How is depression linked to gut microbiota?

Once it was common to think that we need our gut bacteria to resist pathogens that enter our gastrointestinal act. Today scientists discovered a connection between the gut microbiota and development of brain systems. Gut microorganisms can produce and transmit neuroactive chemicals, including serotonin and gamma-aminobutyric acid. Researchers also found antidepressant properties in some probiotics.
The link between the gastrointestinal tract and neural system was discovered decades ago, mainly through patients with anxiety and depression who developed eating disorders and autoimmune diseases. Later it appeared that intestinal microorganisms live in contact with the immune system cells causing psychiatric disorders or protecting systems from the intrusion of pathogens. Bacteria influence the brain function through the vagus nerve that is a direct connection between the brain and the stomach. Apparently, gut microbiota releases certain neurotrophins and proteins that directly impact the brain function.
To define the link between the gut bacteria and anxiety, scientists conducted research on animal testers. Rats given probiotics were less likely to develop anxiety when put to stress. As a consequence, bacteria are considered to be the primary link between the unhealthy diet and depression. The first probiotic treatment of depression appeared yet in the early 20th century, and today the market of non-prescription probiotics in the US can compete with some popular prescription antidepressants.