By Tarah Knaresboro, published on November 1, 2010 - last reviewed on June 9, 2016
The world is full of microorganisms. Up to 40,000 species make their home in the human gut alone. Most, like the lactobacillus found in yogurt, live harmoniously with us. Such probiotics are thought essential for protecting against invasion by disease-causing organisms. Others are necessary for stimulating development of the immune system and maintaining its integrity; still others enable the process of digestion. But the biggest news of all is that some probiotic strains influence the brain, and behavior as well. Scientists are exploring how variations in the composition of gut microbes affect the functioning of the nervous system. The link may be the parasympathetic nervous system, which, via the vagus nerve, connects the brain to many internal organs, providing agents with a pathway for communicating with the brain. Although it's premature to recommend agents and dosages, some specific organisms may have value in alleviating depression, pain, and psychosomatic disorders such as irritable bowel disease.