Intestinal bacteria may play a role in inducing anxiety and depression, according to a recent study.

Researchers from the Farncombe Family Digestive Health Research Institute at McMaster University are the first to explore the role of intestinal microbiota in the altered behavior that is a consequence of early life stress.

"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 study, said in a statement. "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."

For the study, researchers subjected mice to early life stress by separating newborn mice from their mother from day three to 21 for three hours each day.

First, Bercik and his team confirmed that conventional mice with complex microbiota, which had been maternally separated, displayed anxiety and depression-like behavior, with abnormal levels of the stress hormone corticosterone. These mice also showed gut dysfunction based on the release of a major neurotransmitter, acetylcholine.

Then, they repeated the same experiment in germ-free conditions and found that in the absence of bacteria mice which were maternally separated still have altered stress hormone levels and gut dysfunction, but they behaved similar to the control mice, not showing any signs of anxiety or depression.

Next, they found that when the maternally separated germ-free mice are colonized with bacteria from control mice, the bacterial composition and metabolic activity changed within several weeks, and the mice started exhibiting anxiety and depression.

"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 findings are detailed in the journal Nature Communications.