The Role of Microbiota on GI Health

The mammalian microbiota has several important functions. It aids in nutrition by degrading complex nutrients and by synthesizing vitamins. It protects against infectious disease, by either preventing invading pathogenic bacteria from establishing in the GIT, or by conditioning of the mucosal immune system. The microbiota may also influence the development of cancer, by modulation of carcinogens, pre-carcinogens or by activation of immunological responses.

Many factors influence the progression and severity of GI infectious disease. Some examples of this are seen in the interaction between various microorganisms and also in their interaction with dietary factors and the host. A pathogen entering the GIT will meet resistance by the microbiota. An invading pathogen is also faced with the host's immune system as well as host factors such as stomach acids, bile acid and enzymes.

The GI microbiota plays an important role in activation of the innate immune system (17-19). Mucosal immune responses are activated as a result of microorganisms interacting with the gut associated lymph tissue (GALT). Interaction of microbes and antigens with GALT leads to a cascade of responses as outlined in the chapter by Moreau. The host mucosal immune system is important in preventing a pathogen from invading the GIT and the translocation of a pathogen to both the mesenteric lymph nodes (MLN) and the internal organs (20-22). The intestinal microbiota and orally administrated probiotics, prebiotics, and other nutrients may also affect the balance of Th1/Th2 cell response, and the production of pro and anti-inflammatory cytokines (23,24). The oral administration of probiotics to rodents may activate macrophages (25) and natural killer (NK) cells (26), in a similar fashion to when they are administered to humans (27). There are a number of described animal models that make research on human GI microbiota possible and bring to light the effects of the human microbiota on nutrition, immunology, and resistance against infections and other diseases (Table 1).

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