Researchers say stress in early life may trigger gut bacteria alterations that lead to the development of anxiety and depression in adulthood.
Increasingly, researchers are investigating how gut bacteria impact health. In November 2014, for example, Medical News Today reported on a study revealing how gut bacteria influence weight, while another study associated gut bacteria with Parkinson's disease.
According to senior study author Premysl Bercik, associate professor of medicine at the Michael G. DeGrotte School of Medicine at McMaster University in Canada, and colleagues, it has long been known that gut bacteria can also influence behavior.
However, the majority of studies investigating this association have used healthy, normal mice, says Bercik. For their study, the team used two groups of mice; one group had normal gut bacteria while the other group had no gut bacteria.
Some of the mice in each group were subjected to early-life stress, triggered by separation from their mothers for 3 hours daily from the age of 3-21 days.
Neonatal stress changed gut bacteria in mice, inducing anxiety and depression
In mice with normal gut bacteria, the team found stressed mice developed abnormal levels of the stress hormone corticosterone, alongside anxiety and depression-like behavior. What is more, these mice showed impaired gut function.
However, while stressed mice with no gut bacteria still experienced a rise in corticosterone and impaired gut function, they did not develop anxiety and depression-like behavior.
The researchers then colonized stressed germ-free mice with bacteria from stressed mice with normal gut bacteria. They found this triggered anxiety and depression, but this was not the case when they transferred gut bacteria from stressed mice into non-stressed germ-free mice.
"This suggests that in this model, both host and microbial factors are required for the development of anxiety and depression-like behavior," explains Bercik. "Neonatal stress leads to increased stress reactivity and gut dysfunction that changes the gut microbiota which, in turn, alters brain function."
Speaking of the importance of their findings, Bercik says:
"We are starting to explain the complex mechanisms of interaction and dynamics between the gut microbiota and its host. Our data show that relatively minor changes in microbiota profiles or its metabolic activity induced by neonatal stress can have profound effects on host behavior in adulthood."
The team says it is important to determine whether the observations made in this study apply to humans. "For instance, whether we can detect abnormal microbiota profiles or different microbial metabolic activity in patients with primary psychiatric disorders, like anxiety and depression," adds Bercik.
In April, MNT reported on a study published in the journal Cell, in which researchers identified specific gut bacteria that may play an important role in the production of serotonin - a neurotransmitter believed to be responsible for maintaining mood balance.