The invasion-associated type III secretion system of Salmonella typhimurium: common and unique features

Several bacterial pathogens make use of a specialized protein secretion sys tem to inject effector proteins into host cells. This system, commonly refe rred to as type III secretion, is always associated with phenotypes related to intimate interactions between the pathogen and its respective host cell s. The enteric pathogen Salmonella typhimurium utilizes a type III secretio n system to invade nonphagocytic intestinal epithelial cells. Whereas the i nvasion-associated type III system of S. typhimurium has evolved to perform a specific function, many of the components of this system are conserved a mong the type III systems of other bacterial pathogens. This review will di scuss the common and unique features of the S. typhimurium system in relati on to the type III systems of other human pathogens. Topics discussed inclu de the phenotypes associated with various type III systems, the genetic loc i encoding these systems, the components of the type III secretion apparatu s, the effector proteins and the mechanisms by which they enter host cells as well as the mechanisms used to regulate the expression of type III syste ms.