Great, but still, ew. Trillions of bacteria that live in and off the human gut might be the key to fighting obesity, according to a new study published by the journal Science. Scientists took the bacteria from twins—one fat, one slim—and implanted them into mice. The mice with the bacteria from the larger twin grew larger and mice with the smaller twin’s germs stayed thin. The findings are being praised as powerful keys to what we understand about obesity. One researcher not involved with the study said the next step will be to try and treat obese patients with fecal implants, but others urged caution, saying multiple factors, including genetics and diet, influence weight.