Researchers have probed how the trillions of microbiota, the bacteria that live in the gut, influence the mental health, particularly how they affect a person's likelihood of having depression.

A team led by Jeroen Raes of KU Leuven University examined the fecal microbiome of over 1,000 participants from the Netherlands and compared them to diagnoses of depression. They found that people who lack a certain gut bacteria are more likely to experience depression.

The study is part of a bigger effort to study the relationship between gut flora and quality of life.

Depression And The Gut

In the paper published in the journal Nature Microbiology, the researchers identified the two kinds of bacteria that were less common in people who reported that they were depressed: Dialister and Coprococcus. They found that the individuals were depleted of the two bacteria whether they take antidepressants or not.

The researchers also identified the microbiota's capacity to produce or degrade molecules that interact with the nervous system. They found a correlation between quality of life and the potential of the microbiome to synthesize 3,4-dihydroxyphenylacetic acid, the principal metabolite of the neurotransmitter dopamine.

"The relationship between gut microbial metabolism and mental health is a controversial topic in microbiome research," stated Professor Raes in a press release. "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. In our population-level study we identified several groups of bacteria that co-varied with human depression and quality of life across populations."

Does The Microbiome Influence Mental Health

The study, however, does not prove that the gut flora or the lack of certain bacteria cause depression or other mental health issues. It only establishes a correlation that needs to be further probed.

According to Professor Raes, to prove causality, they would need to isolate the bacteria, culture them, and use them in animal models to see if they will trigger symptoms similar to depression.

The researchers also admit that the study has limitations. They could not say whether their findings are also true of the microbiome of people in Asia and Africa.

For now, the study adds to the growing evidence that microbiota shapes certain aspects of the brain and might contribute to conditions such as neurodegenerative diseases.