Two types of bacteria, Coprococcus and Dialister, are depleted in people with depression, researchers report today (February 4) in Nature Microbiology. The study also found that many gut bacteria can produce compounds that act on the nervous system. If confirmed, the results could lead to a deeper understanding of the gut-brain connection, and possibly open avenues to new treatments for mental illness.
“This is the first time this kind of work has been done in such a large scale in humans. Most previous work has been done in animal models,” study coauthor Jeroen Raes, a systems biologist at The Flanders Institute of Biotechnology, tells Forbes. Because most previous studies on a possible connection between gut microbial metabolism and mental health had been done in animals, the relationship has been controversial.
To find out whether the link applies to humans, Raes and his colleagues analyzed the microbiomes of 1,054 people enrolled in a study known as the Flemish Gut Flora project, as well as self-reported and physician-diagnosed depression data on the same subjects. The results revealed several types of bacteria that are negatively or positively correlated with mental health, with Coprococcus and Dialister among those that were more common in people without depression. An analysis of fecal metagenome data also showed that better mental health was associated with the gut microbiome’s ability to produce a metabolite of the human neurotransmitter dopamine called DOPAC.
John Cryan, a neuroscientist at University College Cork in Ireland who was not involved in the study, tells Science that the work is “the real first stab” at determining how a microbe’s metabolites influence mood, and that it pushes the field forward. Still, Raes is cautious, noting in Forbes that “we don't yet know whether these neuroactive compounds produced in the gut can reach the brain. Can they traverse the blood-brain-barrier? Or perhaps they act directly on the vagus nerve in the stomach, which sends signals directly to the brain.”
Adding to the complexity is the fact that not all human gut microbes have been identified yet. Today in Nature Biotechnology, for example, a separate group of researchers announced they had grown bacterial strains from 20 fecal samples from people in the UK and the US. DNA sequencing revealed more than 100 strains that had never been isolated before. “This study has led to the creation of the largest and most comprehensive public database of human health-associated intestinal bacteria,” study coauthor Samuel Forster of the Wellcome Sanger Institute and Hudson Institute of Medical Research says in a statement. “The gut microbiome plays a major role in health and disease. This important resource will fundamentally change the way researchers study the microbiome.”