By signaling to nerve cells in the brain, certain microbes in the gut slow a fruit fly’s walking pace, scientists report. Fruit flies missing those microbes — and that signal — turn into hyperactive speed walkers.

With the normal suite of gut microbes, Drosophila melanogaster fruit flies on foot cover an average of about 2.4 millimeters a second. But fruit flies without any gut microbes zip along at about 3.5 millimeters a second, Catherine Schretter, a biologist at Caltech, and her colleagues report October 24 in Nature. These flies with missing microbes also take shorter breaks and are more active during the day.

“Our work suggests that microbes assist in maintaining a certain level of locomotion,” Schretter says.Fruit flies lacking the microbes (right) move faster and more frequently than flies with a full complement of gut bacteria (left), as shown by their tracks in red.

An enzyme made by Lactobacillus brevis bacteria normally serves as the brakes, the researchers found. When researchers supplied the enzyme, called xylose isomerase, to flies lacking bacteria, the flies began walking at a slower, more normal pace. Xylose isomerase acts on a sugar that’s thought to influence nerve cells in fruit flies’ brains that control walking.

For still mysterious reasons, the bacterial influence on walking speed occurred only in female fruit flies, not males. Studying that difference will be “a very interesting potential direction for this work,” Schretter says.

It’s not known whether bacteria influence movement in people and other mammals, but the results raise that possibility. Other studies hint that gut bacteria may have roles in other behaviors, such as appetite, and even mood