Figure 1. A. Transmission electron micrograph of an A/E lesion formed by rabbit enteropathogenic Escherichia coli (EPEC) infecting rabbit intestinal epithelial cells (micrograph provided by Dr. Ursula Heczko, Biotechnology Laboratory, University of British Columbia). B. Effects of EPEC infection on host intestinal epithelial cells. EPEC initially adheres to the host cell by its bundle-forming pili, which also mediate bacterial aggregation. Following initial attachment, EPEC secretes several virulence factors by a type III-secretion system. Signal transduction events occur within the host, including activation of phospholipase C (PLC) and protein kinase C (PKC), inositol triphosphate (IP₃) fluxes, and Ca²⁺ release from internal stores. The bacterium intimately adheres to the cell by secreting its own receptor, Tir, into the host and binding to it with its outer membrane ligand, intimin. Intimin can also bind ß1-integrins. Several cytoskeletal proteins are recruited to the site of EPEC attachment, including actin, -actinin, talin, and ezrin. Cytoskeletal rearrangements occur following Tir-intimin binding, resulting in the formation of a pedestal-like structure upon which the pathogen resides.

Figure 2. A. Transmission electron micrograph of Salmonella-induced membrane ruffling in polarized Caco-2 epithelial cells. B. Salmonella invasion into host epithelial cells. Salmonella secrete virulence proteins, including SopE and SptP, into host cells by the type III-secretion system. SopE functions as a guanidine exchange factor for small GTP-binding proteins, probably mediating the exchange of GDP for GTP on a Rho subfamily member, CDC42. SptP is a tyrosine phosphatase required for invasion, probably by disrupting the cytoskeleton. Invasion also stimulates phospholipase C (PLC) activity, leading to inositol triphosphate (IP₃) and Ca²⁺ fluxes, which in turn may be involved in cytoskeletal rearrangements leading to membrane ruffling and Salmonella internalization.

Figure 3. A. Immunofluorescence micrograph showing Shigella (red) propelling itself through the cytoplasm by polymerizing actin (green) (Philippe Sansonetti, Institut Pasteur, reprinted with permission from Trends in Microbiology, 1996). B. Shigella-mediated cytoskeletal rearrangements. The outer membrane protein, IcsA, is sufficient to drive actin-based motility of Shigella. IcsA directly binds two proteins, vinculin and neural-Wiskott-Aldrich Syndrome protein (N-WASP). Vinculin undergoes proteolysis within the host cell upon Shigella infection, producing a 90-kDa fragment that can bind to IcsA and to the vasodilator-stimulated phosphoprotein (VASP). VASP in turn can recruit profilin to the bacterial surface, which can provide actin for tail construction. N-WASP binding of IcsA can also recruit profilin to the bacterial surface and may be another means of obtaining monomeric actin for tail formation and subsequent bacterial motility.