Symbiont organisms are composed of a combination of commensal and probiotic organisms thought to be decreased in disease states at the expense of increased colonization by pathobionts. The location of the epithelium between the microbiota and the underlying lamina propria, which contains nerves, blood vessels, lymphatics, stroma and immune cells, allows for bi-directional host-microbe communication to be established and regulated. The microbiota-gut-brain axis is composed of a bi-directional communication system involving the nervous system, the humoral system and the hypothalamic- pituitary-adrenal-axis. Changes to the intestinal microbiota resulting from an enteric pathogen infection are also associated with the development of changes in normal behavior. Patients with inflammatory bowel diseases and irritable bowel syndrome are prone to develop concurrent mood disorders, including anxiety and depression, which are more prevalent under conditions of perceived stress. In human studies, psychological effects of administration of a Lactobacillus/ Bifidobacteria-containing mixture to healthy volunteers were determined.