilin protein subunits arranged in a helical filament extending from the bacterial cell surface. There are two main functional categories: (1) Common/attachment pili (fimbriae) - short, numerous appendages that mediate adherence to host epithelial cells and other surfaces (e.g., Type 1 pili in uropathogenic E. coli bind mannose residues on bladder epithelium; P pili mediate binding in pyelonephritis). Neisseria gonorrhoeae uses pili for attachment to urogenital epithelium and antigenic variation of pilin allows immune evasion, contributing to lack of durable immunity and vaccine difficulty. (2) Sex pili (conjugative pili) - longer, typically one or few per cell, encoded by F plasmid genes, that mediate bacterial conjugation by forming a connection between donor and recipient cells for transfer of genetic material (plasmids, sometimes chromosomal DNA). Clinically, pili are important virulence determinants: loss of pili often correlates with reduced pathogenicity (e.g., colony morphology changes in gonococci). Pili also serve as antigens targeted by the immune system and are sometimes vaccine targets, though antigenic variation (e.g., in Neisseria, via gene conversion) limits efficacy. Understanding pili is important for explaining bacterial adhesion, tissue tropism, biofilm formation, and horizontal gene transfer (spread of antibiotic resistance genes via conjugation).