Type III secretion machines and the pathogenesis of enteric infections caused by Yersinia and Salmonella spp.

Salmonella and Yersinia spp. infect the intestinal tract of humans. Althoug h these organisms cause fundamentally different diseases, each pathogen rel ies on type III secretion machines to either inject virulence factors into the cytosol of eukaryotic cells or release toxins into the extracellular mi lieu. Type III secretion machines are composed of many different subunits a nd export several polypeptides with unique substrate requirements. During S almonella pathogenesis, the type III machine encoded by the Salmonella path ogenicity island (SPI)-1 genetic element functions to cause invasion of the intestinal epithelium, whereas another type III machine (SPI-2) is require d for survival in macrophages. Yersinia enterocolitica and Yersinia pseudot uberculosis employ type III machines to resist macrophage phagocytosis and to manipulate the host's immune response, thereby colonizing intestinal lym phoid tissues. We describe what is known about the pathogenic functions of virulence factors secreted by type III machines. Furthermore, type III secr etion machines may be exploited for the injection of recombinant proteins, a strategy that has already been successfully employed to elicit a cell-med iated immune response.