Overeating, inactivity and genetics are key players in the path to obesity, but there may be a surprising, hidden cause that’s also to blame: intestinal bacteria.

In a recent study, certain combinations of bacteria led mice to develop metabolic syndrome. When the problematic bacterial culture was transferred to healthy mice, they, too, started to exhibit dangerous eating behaviors and associated weight gain.

There’s no clear-cut cause-and-effect here—and researchers are still trying to unravel how those changes in gut bacteria could influence obesity in humans. But if the research pans out, there might be a much more desirable obesity treatment than gastric bypass surgery.