Stress: How social conflict alters gut bacteria

We now know that our brains and guts influence each other — although many of the ways in which they are interlinked remain mysterious. A new study explores how stress related to conflict impacts gut bacteria, asking: is there a difference between how "winners" vs. "losers" are affected?

Research has shown that exposure to stress can affect our bodies in myriad ways, and that this can impact everything from the health of our hearts to that of our guts.

In fact, studies have even discovered that mood disorders are often linked with gastrointestinal discomfort, among other physical symptoms.

But several aspects of the brain-gut relationship remain unclear. For instance, if the stress we are exposed to comes from social conflict, does our final position — as "winners" or "losers" — as we emerge from that situation determine to what degree our microbiome is affected?

Scientists from Georgia State University in Atlanta set out to investigate this problem by looking at physiological changes occurring in Syrian hamsters when they had to deal with stressful situations.

These animals — apart from being a source of joy as adorable pets — lend themselves very well to research about biological responses to social stress. This is because, when placed together, they compete to establish hierarchies, splitting into dominant ("winning") and subordinate ("losing") animals.

Dr. Kim Huhman and colleagues worked with adult male hamsters and looked at how such stressful social situations would alter their gut microbiota. They predicted that hamster "losers" might be the most affected by conflict with the other animals — but their study revealed some surprises.

The results of this project were published in the journal Behavioural Brain Research.

The question of 'winners' vs. 'losers'

Dr. Huhman and team analyzed the gut bacteria of the hamsters both at the beginning of the study, before the animals had been allowed to meet, and at the end, after they had competed to establish a hierarchy in their newly assembled group.

The researchers compared these samples with those taken from a group of control hamsters that were already familiar with each other and so did not have to deal with any social stress.

"We found that even a single exposure to social stress causes a change in the gut microbiota, similar to what is seen following other, much more severe physical stressors, and this change gets bigger following repeated exposures," explains Dr. Huhman.

She adds, "Because 'losers' show much more stress hormone release than do 'winners,' we initially hypothesized that the microbial changes would be more pronounced in animals that lost than in animals that won."

However, the researchers were in for a surprise; when comparing the samples of gut bacteria taken from "winners" to those sourced from their subordinate counterparts, the differences that they were looking for were not there.

Both "winners" and "losers" had much less diverse gut microbiota. In fact, the only notable variation was found in the kinds of bacteria that the hamsters' guts now housed.

"Interestingly," says Dr. Huhman, "we found that social stress, regardless of who won, led to similar overall changes in the microbiota, although the particular bacteria that were impacted were somewhat different in winners and losers."

"It might be that the impact of social stress was somewhat greater for the subordinate animals, but we can't say that strongly."

Dr. Kim Huhman

Another set of samples — those taken from the animals before they had been exposed to social stress — brought a different kind of surprise to the researchers.

They discovered that the original differences in the hamsters' individual populations of gut bacteria could, in fact, predict which ones were likely to succeed in their struggle for dominance and which were likely to lose the "competition."

"It's an intriguing finding that there were some bacteria that seemed to predict whether an animal would become a winner or a loser," Dr. Huhman explains.

"These findings," says co-author Dr. Benoit Chassaing, "suggest that bi-directional communication is occurring, with stress impacting the microbiota, and on the other hand, with some specific bacteria in turn impacting the response to stress."

Future studies, the researchers say, should aim to investigate the potential of a mutual impact of gut bacteria and the response to stress caused by social conflict.