The scientists then transferred the microbes from the surgically-altered mice into mice whose stomachs lacked their own bacteria. Those mice also lost weight while other sterile mice receiving bacteria from either group didn't.
It's not clear what about the bacteria caused weight loss, the study authors said.
"We need to learn a good deal more about the mechanism by which a microbial population changed by gastric bypass exerts its effects," said Lee Kaplan, a study author and director of the Obesity, Metabolism, and Nutrition Institute at Massachusetts General Hospital, in a statement. " The ability to achieve even some of these effects without surgery would give us an entirely new way to treat the critical problem of obesity, one that could help patients unable or unwilling to have surgery."