Acidification-dependent suppression of C. difficile by enterococci in vitro

Clostridioides difficile and Vancomycin-resistant Enterococcus faecium (VRE) are commonly co-isolated from hospitalized patients. We sought to develop a co-culture biofilm model to characterize interactions between these two opportunistic pathogens. Upon growth in biofilm-promoting media containing added glucose, fructose or trehalose, VRE produces sufficient acid to lower the pH and inhibit growth of C. difficile. We found this effect depended on the carbon source, and that acidification by VRE was necessary and sufficient to suppress C. difficile growth in liquid medium and in cecal content extracts from germ free mice. VRE frequently dominates the intestine of patients administered antibiotics which can predispose to the development of C. difficile infection. We reasoned that it may be possible to suppress C. difficile growth during co-infection with VRE by supplementing the mouse diet with a fermentable sugar. A VRE-dominated gut microbiota may convert the sugar to acid, lower the pH and reestablish colonization resistance to C. difficile. Supplementation of the diet of VRE colonized mice with high levels of fructose neither resulted in a lower pH, nor did it prevent colonization by C. difficile. Taken together, these data suggest that VRE can suppress growth of C. difficile by organic acid production in a carbon source-dependent manner in vitro, however, the mammalian intestine may require sophisticated approaches to lower pH therapeutically.