A new study suggests a single strain of bacteria can reconfigure entire communities of microbes that make up the gut microbiota. The findings hint that preventing gut microbial communities from being thrown out of balance by the activities of a few specific bacteria might pave the way for promising treatment options for inflammatory bowel diseases. In healthy individuals, a diverse array of intestinal-dwelling microorganisms respond to environmental stressors such as diet, inflammation and infection. In those with disease, this rich variety of microbiota becomes altered (a poorly understood process also referred to as dysbiosis). Seeking to learn more, Josephine Ni and colleagues characterized the molecular contents of fecal samples from 90 pediatric patients afflicted by Crohn's disease. They determined that children with the condition had increased amounts of the protein building blocks known as amino acids in their guts as compared to healthy individuals. The scientists narrowed in on a bacterial enzyme called urease that helps microbes scavenge nitrogen to make amino acids. Inoculating mice with a single strain of Escherichia coli that had been engineered to make urease caused more severe disease in colitis models. Ni et al. determined that the presence of urease-producing E. coli was sufficient to tip the composition of gut bacterial communities in mice toward an imbalanced state or dysbiosis. The authors suggest that the inconsistent effectiveness of fecal microbiota transplantation for inflammatory bowel disease could be related to variable bacterial urease activity.