Liberation of host heme by Clostridioides difficile-mediated damage enhances Enterococcus faecalis fitness during infection

Toxin production by Clostridioides difficile damages the colonic epithelium and leads to a robust inflammatory response. This disruption of the epithelial barrier markedly alters the nutritional landscape in the C. difficile-infected gut. The impact of toxin-mediated nutritional remodeling during C. difficile infection (CDI) on resident microbiota remains largely unexplored. One group of opportunistic pathogens, the enterococci, thrive during CDI, but it is unclear what strategies they employ to survive in this altered environment. Here, we demonstrate that Enterococcus faecalis, a heme auxotroph, takes advantage of C. difficile toxin-mediated damage to acquire host heme for enhanced fitness. Specifically, heme acquired from the C. difficile-infected gut is used by E. faecalis to populate a heme-dependent cytochrome and aerobically respire. This fitness advantage is specific to C. difficile toxin-mediated damage, as infection with a toxin-null strain of C. difficile does not provide E. faecalis with a fitness advantage. Finally, targeted disruption of the E. faecalis cytochrome (cydABDC) operon leads to a fitness defect in the C. difficile-infected gut. Together, this work demonstrates that C. difficile toxin remodels the gut ecosystem and improves the fitness of E. faecalis in a cydABDC-dependent manner. These data further highlight growing evidence of a cooperative partnership between C. difficile and enterococci that has implications on susceptibility to and severity of CDI.