Pilus biogenesis at the inner and outer membranes of Gram-negative bacteria

Pili are critical virulence factors of many Gram-negative pathogens. These surface structures provide bacteria with a link to their outside environments, allowing bacteria to interact with their hosts, other surfaces or with each other. They can even provide a conduit for the exchange of information. The surface chemistries and other biophysical properties of pili are uniquely adapted to the environmental challenges faced by bacteria. The assembly of these surface structures presents a molecular engineering challenge, which bacteria have solved in a variety of unique ways. In this review, we examine recent advances in our structural understanding of various Gram-negative pilus systems and discuss their functional implications.