It is estimated that more than 400 bacterial species inhabit the human intestinal tract. Among these, only 30 to 40 species constitute some 99% of the mass of intestinal flora (Figure 1). Although environmental factors and physiological interactions can modulate the distribution of the microflora, diet appears to be the major factor that regulates the frequency and concentration of individual species of microorganisms that colonize the gut.
Figure 1) Distribution of bacterial species in the gastrointestinal tract
Bacteria entering the mouth are washed with saliva into the stomach. Most bacteria are destroyed by gastric acid, resulting in a very sparse bacterial population in the upper small bowel because only the most acid-resistant organisms survive transit through the stomach. The small intestine constitutes a zone of transition between the sparsely populated stomach and the luxuriant bacterial flora of the colon. In addition, bile acids, bicarbonate, lactozyme, mucins, peristalsis and antimicrobial peptides all contribute to the relative scarcity of bacterial colonization in the small intestine. In the colon, all available habitats are occupied by indigenous microorganisms. At least three major bacterial habitats have been described: the lumen of the gastrointestinal tract, the mucus gel that overlies the epithelium and the adherence of bacteria to receptors on mucosal epithelial cells.