The genetics of the fat and skinny mice: identical. Their ages: all the same. The food in front of them: the same bland pellets in every cage. But the billions of bacteria teeming through each mouse’s intestines: vastly different.

The variation in gut bacteria between lean and obese mice isn’t just a consequence of their health, it’s a cause, according to work by Rob Knight, an HHMI early career scientist at the University of Colorado at Boulder, Emory University pathologist Andrew Gewirtz, and their colleagues. Transferring gut bacteria from obese mice with insulin resistance and metabolic syndrome to healthy mice makes the healthy mice develop metabolic syndrome, they found. Giving the mice antibiotics before the bacterial transfer can prevent the syndrome. Moreover, the researchers discovered that the changed microbiota doesn’t just affect molecules inside the mouse gut, it also affects outward behavior: the mice eat more than their healthy counterparts who didn’t receive transferred bacteria.

“Out of all our studies on obesity and microbes, this was the most shocking thing to me,” says Ruth Ley, one of Knight and Gewirtz’s collaborators in data analysis at Cornell University. “To have microbes actually affecting behavior.”

Illustrations: Helicobacter: dieaktivisten.de/UFZ; E. coli and Campylobacter: Creative Commons