Effects of Enteric Microbiota in the Healthy Intestine

Experiments with germ-free and re-colonized animals demonstrate beneficial effects of the resident microbiota (20). The commensal bacteria act as a defense against infection using several mechanisms, including competition for nutrients, the production of antimicrobial factors against pathogens, such as lactic acid and bacteriocins, and blockage or antagonism of adhesion sites.

In addition, the integrity of the mucosa requires cell signaling between the microbiota, epithelium, and mucosal immune system (7). Without the microbiota, mucosal associated lymphoid tissue is underdeveloped and cell mediated immunity is defective. The enteric microbiota plays an important role in immune system education by fine-tuning T-cell repertoires and Th1/Th2 cytokine profiles (11). Compared with conventional animals, germ-free animals have reduced mucosal cell turnover, cytokine production, mucosal associated lymphoid tissue and lamina propria cellularity leading to an ineffective cell mediated immunity, decreased vascularity and less muscle wall thickness (25-27). There are also differences in intraepithelial lymphocytes (28,29). The intestinal microbiota primes the mucosal immune response and keeps it in a state of "controlled physiological inflammation" (26). Induction and/or maintenance of oral tolerance to ingested antigens also require microbial colonization of the gastrointestinal tract in early life.

Understanding the influence of the gastrointestinal microbiota has prompted interest in the therapeutic modification of the enteric microbiota with probiotics or prebiotics.

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