Hundreds of species of bacteria call the human gut their home. This gut “microbiome” influences our physiology and health in ways that scientists are only beginning to understand. Now, a new study suggests that gut bacteria can even mess with the mind, altering brain chemistry and changing mood and behavior.

John Cryan and colleagues at McMaster University in Canada fed mice a broth containing a benign bacterium, Lactobacillus rhamnosus. The scientists chose this particular bug partly because they had a handy supply and also because related Lactobacillus bacteria are a major ingredient of probiotic supplements and very little is known about their potential side effects, Cryan says.

In this case, the side effects appeared to be beneficial. Mice whose diets were supplemented with L. rhamnosus for 6 weeks exhibited fewer signs of stress and anxiety in standard lab tests, Cryan and colleagues reported yesterday in the Proceedings of the National Academy of Sciences.

http://news.sciencemag.org/sciencenow/2011/08/mind-altering-bugs.html