Scientists say that bacteria in the intestines could be responsible for triggering anxiety and depression. In this photo, An Albanian villager sits in his house which was partially destroyed by a landslide in the village of Bogo, district of Kavaje some 50 km (31 miles) from Tirana on March 22, 2009. Reuters/Arben Celi

Anxiety and depression could be linked to the balance of certain types of bacteria in the intestines, according to a new research. Scientists from McMaster University in Canada found that anxious and depressive behavior in mice caused by stress in early life appeared to be triggered only in the presence of certain gut microbes.

While scientists have long known that gut bacteria has behavioral effects, previous research has only focused on healthy mice. The results of the latest study were published in the journal Nature Communications on Tuesday.

"We have shown for the first time in an established mouse model of anxiety and depression that bacteria play a crucial role in inducing this abnormal behavior," Premysl Bercik, senior author of the paper, said in a press release. "But it's not only bacteria, it's the altered bi-directional communication between the stressed host -- mice subjected to early life stress -- and its microbiota, that leads to anxiety and depression."

The team induced stress in mice by separating them from their mothers at an early age for hours at a time. Bercik and his team confirmed that when mice with complex microbiota, which means it contains germs, were maternally separated, they showed signs of anxiety and depression, in particular, elevated levels of the stress hormone corticosterone. Such mice also showed gut dysfunction related to the release of the neurotransmitter acetylcholine.

However, mice that had no complex microbiota in their guts, but were still separated from their mothers, showed no signs of depression or anxiety.

Also, mice with germ-free microbiota, when colonized with bacteria from the control group mice, were found to show significant changes in the metabolic activity and bacterial composition of their guts, and began displaying signs of anxiety and depression within a few weeks.

"However, if we transfer the bacteria from stressed mice into non stressed germ-free mice, no abnormalities are observed. This suggests that in this model, both host and microbial factors are required for the development of anxiety and depression-like behavior. Neonatal stress leads to increased stress reactivity and gut dysfunction that changes the gut microbiota which, in turn, alters brain function," Bercik said.

The scientists called for further research to see if the conclusions from their study would apply to human beings and, if so, whether therapies that targeted gut bacterial compositions could be used to treat these conditions.