Glycine fermentation by C. difficile promotes virulence and spore formation, and is induced by host cathelicidin

Clostridioides difficile is a leading cause of antibiotic-associated diarrheal disease. C. difficile colonization, growth, and toxin production in the intestine is strongly associated with its ability to use amino acids to generate energy, but little is known about the impact of specific amino acids on C. difficile pathogenesis. The amino acid glycine is enriched in the dysbiotic gut and is suspected to contribute to C. difficile infection. We hypothesized that the use of glycine as an energy source contributes to colonization of the intestine and pathogenesis of C. difficile. To test this hypothesis, we deleted the glycine reductase (GR) genes grdAB, rendering C. difficile unable to ferment glycine, and investigated the impact on growth and pathogenesis. Our data show that the grd pathway promotes growth, toxin production, and sporulation. Glycine fermentation also had a significant impact on toxin production and pathogenesis of C. difficile in the hamster model of disease. Furthermore, we determined that the grd locus is regulated by host cathelicidin (LL-37) and the cathelicidin-responsive regulator, ClnR, indicating that the host peptide signals to control glycine catabolism. The induction of glycine fermentation by LL-37 demonstrates a direct link between the host immune response and the bacterial reactions of toxin production and spore formation.