Depression and quality of life appear to be associated with certain species of bacteria found in the human gut, according to a new study by Belgian researchers. The authors found that two groups of bacteria, Coprococcus and Dialister, were depleted in people suffering from depression. The findings are reported in Nature Microbiology.

The research team found this correlation by combining the fecal microbiome data of 1,054 individuals enrolled in the Flemish Gut Flora Project with their mental health diagnoses. They then validated the finding by studying an independent group of 1,063 people from the Dutch LifeLinesDEEP cohort and a group of clinically depressed patients at the University Hospitals Leuven.

“The relationship between gut microbial metabolism and mental health is a controversial topic in microbiome research. 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," senior author Jeroen Raes, from the Flanders Institute for Biotechnology and the Catholic University of Leuven, said in a statement. “In our population-level study we identified several groups of bacteria that co-varied with human depression and quality of life across populations."

The team also created a computational toolbox that was used to identify which species of gastrointestinal bacteria can produce neuroactive compounds. They analyzed 532 genomes belonging to gut microbes and found that several bacteria can produce and interact with neurotransmitters. An example is the interaction of bacteria in the gastrointestinal tract with serotonin and dopamine. Some can even produce precursors to these important molecules.

“Many neuroactive compounds are produced in the human gut. We wanted to see which gut microbes could participate in producing, degrading, or modifying these molecules,” explained lead author Mireia Valles-Colomer, a graduate researcher at the Flanders Institute for Biotechnology and the Catholic University of Leuven. 

“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. 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.”

The team is very upfront in saying that this study doesn’t test for causality or directionality between depression and gut flora. Hopefully, further study will expand on this fascinating association.