Bacteria in your belly can put your brain in a bad ballgame. Specifically, bacteria in the intestines are linked with poor mental states such as depression and anxiety.

This is what experts from the Farncombe Family Digestive Health Research Institute at Canada's McMaster University discovered when they studied mice that have been exposed to early stress.

Findings of the study were published July 28 in the online journal Nature Communications.

Intestinal bacteria has, for some time now, been known to affect behavior, according to Premysl Bercik, an associate professor of medicine at the Michael G. DeGroote School of Medicine in McMacster and the study's senior author. However, previous studies have only looked at bacteria's effects on healthy, normal mice.

The team's latest study is the first to monitor how intestinal microbiota affects behavior coming from early life stress.

In the experiment, the researchers separated newborn pups from their mothers, to subject them to early life stress. From when the mice were three days old up to when they turned three weeks old, they were consistently separated from their mothers every day for three hours.

The team observed behaviors that showed anxiety and depression, with normal stress hormone corticosterone levels on maternally separated mice with complex microbiota. The researchers also noticed gut dysfunction in the mice, in the release of acetylcholine, which is a major neurotransmitter.

The researchers then repeated the same experiment but in germ-free situations. This time, they found that where there is no bacteria, the maternally separated mice also had altered levels of the stress hormone and gut dysfunction, however nothing close to anxiety or depression.

The team also tried mixing the mice with varying stress and bacteria levels. When they allowed bacteria from a control healthy mouse to colonize the maternally separated germ-free mice, they noticed that within several weeks, there were changes in the composition of bacteria and metabolic activity, leading to the mice starting to exhibit anxiety and depression.

Finally, the researchers observed that when bacteria from the stressed mice were incorporated with the nonstressed and germ-free mice, no changes occurred.

Anxiety and depression-like behavior, according to the study, is therefore affected by both host and microbial factors.

Bercik emphasized how this study led researchers to better understand the link between microbiota and psychiatric disorders.

"It would be important to determine whether this also applies to humans," he said. "For instance, whether we can detect abnormal microbiota profiles or different microbial metabolic activity in patients with primary psychiatric disorders, like anxiety and depression."

Photo: Sander van der Wel | Flickr

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