Shigella flexneri effectors OspE1 and OspE2 mediate induced adherence to the colonic epithelium following bile salts exposure

Shigella flexneri is a Gram-negative pathogen that invades the colonic epithelium. While invasion has been thoroughly investigated, it is unknown how Shigella first attaches to the epithelium. Previous literature suggests that Shigella utilizes adhesins that are induced by environmental signals, including bile salts, encountered in the small intestine prior to invasion. We hypothesized that bile would induce adherence factors to facilitate attachment to colonic epithelial cells. To test our hypothesis, S. flexneri strain 2457T was subcultured in media containing bile salts, and the ability of the bacteria to adhere to the apical surface of polarized T84 epithelial cells was measured. We observed a significant increase in adherence, which was absent in a virulence plasmid-cured strain and a type-III secretion system mutant. Microarray expression analysis indicated that the ospE1/ospE2 genes were induced in the presence of bile, and bile-induced adherence was lost in a ΔospE1/ΔospE2 mutant. Further studies demonstrated that the OspE1/OspE2 proteins were localized to the bacterial outer membrane following exposure to bile salts. The data presented are the first demonstration that the OspE1/OspE2 proteins promote initial adherence to the intestinal epithelium. The adhesins required for Shigella attachment to the colonic epithelium may serve as ideal targets for vaccine development.