Mechanisms of maintenance of intestinal homeostasis by autochthonic microbiota and probiotics

Intestinal microbes are taxonomically diverse and constitute an ecologically dynamic microbiom interactively performing various physiological and physiopathological processes. It has been proposed that normal intestinal microbiotas play a critical role in the host's metabolic homeostasis and immune tolerance. The modulation of intestinal microbiota populations by prebiotics, probiotics, and synbiotics may be beneficial for the host's health. Under certain conditions, the intestinal microbiota and the host's homeostasis can be restored by introducing bacteria that co-mediate anti-inflammatory responses. Commensal microbes and probiotics exert their beneficial effect by at least three mechanisms. These include - the maintenance of the epithelial barrier function and the attenuation of changes in intestinal permeability through effects on tight junction, decreasing paracellular permeability, providing innate defense against pathogens, and enhancing the physical impediment of the mucous layer, - competitive exclusion by the application of probiotic bacteria stabilizing the indigenous microflora, - immunomodulatory capacity, affecting a variety of signaling pathways with modulation of proper immune, inflammatory and allergic responses. The epithelial gut barrier faces important challenges, since its function is to prevent pathogens and harmful elements of the gut lumen from penetrating into the internal environment. Competitive exclusion treatment can increase resistance to pathogen colonization and control intestinal disturbance. The dominance of symbiotic and probiotic bacteria among the gut microbiota favors a tolerogenic immune response. The release of secretory IgA stabilizes tight junctions between cells of the epithelial layer as well as hampers pathogens and symbionts invading deeper layers. The understanding of these vital processes may help to protect the host against infection, prevent chronic inflammation, and maintain mucosal integrity.