Perhaps one of the most compelling, and little understood, areas of gut microbiome science is the strange relationship between gut bacteria and mental health. Researchers are rapidly uncovering new insights into the gut-brain connection, discovering potential microbiome influences on everything from PTSD to brain inflammation. However, a new study into the correlation between gut bacteria and depression has revealed an intriguing association between low levels of specific bacteria and increased rates of depression.

"The relationship between gut microbial metabolism and mental health is a controversial topic in microbiome research," says Jeroen Raes from the VIB Center for Microbiology and the University of Leuven who is one of the authors on the new study. "The notion that microbial metabolites can interact with our brain – and thus behavior and feelings – is intriguing, but gut microbiome-brain communication has mostly been explored in animal models, with human research lagging behind."

Utilizing data from the large and ongoing Flemish Gut Flora Project, the researchers assessed the association between fecal microbiome data and diagnosed clinical depression. The results found two specific bacteria significantly stood out as being consistently seen in low levels among subjects suffering from depression.

Coprococcus and Dialister, two different bacterial families, were identified as major quality of life markers. As well as being found in depleted levels in depressed subjects, increased levels of the bacteria were strongly associated with higher quality of life indicators. Interestingly, a bacterial enterotype known as Bacteroides2 was found to be more prevalent in depressed subjects. This particular family of bacteria has previously been associated with the inflammatory bowel condition Crohn's disease.

"This finding adds more evidence pointing to the potentially dysbiotic nature of the Bacteroides2 enterotype we identified earlier," explains Raes. "Apparently, microbial communities that can be linked to intestinal inflammation and reduced wellbeing share a set of common features."

Although this study makes no claims about a causal link between the bacteria and depression at this stage, the researchers did conduct a comprehensive genome analysis tracking neuroactive potential in over 500 different microbial species. The idea was to catalog the neuroactivity of certain bacteria by understanding what chemical compounds they have the ability to produce or degrade.

"Our toolbox not only allows to identify the different bacteria that could play a role in mental health conditions, but also the mechanisms potentially involved in this interaction with the host," says Mireia Valles-Colomer, another researcher working on the project. "For example, we found that the ability of microorganisms to produce DOPAC, a metabolite of the human neurotransmitter dopamine, was associated with better mental quality of life."

Ultimately, this new research does raise more questions than it answers. If, for example, certain gut bacteria are found to stimulate the production of neuroactive mood-affecting chemicals, what mechanistic pathway exists to produce any effects on the brain?

It is still extraordinarily early days in the study of gut bacteria's effects on mental health. A very small recent human trial into a specific probiotic supplement's influence on depression revealed minor but positive results. Understanding causation, however, is still the greatest challenge for much of this research, and even in the earlier probiotic study it was unclear whether any mental health improvements were directly related to microbiome alterations, or were simply a secondary effect generated by a reduction in broader inflammatory symptoms.

This new research pushes the microbiome research field a step forward, offering a broader understanding into how certain bacteria may be producing chemicals that can alter mood in human subjects.

The new research was published in the journal Nature Microbiology.

Source: VIB