Updated project metadata. Intestinal microbial dysbiosis is associated with Crohn’s disease (CD). However, the mechanisms leading to the chronic mucosal inflammation that characterizes this disease remain unclear. To evaluate causality and mechanisms of disease, we conducted a systems level study of the interactions between the gut microbiota and host in new-onset pediatric patients. We report an altered host proteome in CD patients indicative of impaired mitochondrial functions. A downregulation of mitochondrial proteins implicated in H2S detoxification was observed, while the relative abundance of H2S microbial producers was increased. Network correlation analysis identified Atopobium parvulum as the central hub of H2S producers. Gnotobiotic and conventionalized colitis-susceptible interleukin-10-deficient (Il10-/-) mice demonstrated that A. parvulum induced colitis, a phenotype requiring the presence of the intestinal microbiota. Administration of bismuth, a H2S scavenger, prevented A. parvulum-induced colitis in Il10-/- mice. This study identified host-microbiota interactions that are disturbed in CD patients providing mechanistic insights on CD pathogenesis.