A new study done at McMaster University and published in Science Transational Medicine showed that gut bacteria not only have an influence on intestinal function, but also brain function. Researchers transplanted fecal material from humans with and without IBS-D (irritable bowel syndrome with diarrhea) into mice.
From Science Daily (March 1, 2017):
"The researchers found that aspects of the illness that were impacted through fecal transplants included gastrointestinal transit (the time it takes for food to leave the stomach and travel through the intestine); intestinal barrier dysfunction; low grade inflammation; and anxiety-like behavior."
I find it particularly interesting that they mention intestinal barrier dysfunction, otherwise known as "leaky gut" or intestinal permeability. This is an obvious mechanism by which gut bacteria, or more specifically, endotoxin from gut bacteria, can reach and influence the brain.
The authors also mentioned that since evidence suggests that intestinal bacteria alter brain function, they may also play a role in disorders such as Autism, Parkinson's, and Multiple Sclerosis.
From a causal perspective, then, optimizing the health of the intestinal tract, including the bacterial balance and intestinal barrier function, should be a consideration when looking to optimize brain health.
Transplantation of fecal microbiota from patients with irritable bowel syndrome alters gut function and behavior in recipient mice. Science Translational Medicine, 2017; 9 (379): eaaf6397 DOI: 10.1126/scitranslmed.aaf6397
McMaster University. "Intestinal bacteria alter gut and brain function, study shows." ScienceDaily. ScienceDaily, 1 March 2017. <www.sciencedaily.com/releases/2017/03/170301142503.htm>.