Scientists have shown for the first time that there is a way to model how the gut bacteria in a mouse can have an active role in causing anxiety and depressive-like behaviors.

It has been shown that the gut microbiome can affect many aspects of our health, but most of this research has been performed on mice that were healthy. The scientists from McMaster University wanted to test how the gut microbiome affected stressed mice. The results have been published in nature communications.

The baby mice were stressed from 3 to 21 days old by being separated from their mother for 3 hours each day.

This experiment was conducted with mice that had different gut bacterial conditions. One group of mice was grown completely free of bacteria in their guts and kept in a sterile room to prevent bacteria from affecting their behaviors (germ-free mice). The other group were regular mice that were exposed to an ordinary, complex range of bacteria. The last group was a germ-free control group that hadn't been separated from their mothers.

The baby mice with normal gut microbiomes that had been subjected to early-life stress showed an unusual increase in the stress hormone corticosterone. They also exhibited signs of depression as well as anxiety. The germ-free mice, meanwhile, behaved similarly to the control mice, showing no symptoms of anxiety or depression. It is interesting to note that these mice also had elevated levels of corticosterone, just not symptoms of depression. Naturally, the control group showed no elevated stress hormone or altered behavior.

These results indicate that the bacteria in the environment are contributing to behavior associated with anxiety and depression. Next they exposed the germ-free mice to bacteria taken from the stressed group. As the bacterial composition of the germ-free mice changed, so did their metabolic activity and their behavior. After a few weeks, the previously symptom-free mice were now showing signs of depression.

Finally, the researchers wanted to see how the control group reacted when they were exposed to bacteria from the stressed mice. In this situation, the mice didn't start showing symptoms of depression at all.

Premysl Bercik, the lead author of the study from McMaster University, concluded that stress shortly after birth in mice, alongside the microbiome associated with stress, can lead to depression later in life.

It would be interesting to see if this relationship also effects humans. It could have interesting implications for the future direction of anxiety and depression treatments. Bercik told IFLScience that the next stage for his team will be to gather human data.

"We need to obtain some human data to be able to say with confidence that bacteria are really inducing anxiety or depression. I think it’s very likely that for at least a percentage of patients with psychiatric disorders, the microbiota is playing an important role. However, so far, the data is missing.”