Researchers examined the performance of germ-free mice, who lack gut bacteria, on a kind of maze used to test anxiety-like behaviors. The maze is in the shape of a plus with two open and two closed arms; normally, mice will avoid open spaces to minimize the risk of being seen by predators.
Normal mice, as expected, spent far more time in the closed arms when placed in the maze. The germ-free mice, however, entered the open arms far more often, spending significantly more time there than in the closed arms.
According to the study in Neurogastroenterology & Motility, when they examined the animals' brains, they found that:
"these differences in behavior were accompanied by alterations in the expression levels of several genes in the germ-free mice. ... Bacteria colonize the gut in the days following birth, during a sensitive period of brain development, and apparently influence behavior by inducing changes in the expression of certain genes."