Several animal studies have suggested that the gut microbiome impacts some of the neurobiological features of depression. For example, transfer of gut microbiota from human participants with depression to non-depressed rats evoked behavioral and physiological changes consistent with depression in recipients (2). This effect appears to be consistent with other neurological disorders such as anxiety, and also metabolic conditions like obesity.
While this is certainly enough evidence to argue that the gut microbiota may influence the nervous system in rodents, few studies have systematically analyzed this association in humans. Most studies to date included very small sample sizes, making it difficult to assess the reproducibility or validity of the findings. To address this information gap, a recently published study compared depression scores with gut microbiome diversity and composition in over 2500 individuals, while controlling for lifestyle factors and medication use (2). Mendelian randomization was then performed to determine whether a causal relationship exists between depression and certain gut microbiota.
Following statistical analysis, 12 different genera and one microbial family, Ruminococcaceae, were found to be associated with depressive symptoms. Four taxa in particular, Sellimonas, Eggerthella, Lachnoclostridium, and Hungatella, were found to be elevated in individuals with more severe symptoms of depression, whereas the remaining eight taxa were depleted in depressed patients. Additionally, a more diverse gut microbiome was also significantly associated with depressed symptoms, suggesting a multitude of factors are likely involved.
Previous studies have also shown that the intestinal bacterial strains Eggerthella, Subdoligranulum, and Coprococcus have been linked to depression, aligning with the findings of this paper. A recent fecal transplantation study found that Coproccocus was depleted in rats that exhibited depressed behavior upon fecal transplant from depressed human patients, suggesting a causal effect. Additionally, eight separate studies have found Eggerthella to be consistently elevated in patients with depression and anxiety, and MR analysis also suggests a causal link between Eggerthella and depression. However, further investigation is required to better understand the gut-brain connection.