The terms intestinal "microflora" or "microbiota refer to the microbial ecosystem colonizing the gastrointestinal tract. Recently developed molecular biology instruments suggest that a substantial part of bacterial communities within the human gut still have to be described. The relevance and impact of resident bacteria on the host physiology and pathology are, however, well documented. The main functions of intestinal microflora include (1) metabolic activities translating into energy and nutrients uptake, and (2) host protection against invasion by foreign microorganisms. Intestinal bacteria play an essential role in the development and homeostasis of the immune system. Lymphoid follicles within the intestinal mucosa are the main areas for immune system induction and regulation. On the other hand, there is evidence implicating intestinal microbiota in certain pathological processes including multi-organ failure, colon cancer, and inflammatory bowel disease.