Secretion systems in Gram-negative bacteria: structural and mechanistic insights

Bacteria have evolved a remarkable array of sophisticated nanomachines to export various virulence factors across the bacterial cell envelope. In recent years, considerable progress has been made towards elucidating the structural and molecular mechanisms of the six secretion systems (types I-VI) of Gram-negative bacteria, the unique mycobacterial type VII secretion system, the chaperone-usher pathway and the curli secretion machinery. These advances have greatly enhanced our understanding of the complex mechanisms that these macromolecular structures use to deliver proteins and DNA into the extracellular environment or into target cells. In this Review, we explore the structural and mechanistic relationships between these single- and double-membrane-embedded systems, and we briefly discuss how this knowledge can be exploited for the development of new antimicrobial strategies.