Harvard scientists link gut bacteria to depression through hidden inflammation trigger

Scientists have increasingly recognized that the gut microbiome plays an important role in overall health, including the brain. However, researchers are still working to identify which specific bacteria are involved in disease and exactly how they influence the body.

One bacterium in particular, Morganella morganii, has been linked in several studies to major depressive disorder. Until recently, though, it was unclear whether this microbe contributes to depression, whether depression changes the microbiome, or whether another factor explains the connection.

Researchers at Harvard Medical School have now identified a biological mechanism that strengthens the case that M. morganii can affect brain health. Their findings offer a clearer explanation of how this bacterium may influence depression.

Published in the Journal of the American Chemical Society, the study points to an inflammation-triggering molecule and suggests a possible new target for diagnosing or treating certain cases of depression. It also provides a framework for studying how other gut microbes may shape human health and behavior.

The researchers discovered that an environmental contaminant called diethanolamine, or DEA, can sometimes replace a sugar alcohol in a molecule produced by M. morganii in the gut.

This altered molecule behaves very differently from the normal version. Instead of remaining harmless, it activates the immune system, prompting the release of inflammatory proteins known as cytokines, especially interleukin-6 (IL-6).

This chain of events provides a potential explanation that links M. morganii to depression. Chronic inflammation is known to play a role in many diseases and has also been associated with major depressive disorder.

Previous research supports this connection. Studies have linked IL-6 to depression and have also associated M. morganii with inflammatory conditions such as type 2 diabetes and inflammatory bowel disease (IBD).

Their findings also add weight to the idea that depression, or at least some forms of it, may involve the immune system. This raises the possibility that treatments targeting immune responses, such as immune-modulating drugs, could be effective for some patients.

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