The "psychobiome" is bacteria in your gut that affects how you think and act

An array of scientific evidence suggest that in some cases, the bacteria in your gutyour microbiomecould be tied to neurological and psychological disorders and differences, from anxiety and autism to Parkinson's and schizophrenia. The journal Science published a survey of the field and the Cambridge, Massachusetts start-up Holobiome that hopes to use insight into this "psychobiome" to develop treatments for depression, insomnia, and other conditions with a neurological side to them. From Science:

For example, many people with irritable bowel syndrome are also depressed, people on the autism spectrum tend to have digestive problems, and people with Parkinsons are prone to constipation.

Researchers have also noticed an increase in depression in people taking antibioticsbut not antiviral or antifungal medications that leave gut bacteria unharmed. Last year, Jeroen Raes, a microbiologist at the Catholic University of Leuven, and colleagues analyzed the health records of two groupsone Belgian, one Dutchof more then 1000 people participating in surveys of their types of gut bacteria. People with depression had deficits of the same two bacterial species, the authors reported in April 2019 in Nature Microbiology.

Researchers see ways in which gut microbes could influence the brain. Some may secrete messenger molecules that travel though the blood to the brain. Other bacteria may stimulate the vagus nerve, which runs from the base of the brain to the organs in the abdomen. Bacterial molecules might relay signals to the vagus through recently discovered neuropod cells that sit in the lining of the gut, sensing its biochemical milieu, including microbial compounds.