New research from a team of French scientists published in the journal Nature Communications has found a connection between chronic stress, changes in the microbiome leading to decreased activity in the hippocampus, and depressive behaviour.

The researchers began by investigating microbiome changes in a mouse model of stress-induced depression. As well as initially examining the microbiome differences between healthy animals and animals with mood disorders, the researchers discovered those mood disorders could be transferred to the healthy animals via faecal transplants.

“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,” explained Pierre-Marie Lledo, one of the study’s authors.

The next step was to investigate the connection between gut bacteria alterations and depressive behaviours. It was here the researchers discovered changes in the depressed animals’ endocannabinoid system.

Changes to endocannabinoid signalling, particularly in the hippocampus, have been linked to depression and mood disorders, and in this new study, the researchers found microbiome alterations were associated with diminished endocannabinoid signalling in the hippocampus.

The researchers found stress-induced changes to the gut microbiome resulted in a decrease of certain fatty acid metabolites known to be precursors to endocannabinoids. This suggests a direct mechanism by which bacteria in our gut can possibly lead to mood disorders.

“In sum, our data shows that microbiota dysbiosis induced by chronic stress affects lipid metabolism and the generation of eCBs [endocannabinoids], leading to decreased signalling in the eCB system and reduced adult neurogenesis in the hippocampus.

“This might be the pathway, at least in part, that links microbiota dysbiosis to mood disorders, which in turn, may affect the composition of the gut microbiota through physiological adjustments and modulation of the immune system,” said the researchers.

The final stage of the research saw the team investigate the role of a particular strain of the Lactobacilli genus bacteria that seemed to be especially reduced in the depressed animals. 

Supplementation of that particular strain was enough to increase endocannabinoid brain levels and alleviate depressive behaviours in the animals.