Proinflammatory Bacteria and Their Role in Hippocampal Changes Associated with Depression

A recent neuroimaging study has linked gut bacteria with brain function changes in unmedicated patients with major depressive disorder (MDD). The research, published in Translational Psychiatry, reveals that individuals with higher levels of certain gut bacteria, which cause inflammation, show altered connectivity in the hippocampus. This brain region is critical for memory and emotion.

The hippocampus, shaped like a seahorse, is located in the temporal lobe of the brain. It plays a vital role in forming memories, learning, and navigating spaces. It connects memories to emotions and helps convert short-term memories to long-term ones. In people with depression, the hippocampus sometimes shows reduced volume and impaired cell growth, affecting memory and emotional regulation.

Researchers Shu Xiao and colleagues examined the relationship between gut bacteria and hippocampal connectivity in 49 MDD patients and 44 healthy individuals. Participants were aged between 18 and 55 and were carefully screened for other health issues to ensure consistency. All MDD patients were off medication during the study. Researchers analyzed fecal samples to identify gut bacteria and used MRI scans to assess brain connectivity.

The study found significant differences in gut bacteria between MDD patients and healthy controls. MDD patients had lower species richness of gut bacteria and a higher abundance of harmful bacteria from the Enterobacteriaceae family. They also had fewer beneficial bacteria like Prevotella, which help reduce inflammation.

These findings support the idea that gut bacteria can influence the onset of MDD. An imbalance in gut bacteria may harm the hippocampus by promoting inflammation, which can interfere with memory and emotional control. Patients with longer histories of MDD showed notably lower levels of Prevotella, indicating a potential link between illness duration and gut bacteria changes.

Additionally, researchers combined gut bacteria data and brain connectivity findings to create a model that could accurately distinguish MDD patients from healthy individuals. This could help in diagnosing MDD and identifying those at risk.

The study highlights the connections between gut bacteria and brain function but does not establish a cause-and-effect relationship. The small sample size and recruitment from a single hospital also limit the conclusions. Further research with broader participant pools is necessary for more reliable results.

The paper, “Gut proinflammatory bacteria is associated with abnormal functional connectivity of hippocampus in unmedicated patients with major depressive disorder,” was authored by a team led by Shu Xiao.