The gut microbiome – the world of bacteria living in our digestive system – doesn’t just exist to give us stomach aches or to help us break down food. Research is rapidly emerging from the scientific community that suggests these little critters have a huge impact on our behavior, including (potentially) on our response to fear.

A new study led by the University of California Los Angeles appears to have found evidence of yet another unusual link between your stomach and your brain. Namely, a selection of gut microbes seem to be linked to regions of the brain associated with mood and general behavior, the first time such a mechanism has been found in healthy humans.

"Although rodent models have demonstrated effects of the gut microbiota on emotional, nociceptive, and social behaviors, there is little translational human evidence to date," the authors of the paper explain. "In this study, we identify brain and behavioral characteristics of healthy women clustered by gut microbiota profiles."

The team collected fecal matter from 40 different women, within which a microcosm of their gut microbiome would be contained. As these were being profiled, the same women were hooked up to a magnetic resonance imaging (MRI) scanner and shown various images of individuals, environments, situations or objects that were designed to provoke emotional responses.

As explained in the journal Psychosomatic Medicine, the team found that there were two primary groups of bacteria that appeared to have some effect on the constitution of the brain.

The first, the Prevotella, were found most commonly within seven of the women. These participants’ brains showed a greater connectivity between the emotional, attentional, and sensory brain regions, while having smaller and less active hippocampi, the region of the brain that is related to emotional regulation, consciousness and the consolidation of short-term memories into long-term ones.

These women appeared to experience profoundly negative emotions, including those related to distress and anxiety, when viewing negative images.

The second bacterial group, the Bacterioids, were more prevalent in the other 33 women. Consequently, they had a very different type of brain. The regions of the brain linked to problem-solving and complex information processing had more gray matter than the other group of women. Their hippocampi were also more voluminous and active.

These subjects, in contrast to the Prevotella-prominent women, were less likely to experience negative emotions when being shown negative imagery.

This research is indubitably fascinating, but as with plenty of these studies, it merely proves that a strong correlation between cognition and the gut microbiome exists. The causal mechanisms are deeply uncertain at this point, and this particular study involved an incredibly small sample size.

A far more diverse population may come up with very different results. The prevalence of Prevotella, for example, varies wildly in the gut microbiomes of, say, European and African children.

In any case, the idea that certain gut bacteria not only influence thought processes, but the physical structure of the brain itself, is, for lack of a better word, mind-boggling.

A separate analysis back in 2015 points out that "the human gut harbors a dynamic and complex microbial ecosystem, consisting of approximately 1 kg of bacteria in the average adult," which, rather incredibly, is "approximately the weight of the human brain."

Make no mistake about it: our gut microbes are a major part of our biological framework, and the more we understand about them and their links to other parts of our constitution, the better.