“Meanwhile, researchers were starting to uncover ways that bacteria in the gut might be able to get signals through to the brain. Pettersson and others revealed that in adult mice, microbial metabolites influence the basic physiology of the blood–brain barrier4. Gut microbes break down complex carbohydrates into short-chain fatty acids with an array of effects: the fatty acid butyrate, for example, fortifies the blood–brain barrier by tightening connections between cells (see ‘The gut–brain axis’).
“Recent studies also demonstrate that gut microbes directly alter neurotransmitter levels, which may enable them to communicate with neurons. For example, Elaine Hsiao, a biologist now at the University of California, Los Angeles, published research5 this year examining how certain metabolites from gut microbes promote serotonin production in the cells lining the colon — an intriguing finding given that some antidepressant drugs work by promoting serotonin at the junctions between neurons. These cells account for 60% of peripheral serotonin in mice and more than 90% in humans.
“Like the Karolinska group, Hsiao found that germ-free mice have significantly less serotonin floating around in their blood, and she also showed that levels could be restored by introducing to their guts spore-forming bacteria (dominated by Clostridium, which break down short-chain fatty acids). Conversely, mice with natural microbiota, when given antibiotics, had reduced serotonin production. “At least with those manipulations, it’s quite clear there’s a cause–effect relationship,” Hsiao says.”
Read the full article from Nature by Peter Andrey Smith: The Tantalizing Links Between Gut Microbes and the Brain