The many roles of commensal gut bacteria in the gut are by now widely appreciated. Those roles can be for better or for worse: The composition of gut bacteria affects the ability to metabolize food and so, weight. Regulation of the gut immune system affects diseases like inflammatory bowel disease. It also plays both sides of the fence in cancer, with some species contributing to colon cancer and others wreaking havoc through their metabolism of certain chemotherapies. (See BioWorld Today, Aug. 26, 2009, and Nov. 5, 2010.)