Long overview of research. This thread of biological research is part of a general recent scientific trend that shows the astounding complexity of life....
Hundreds of trillions of microbes live in the human gut, with 300 times the total DNA as humans. The products of this “forgotten organ” include large amounts of DNA that are critical to create necessary human nutrients, for essential metabolism and to develop the most effective immune system. Recent research shows dramatic effects of microbe products from the gut on mental function—depression, stress, autism, and degenerative illness. In humans, many studies show microbes affect anxiety, mood, depression and social behavior. Direct effects are through secreted products, stimulation of the enteric nervous system and travel of microbes into the brain, while indirect factors are microbes’ influence on immune function affecting behavior. Microbes produce molecules that transform into hormones and neurotransmitters or they produce neurotransmitters themselves. Microbes effect on the brain includes fetal development and neurotransmitter function.