The intestinal microbial ecosystem includes diverse species of bacteria. These bacteria can produce bioactive metabolites that directly affect the host’s immune system. An imbalance of microbial metabolites has been implicated in the pathogenesis of chronic disorders, including ulcerative colitis. In a recent article published in PNAS, a team led by researchers at Osaka University uncovered a molecular mechanism by which aggravation of colitis is inhibited through regulation of intestinal levels of a microbial metabolite, adenosine triphosphate (ATP), that affects certain immune cells and facilitates inflammatory responses.