FIGURE 1.

The bi-directional microbiome-gut-brain axis in health and disease. (A) Brain dysfunction may lead to the disruption of brain-to-gut-to-microbiome signals, host behaviors and dietary habits that ultimately affect the normal colonization of the gut microbiome. Host genes may influence intestinal function and the microenvironment which may then impact on the structure/composition of the microbiome. The origins of brain disorders may be also linked to host genetics. A pre-existing abnormality in the gut microbiome may lead to heightened inflammation and cause disruption of the gut-to-brain communication, aggravating brain dysfunction. (B) Normalizing microbial community structure and function would augment host immunity and improve metabolism that would counter-balance the detrimental effects of brain dysfunction, in spite of dysbiosis not being the primary site of pathology. Rehabilitation of brain pathology would normalize brain-to-gut-to-microbiome signals and lead to a diminished inflammatory response, contributing to the improvement of the gut community. However, current treatments have side-effects that may include a disruption of the gut microbiome and normal gut health.