A study by Vanessa Eeckhaut et al1 shows that subjects with inflammatory bowel disease (IBD) have lower faecal counts of Butyricicoccus pullicaecorum, that this bacterium attenuates trinitrobenzenesulfonic acid-induced colitis in rats and that the supernatant of its culture strengthens the epithelial barrier of Caco-2 cells. More generally, it is now established that some of the bacteria that dominate the intestinal mucosal and luminal ecosystems of healthy subjects are under-represented in IBD.1 ,2 It is possible that these findings are partly due to confounding factors such as IBD treatments (including low residue diet), and smoking, and this requires attention. This raises the question whether those microbes might be used as diagnostic or theragnostic markers. Developing new (bacterial) diagnostic markers of IBD is a long way off, which requires establishing their specificity, sensitivity, and predictive positive and negative values to answer relevant clinical questions, especially …