Bacteroides spp. modulate Shigella flexneri virulence

Shigella flexneri causes bacterial dysentery in humans, leading to high rates of morbidity and mortality each year. While much is known about the invasive process and intracellular lifecycle of S. flexneri, its lifecycle in the colon is less well understood. Interactions with members of the gut microbiota in the colon are potentially an important aspect of S. flexneri pathogenesis, but these interactions have not been previously characterized. To understand the impact of the gut microbiota on S. flexneri, we examined the effect of Bacteroides thetaiotaomicron (Bt) and Bacteroides fragilis (B. fragilis) on S. flexneri growth and virulence. Conditioned medium collected from Bt or B. fragilis reduced S. flexneri invasion by repressing S. flexneri virulence gene and protein expression. Repression of S. flexneri virulence protein expression was not caused by Bacteroides secreted small molecules or proteins, but was linked to Bt outer membrane vesicles (OMVs) that fuse to S. flexneri. Bt OMVs repress S. flexneri invasion and virulence gene expression. Repression of S. flexneri virulence protein levels was due to the Bt derived lipids that comprise the OMVs. This effect is specific to Bt lipids, as an equal amount of lipids from Escherichia coli had no effect on S. flexneri virulence protein levels. Lastly, despite having no effect on gene expression of S. flexneri’s master virulence regulator virF, Bt conditioned medium (CM) and Bt OMVs reduced the protein levels of an epitope tagged VirF, suggesting that Bt CM and Bt OMVs regulate VirF post-transcriptionally. Together, these data demonstrate that interspecies interactions with prominent members of the gut microbiota modulate S. flexneri virulence.Microbiolog