toxB gene on pO157 of enterohemorrhagic Escherichia coli O157 : H7 is required for full epithelial cell adherence phenotype

Adherence of enterohemorrhagic Escherichia coli (EHEC) to the intestinal ep ithelium is critical for initiation of a bacterial infection. An in vitro i nfection study previously indicated that EHEC bacteria initially adhere dif fusely and then proliferate to develop MC, a process that is mediated by va rious secreted proteins, such as EspA, EspB, EspD, Tir, and intimin, as wel l as other putative adherence factors. In the present study, we investigate d the role of a large 93-kb plasmid (pO157) in the adherence of O157:H7 (O1 57Sakai) and found the toxB gene to be involved in the full adherence pheno type. A pO157-cured strain of O157Sakai (O157Cu) developed microcolonies on Caco-2 cells; however, the number of microcolonies was lower than that of O157Sakai, as were the production and secretion levels of EspA, EspB, and T ir. Introduction of a mini-pO157 plasmid (pIC37) composed of the toxB and o ri regions restored full adherence capacity to O157Cu, including production and secretion of the proteins. In contrast, introduction of a pO157 mutant possessing toxB::Km into O157Cu could not restore the full adherence pheno type. Expression of truncated versions of His-tagged ToxB also promoted Esp B production and/or secretion by O157Cu. These results suggest that ToxB co ntributes to the adherence of EHEC to epithelial cells through promotion of the production and/or secretion of type III secreted proteins.