A comprehensive guide to pilus biogenesis in Gram-negative bacteria

Pili are crucial virulence factors in many Gram-negative pathogens; these surface structures enable bacteria to interact with, and attach to structures in, their environment and can also facilitate horizontal gene transfer. In this Review, Waksmanet al. examine recent advances in our structural understanding of various pilus systems in Gram-negative bacteria and discuss their functional implications. Pili are crucial virulence factors for many Gram-negative pathogens. These surface structures provide bacteria with a link to their external environments by enabling them to interact with, and attach to, host cells, other surfaces or each other, or by providing a conduit for secretion. Recent high-resolution structures of pilus filaments and the machineries that produce them, namely chaperone–usher pili, type IV pili, conjugative type IV secretion pili and type V pili, are beginning to explain some of the intriguing biological properties that pili exhibit, such as the ability of chaperone–usher pili and type IV pili to stretch in response to external forces. By contrast, conjugative pili provide a conduit for the exchange of genetic information, and recent high-resolution structures have revealed an integral association between the pilin subunit and a phospholipid molecule, which may facilitate DNA transport. In addition, progress in the area of cryo-electron tomography has provided a glimpse of the overall architecture of the type IV pilus machinery. In this Review, we examine recent advances in our structural understanding of various Gram-negative pilus systems and discuss their functional implications.