Bacterial Pili and Fimbriae

Abstract

Abstract Bacterial proteinaceous filaments termed pili or fimbriae are nonflagellar, hair‐like structures protruding from the cell surface that are critical for bacterial virulence and fitness. Present in both Gram‐negative and Gram‐positive bacteria, pili are involved in many processes such as conjugation, adherence, twitching motility, biofilm formation and immunomodulation. Considerably diverse and complex, Gram‐negative pili are formed by noncovalent polymerisation of various pilin subunits; many of these pili require chaperones and usher proteins for their assembly. In contrast, fewer pilus systems have been described for the Gram‐positive counterparts; notably well studied are the heterotrimeric or ‐dimeric pili that are covalently assembled by a transpeptidase enzyme called sortase. Furthermore, type IV pili have been identified in several Gram‐positive bacteria, especially in clostridia. Key Concepts Noncovalent pilus polymerisation is a general mechanism of pilus assembly in Gram‐negative bacteria, including the chaperone/usher assembly pathway and type IV pilus pathway, which generates noncovalent pilus polymers. Sortase‐mediated pilus assembly is a general mechanism of pilus assembly in Gram‐positive bacteria that involves a transpeptidase enzyme named sortase, which cleaves pilin precursors at sorting signals between threonine and glycine residues and utilises the side‐chain amino groups of pilin motif sequences to generate covalent links between pilin subunits. The sorting signal consists of an LPXTG motif followed by a hydrophobic domain and a positively charged tail. The pilin motif is an 11 amino acid sequence of WxxxVxVYPKN located near the N ‐terminus of major pilin subunits, in which the electron‐donating lysine residue forms an isopeptide bond with the threonine residue generated from the cleavage of the LPXTG motif of adjacent pilin subunits. Tissue tropism is referred to as the ability of a pathogen to adhere specifically to some particular epithelial cells. Phase variation involves switching of surface antigens such as pili that allows a pathogen to evade the host immune system. Immunomodulation is a process of changing the host's immune system by certain molecules known as immunomodulators including pili that can activate or suppress immune cells. Dental plaque is one of the most complex bacterial biofilms that is formed by sequential colonisation of initial colonisers such as Actinomyces spp. and oral streptococci and late colonisers; this process involves Actinomyces fimbriae. Twitching motility mediated by pili allows translocation between mucosal surfaces, colonisation of host tissues and establishment of biofilms by a pathogen.