Researchers at the Farncombe Family Digestive Health Research Institute, McMaster University recently published a study claiming that intestinal bacteria are key participants in inducing anxiety and depression. These findings are the first to associate intestinal microbiota with alterations in behavior due to early life stresses.

The Study: Bacteria Affect Mood

The fact that intestinal bacteria affect mood and behavior, has been established for some time now. Much of the experiments conducted in such studies used healthy mice.

Senior Author Premysl Bercik, Associate Professor of Medicine with McMaster’s Michael G. DeGroote School of Medicine highlighted that for this research, mice which were exposed to early life stresses using the technique of ‘maternal separation’. This involved separating newborn mice from their mothers (from day three to 21) for three hours every day, after which they were reunited.

Performing The Experiment and Observing Results

To begin with, Bercik and his team of researchers confirmed that conventional mice, having complex microbiota, displayed behavioral symptoms of anxiety and depression after exposure to maternal separation. Moreover, they had abnormal levels of corticosterone (stress hormone) and exhibited gut dysfunction based on the release of acetylcholine, a significant neurotransmitter.

Following these findings, the team then repeated the experiment in germ-free conditions. Results showed that under such circumstances, mice exposed to maternal separation still expressed altered levels of corticosterone and gut dysfunction, but their behavior was similar to normal (control) mice – no displays of anxiety or depression.

To further confirm their hypothesis, they colonized germ-free mice that had been maternally separated with bacteria from control mice. Changes in metabolic activity and bacterial composition were observed within weeks, and behavioral symptoms of depression and anxiety started to manifest.

However, when bacteria from maternally separated mice were transferred into non-stressed germ-free mice, no behavioral abnormalities were observed. Bercik suggests that this indicates the need for both, host and microbial factors, for developing depression and anxiety-like behavior.

“Neonatal stress leads to increased stress reactivity and gut dysfunction. This changes gut microbiota which, in turn, alters brain function”, he explains in the study published in the journal Nature Communications.

Implications And Prognosis

Bercik stated that the study not only showed the role of intestinal bacteria in inducing anxiety and depression, but also highlighted the distorted bi-directional communication between the stressed host and intestinal microbiota that leads to altered behavior. He added that considerably minor changes in the composition of intestinal microbiota, or its metabolic activity, brought on by neonatal stresses, could have a profound impact on behavior in adulthood.

Bercik also claims that this newly found dynamic could be vital for understanding psychiatric disorders in humans from a new perspective. For example, if the findings were extended to humans, abnormal profiles of intestinal microbiota could help diagnose primary psychiatric disorders, such as depression and anxiety.