In a Scientific American podcast, Karen Hopkins reports on a new study that shows that a person's gut bacteria may affect their brain. In the study, which appears in Gastroenterology, researchers found that treating adult mice with oral antibiotics changed not only the bacterial community in their digestive tract, but their behavior and brain chemistry as well, Hopkins says. The normally shy mice became "less cautious and less anxious," she adds, and when they were taken off the drugs, their behavior returned to normal along with their "original intestinal balance." In order to confirm the association between gut and brain, the researchers took mice raised in germ-free cages and fed them bacteria from mice that were more active, Hopkins says. Indeed, the mice began to exhibit more energy and bold behavior. "So if you react to someone in a hostile way, you might literally be giving them a bellyful," Hopkins adds.