A healthy community of gut bacteria contributes to normal brain function and reduces symptoms of depression, a French study has shown.

Depressive-like behaviours can be caused by a reduction in some metabolites created by an imbalance in the gut bacterial community, according to the research by Institut Pasteur, Inserm and the CNRS.

Known as the gut microbiota, the bacterial population in the gut is the body’s largest reservoir of bacteria. Previous studies have already suggested that the host and the gut microbiota are systems with mutually beneficial interactions.

Now this team of French scientists have demonstrated a relationship between damage to the gut microbiota and mood disorders.

They have established a correlation between the gut microbiota and the efficacy of fluoxetine, which is a molecule used frequently as an antidepressant. Some mechanisms which govern depression, do however remain unknown.

The team found that a change to the gut microbiota brought on by chronic stress might lead to depressive-like behaviours in animal models.

This was caused by a reduction of small molecules called lipid metabolites resulting from metabolism in the blood and the brain. Called endocannabinoids, these lipid metabolites are responsible for coordinating a communication system, which is hindered majorly by a reduction in metabolites.

Endocannabinoids bind to receptors which are also the main destination of the most known active component of cannabis called THC.

Symptoms of depression were brought on through an absence of endocannabinoids in a region of the brain which helps to form memories and emotions, the researchers found.

The research team arrived at this conclusion after studying the microbiotas of healthy animals as well as animals with mood disorders.

Pierre-Marie Lledo is Head of the Perception and Memory Unit at the Institut Pasteur (CNRS/Institut Pasteur) and took part in the study. He said: “Surprisingly, simply transferring the microbiota from an animal with mood disorders to an animal in good health was enough to bring about biochemical changes and confer depressive-like behaviours in the latter.”

Some bacterial species were identified that were significantly reduced in animals with mood disorders. Normal levels of lipid derivatives were restored by an oral treatment with the same bacteria and this lifted the depressive-like symptoms, according to the study. The scientists said that these bacteria could act as an antidepressant, in which case it would be a treatment known as a ‘psychobiotic’.

Fellow joint researcher Gérard Eberl, who is Head of the Microenvironment and Immunity Unit (Institut Pasteur/Inserm), said: “This discovery shows the role played by the gut microbiota in normal brain function.”

Lipids vital for brain function disappear when there is an imbalance in the gut bacterial community, this can lead to the emergence of depressive-like behaviours. The researchers argue that this study has shown that the use of specific bacteria might be a promising method for restoring a healthy microbiota and a more effective treatment for mood disorders.

The study was published in the December edition of the journal Nature Communications.

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