Abstract: As we know that some pathogenic microorganisms not only influence host brain and behaviors, but also cause psychiatric disorders. Recent researches indicate that beneficial symbiotic microorganisms, which also known as probiotics, may improve host health both physically and mentally. Gut microorganisms are involved in the development of the neural system and behavioral pattern of mice. They are also related to chronic gastrointestinal disorders and chronic fatigue syndrome. Some central nervous system diseases, such as hepatic encephalopathy and multiple sclerosis may be induced by the disturbance of gut micro-ecosystems. Depression and autism also may be related to the status and proportion of gut microorganisms. The observations above are consistent with the hygiene hypothesis. Balanced symbiotic microorganisms could play an important role in human physical and mental health. Although the mechanisms still remain to be clarified, various hypotheses and research models will give us a better understanding of the role of gut-brain axis in host health. We believe that the relation of symbiotic microorganisms and host health will become a promising research subject in psychology.

Key words: Symbiotic microorganisms, depression, second brain, inflammation, the hygiene hypothesis