A species of gut bacteria called Clostridium ramosum, coupled with a high-fat diet, may cause animals to gain weight, researchers report. They observed that mice harboring human gut bacteria including C. ramosum gained weight when fed a high-fat diet. Mice that did not have C. ramosum were less obese even when consuming a high-fat diet, and mice that had C. ramosum but consumed a low-fat diet also stayed lean.