Scientists have shown that brain levels of serotonin -- the 'happy hormone' -- are regulated by the amount of bacteria in the gut during early life. The research shows that normal adult brain function depends on the presence of gut microbes during development. Serotonin, the major chemical involved in the regulation of mood and emotion, is altered in times of stress, anxiety and depression and most clinically effective antidepressant drugs work by targeting this neurochemical.