Milk-derived peptone reduces EHEC virulence by inhibiting the type III secretion system

The link between diet and overall human health is well established, with certain diets known to promote better health and be associated with a reduced risk of developing chronic diseases. Previous research has also demonstrated the direct effects of diet on microbiome composition, diversity, and fitness, which can, in turn, alter colonization resistance and the immune response against enteric pathogens. However, much less is known regarding the direct effect of diet on the virulence of pathogenic bacteria. To examine the effect of diet on bacterial virulence while maintaining constant macronutrient composition, we used peptones from different sources (plant- and animal-based) to simulate various dietary protein sources. The peptones were examined for their effect on the virulence of enterohemorrhagic Escherichia coli (EHEC). We found that bacteria grown with peptone derived from casein - the main protein component of milk - exhibit a significant reduction in their type III secretion system (T3SS) activity. This effect manifested as a significant reduction in bacterial adherence to host cells and limited translocation activity of the T3SS effector Tir. We additionally found evidence suggesting that milk-derived peptone causes a shift in bacterial behavior from hyper-virulent, adherent bacteria to a more motile state. Finally, we observed that the inhibitory effects of milk-derived peptone extend beyond EHEC to other T3SS-possessing pathogens. While the specific inhibitory components in casein remain to be identified, our findings provide a foundation for developing casein-based, non-antibiotic therapies against bacterial diarrheagenic pathogens.