There are open lines of communication between brains and the bowels and, in mice these channels allow an individual’s gut bacteria to steer their behaviour, according to Javier Bravo at University College Cork in Ireland. He fed mice with a probiotic bacterium called Lactobacillus rhamnosus, often found in yogurts and dairy products. The bacterial menu changed the levels of signalling chemicals in the rodents’ brains, and reduced behaviours associated with stress, anxiety and depression.

Other trials have found that probiotics can help to alleviate the mood symptoms that accompany irritable bowel or chronic fatigue syndrome. To that end, Bravo wanted to see if L.rhamnosus could influence the brains of normal, healthy animals.

Bravo found that the mice, after regularly eating Lactobacillus, were more likely to spend time in the exposed parts of a maze (a common test for anxiety symptoms) than those who ate bacteria-free meals. They were also less likely to drift motionlessly when plopped into water (a common test for depressive symptoms). And during stressful situations, they built up lower levels of stress hormones.

The bacteria also boosted the role of GABA, a restraining chemical that downplays the buzzing of excitable neurons. GABA works by docking with receptor proteins, and Bravo found that Lactobacillus increased the numbers of these receptors in parts of the brain associated with learning, memory and emotional control.

The bacteria in an animal’s gut can control what happens in its brain, on the other side of the body, as the vagus nerve transmits information from the gut and other visceral organs to the brain. When Bravo severed the vagus nerve in his mice, Lactobacillus lost all of its influence. It changed neither the rodents’ behaviour nor their GABA receptor levels.