One of the most groundbreaking discoveries was the discovery of the gut-brain axis—a two-way biochemical communication system between the gastrointestinal tract and the central nervous system. It turns out that gut microbes can directly influence our behavior, mood, stress levels, and cognitive function. Bacteria produce a huge number of neurotransmitters—signaling molecules of the nervous system. For example, approximately 90% of all serotonin (the “hormone of joy and well-being”) in the body is synthesized in the gut, with the active participation of the microbiota. These signals travel to the brain via the vagus nerve and circulatory system, modulating our emotional responses.