The intestine harbors a dense, delicately balanced microbial population that mainly consists of bacteria (O'Hara & Shanahan, 2006). In general, the gut microbiota plays three major roles in the host's physiology: breaks down and utilizes dietary fiber that otherwise the host would not be able to, such as resistant polysaccharide; prevents adherence of pathogenic bacteria by competing for ecological niches and nutritional components; and synthesizes certain compounds such as vitamins, serotonin, and dopamine that helps promote the host's homeostasis. Except for animals artificially bred germ-free, all animal species possess to a microbiota in the gut. The microbiota is found at all locations in the intestinal tract, but its communities and their