EspFU, a type III-translocated effector of actin assembly, fosters epithelial association and late-stage intestinal colonization by E. coli O157:H7

Abstract

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 induces filamentous actin-rich “pedestals” on intestinal epithelial cells. Pedestal formation in vitro requires translocation of bacterial effectors into the host cell, including Tir, an EHEC receptor, and EspF_U, which increases the efficiency of actin assembly initiated by Tir. While inactivation of espF_U does not alter colonization in two reservoir hosts, we utilized two disease models to explore the significance of EspF_U-promoted actin pedestal formation. EHEC ΔespF_U efficiently colonized the rabbit intestine during co-infection with wild type EHEC, but co-infection studies on cultured cells suggested that EspF_U produced by wild type bacteria might have rescued the mutant. Significantly, EHECΔespF_U by itself was fully capable of establishing colonization at 2 days post-inoculation but unlike wild type, failed to expand in numbers in the cecum and colon by 7 days. In the gnotobiotic piglet model, an espF_U deletion mutant appeared to generate actin pedestals with lower efficiency than wild type. Furthermore, aggregates of the mutant occupied a significantly smaller area of the intestinal epithelial surface than those of the wild type. Together, these findings suggest that, after initial EHEC colonization of the intestinal surface, EspF_U, may stabilize bacterial association with the epithelial cytoskeleton and promote expansion beyond initial sites of infection.