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Over the past several years

, there has been a barrage of studies about how the vast population of microbes that live in the gut can influence our mind and mood, evidenced by all the products in the grocery store with "probiotic" now slapped on their labels and the wave of interest in "good" bacteria. Researchers out of McMaster University in Canada, in a study in the Proceedings of the National Academy of Sciences, have shown not only that introducing new kinds of bacteria could actually reduce anxiety, but also how the bacteria could be creating that effect.

Paul Forsythe, an assistant professor in the Division of Respirology at McMaster, was one of the study's co-authors. He and his team fed mice a bacteria called Lactobacillus rhamnosus, and then monitored their behavior to gauge their anxiety level. Lactobacillus is a common bacterium. Strains of Lactobacillus are used in the making of food such as yogurt, cheese and breads. Other strains are found naturally in the human gut. However, the strain used in this experiment, researchers said, is different from any of those.

"We were interested in how changes in gut bacteria might influence the brain and behavior," Forsythe says. So, after giving the mice this strain of Lactobacillus, the researchers put them in a stressful situation, such as swimming in a bucket in which they had no visible means of escape. Researchers left the mice in there until they stopped swimming. Once the test was over, the mice's levels of corticosterone, a stress hormone similar to humans' cortisone, was measured. In mice that were fed the Lactobacillus, the stress level measured by this hormone increased much less than it did in mice who didn't receive the new microbes.

However, the really interesting part of this study was the second step, in which Forsythe and colleagues tried to figure out how bacteria in the gut might have an effect on the brain. Previous studies have shown that certain bacteria can stimulate nerve activity in the stomach. To test that, the researchers severed the vagus nerve, which connects the stomach to the brain. When they ran the same test on the mice with the nerve no longer working, the Lactobacillus had no effect on anxiety.

Forsythe says that the results suggest that the bacteria use the vagus nerve to send signals to the brain. These signals cause a change in the levels of gamma-aminobutyric acid, commonly called GABA, which influences calmness.

Forsythe and John Bienenstock, another of the study's coauthors, say that their mouse test is a long way from showing that mind-altering microbes could have a future in treating conditions such as severe anxiety in humans. But that's what they're hoping for. "The long-term effect of this is the possible uses of bacteria or their products in helping alleviate stress and possibly anxiety or depression," Bienenstock says. "It changes the way we look at these conditions and how they may be treated and may offer some new approaches."

But one of the main issues now—besides that long road of clinical trials that any medication faces to try to suss out its effectiveness, safety and possible side effects—is finding out whether any other kinds of bacteria have the same effect, and whether that effect would translate to humans. Mark Lyte, a microbiologist from Texas Tech Health Sciences Center, tells PM that the strain of bacteria used in this study is not commercially available, and when the researchers tested more common ones, they didn't get the same results. "What they're saying is that this is such a unique strain," Lyte said. "[They] tried another common one, and it didn't do what this does. Will others work? The jury is out. People shouldn't just run out to the store and get probiotics, though."

Nevertheless, Lyte was impressed—it's the first time potential probiotic bacteria have shown a direct effect on brain chemistry. "[It's] a critical step in demonstrating the use of probiotics and bringing them into the human clinic."

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