Biofilm Formation and Virulence of Shigella flexneri Are Modulated by pH of Gastrointestinal Tract

Shigella infection remains a public health problem in much of the world. Classic models of Shigella pathogenesis suggest that microfold epithelial cells in the small intestine are the preferred initial site of invasion. However, recent evidence supports an alternative model in which Shigella primarily infects a much wider range of epithelial cells that reside primarily in the colon. Here, we investigated whether the luminal pH difference between the small intestine and the colon could provide evidence in support of either model of Shigella flexneri pathogenesis. Because virulence factors culminating in cellular invasion are linked to biofilms in S. flexneri, we examined the effect of pH on the ability of S. flexneri to form and maintain adherent biofilms induced by deoxycholate. We showed that a basic pH (as expected in the small intestine) inhibited formation of biofilms and dispersed preassembled mature biofilms, while an acidic pH (similar to the colonic environment) did not permit either of these effects. To further elucidate this phenomenon at the molecular level, we probed the transcriptomes of biofilms and S. flexneri grown under different pH conditions. We identified specific amino acid (cysteine and arginine) metabolic pathways that were enriched in the bacteria that formed the biofilms but decreased when the pH increased. We then utilized a type III secretion system reporter strain to show that increasing pH reduced deoxycholate-induced virulence of S. flexneri in a dose-dependent manner. Taken together, these experiments support a model in which Shigella infection is favored in the colon because of the local pH differences in these organs.