A study published by Biological Psychiatry studied the neurological effects of gut bacteria typically part of the obese microbiome, because obesity, depression, and the microbiome have each been associated with one another. To do this, 8-week-old male mice were fed either a regular chow diet or a high-fat diet. The microbiomes of these mice were then transplanted into 3-month-old male mice that were on a regular chow diet and antibiotics (the antibiotics were used in place of germ free mice to keep their gut populations low). 16S sequencing eventually showed successful transplantation of the donor microbiome to the recipient mice.

The recipient mice were subjected to anxiety, exploratory, stereotypical behavioral testing, as well as memory testing, all of which are common techniques that test for anxiety and depression in mice. In addition, the mice’s microbiomes and blood were sampled, and the mice’s guts and brains were investigated post-mortem.

Results of the experiment showed that the recipient mice, which were raised conventionally, showed significant disruption of mental behavior after harboring the gut microbiome of obese mice that eat a high-fat diet. Furthermore, these mice had lower microbiome diversity, higher gut permeability (i.e. leaky guts), and higher levels of overall inflammation and brain inflammation than mice with the normal chow transplants. It is not understood exactly how gut bacteria affect behavior, but it is further evidence of the importance of the gut-brain axis and the potential value of prebiotic and probiotic therapeutics for mental health.