A bacterial toxin disarms gut defenses against inflammation

Koch’s postulates, formulated in 1884 by Robert Koch and Friedrich Loeffler, state that a microorganism causes a disease if it is found in all disease cases, can be isolated and grown in culture, causes the disease when introduced into a healthy host, and can be reisolated from that host (1). These criteria have been revised to account for “unculturable” microbes, such as viruses, and to recognize microbial genes as disease agents rather than the microbes themselves (1, 2). No microorganism that fits Koch’s revised postulates has been identified as the cause of ulcerative colitis (3). On page 805 of this issue, Jiang et al. (4) report that a bacterial strain isolated from patients with ulcerative colitis drives gut inflammation in mice by producing aerolysin, which is toxic to protective immune cells. The findings suggest a previously unidentified mechanism by which a microbe might contribute to the progression of ulcerative colitis.