Our laboratory studies the functional interface between microbial pathogens and their hosts. In particular, we are examining the interaction of the enteropathogenic bacteria Salmonella enterica and Campylobacter jejuni with the cells of the intestinal epithelium. In the case of Salmonella, our studies center around the function of an organelle, the type III secretion system, which delivers bacterial proteins into the host cell. These bacterial proteins modulate a variety of cellular functions by mimicking the activity of host cell proteins. Among the bacterial effector proteins, there is a subset that modulates the function of the Rho-family GTPases Cdc42 and Rac. This subset includes two exchange factors and a GAP for Rac and Cdc42. Through the reversible activation of Cdc42 and Rac, Salmonella modulates the actin cytoskeleton to gain access into host cells. In addition, Salmonella injects other proteins such as a tyrosine phosphatase, and inositol phosphate phosphatase, an actin-binding protein and a protease. Efforts in the laboratory are aimed at characterizing the function of these and other bacterial proteins that modulate cellular functions as well as the type III secretion organelle itself. In the case of C. jejuni, efforts are aimed at understanding how these bacteria enter intestinal epithelial cells and regulate cell cycle progression.