Clostridioides difficile uses amino acids associated with gut microbial dysbiosis in a subset of patients with diarrhea.

The gut microbiota plays a critical role in pathogen defense. Studies using antibiotic-treated mice reveal mechanisms that increase susceptibility to Clostridioides difficile infection (CDI), but risk factors associated with CDI in humans extend beyond antibiotic use. Here, we studied the dysbiotic gut microbiota of a subset of patients with diarrhea and modeled the gut microbiota of these patients by fecal transplantation into germ-free mice. When challenged with C. difficile, the germ-free mice transplanted with fecal samples from patients with dysbiotic microbial communities showed increased gut amino acid concentrations and greater susceptibility to CDI. A C. difficile mutant that was unable to use proline as an energy source was unable to robustly infect germ-free mice transplanted with a dysbiotic or healthy human gut microbiota. Prophylactic dietary intervention using a low-proline or low-protein diet in germ-free mice colonized by a dysbiotic human gut microbiota resulted in decreased expansion of wild-type C. difficile after challenge, suggesting that amino acid availability might be important for CDI. Furthermore, a prophylactic fecal microbiota transplant in mice with dysbiosis reduced proline availability and protected the mice from CDI. Last, we identified clinical risk factors that could potentially predict gut microbial dysbiosis and thus greater susceptibility to CDI in a retrospective cohort of patients with diarrhea. Identifying at-risk individuals and reducing their susceptibility to CDI through gut microbiota-targeted therapies could be a new approach to preventing C. difficile infection in susceptible patients.

Human-derived gut microbiota modulates colonic secretion in mice by regulating 5-HT3 receptor expression via acetate production.

Serotonin [5-hydroxytryptamine (5-HT)], an important neurotransmitter and a paracrine messenger in the gastrointestinal tract, regulates intestinal secretion by its action primarily on 5-HT3 and 5-HT4 receptors. Recent studies highlight the role of gut microbiota in 5-HT biosynthesis. In this study, we determine whether human-derived gut microbiota affects host secretory response to 5-HT and 5-HT receptor expression. We used proximal colonic mucosa-submucosa preparation from age-matched Swiss Webster germ-free (GF) and humanized (HM; ex-GF colonized with human gut microbiota) mice. 5-HT evoked a significantly greater increase in short-circuit current (ΔIsc) in GF compared with HM mice. Additionally, 5-HT3 receptor mRNA and protein expression was significantly higher in GF compared with HM mice. Ondansetron, a 5-HT3 receptor antagonist, inhibited 5-HT-evoked ΔIsc in GF mice but not in HM mice. Furthermore, a 5-HT3 receptor-selective agonist, 2-methyl-5-hydroxytryptamine hydrochloride, evoked a significantly higher ΔIsc in GF compared with HM mice. Immunohistochemistry in 5-HT3A-green fluorescent protein mice localized 5-HT3 receptor expression to enterochromaffin cells in addition to nerve fibers. The significant difference in 5-HT-evoked ΔIsc between GF and HM mice persisted in the presence of tetrodotoxin (TTX) but was lost after ondansetron application in the presence of TTX. Application of acetate (10 mM) significantly lowered 5-HT3 receptor mRNA in GF mouse colonoids. We conclude that host secretory response to 5-HT may be modulated by gut microbiota regulation of 5-HT3 receptor expression via acetate production. Epithelial 5-HT3 receptor may function as a mediator of gut microbiota-driven change in intestinal secretion.NEW & NOTEWORTHY We found that gut microbiota alters serotonin (5-HT)-evoked intestinal secretion in a 5-HT3 receptor-dependent mechanism and gut microbiota metabolite acetate alters 5-HT3 receptor expression in colonoids.View this article's corresponding video summary at https://www.youtube.com/watch?v=aOMYJMuLTcw&feature=youtu.be.