pilus

Pili (fimbriae) are thin, filamentous surface structures composed of pilin protein subunits, distinct from flagella (which are for motility). They are found primarily in gram-negative bacteria, though some gram-positive organisms have analogous structures.

Types and functions: 1. Common pili (fimbriae) - Short, numerous appendages that mediate adherence to host epithelial cells and surfaces, critical for the first step of colonization and biofilm formation. Examples: Type 1 pili in E. coli (mannose-sensitive, bind uroepithelium, important in UTI pathogenesis), P pili (mediate binding to Gal-Gal disaccharide, associated with pyelonephritis).

2. Sex pili (F pili) - Longer, less numerous structures encoded by the F plasmid, involved in bacterial conjugation. They form a connection between donor (F+) and recipient (F-) cells, allowing transfer of genetic material including plasmids carrying antibiotic resistance genes (a major mechanism of horizontal gene transfer contributing to antibiotic resistance spread).

Clinical significance: - Neisseria gonorrhoeae uses pili for attachment to mucosal surfaces and also for antigenic variation, allowing immune evasion (contributes to lack of durable immunity and vaccine difficulty). - Pili are important virulence factors and potential vaccine targets. - Type IV pili (found in Neisseria, Pseudomonas) also enable a form of surface motility called twitching motility. - Antigenic variation of pilin proteins allows pathogens to evade host antibody responses.

High-yield board associations: - E. coli type 1 pili → UTI (bind mannose residues on urinary epithelium) - P pili → pyelonephritis - N. gonorrhoeae pili → attachment, antigenic variation, immune evasion - Sex pilus (F pilus) → bacterial conjugation, plasmid/resistance gene transfer