HUMAN COLONIC MICROBIOTA - ECOLOGY, PHYSIOLOGY AND METABOLIC POTENTIAL OF INTESTINAL BACTERIA

In both health and disease, the colonic microbiota plays an important role in several areas of human physiology. This complex assemblage of microorganisms endows great metabolic potential on the large intestine , primarily through its degradative abilities. Many hundreds of differ ent types of bacteria, varying widely in physiology and biochemistry, exist in a multitude of different microhabitats in the lumen of the la rge gut, the mucin layer and on mucosal surfaces. Both microbiota and host obtain clear benefits from association. For example, growth subst rates from diet and body tissues, together with a relatively stable en vironment for bacteria to proliferate are provided by the host, which in turn has evolved to use butyrate, a bacterial fermentation product, as its principal source of energy for epithelial cells in the distal bowel. The main sources of carbon and energy for intestinal bacteria a re complex carbohydrates (starches, nonstarch polysaccharides). Carboh ydrate metabolism is of great importance in the large intestine, since generically, and in terms of absolute numbers, the vast majority of c ulturable microorganisms are saccharolytic. The amounts and types of f ermentation products formed by colonic bacteria depend on the relative amounts of each substrate available, their chemical structures and co mpositions, as well as the fermentation strategies (biochemical charac teristics and catabolite regulatory mechanisms) of bacteria participat ing in depolymerization and fermentation of the substrates. Protein br eakdown and dissimilatory amino acid metabolism result in the formatio n of a number of putatively toxic metabolites, including phenols, indo les and amines. Production of these substances is inhibited or repress ed in many intestinal microorganisms by a fermentable source of carboh ydrate. Owing to the anatomy and physiology of the colon, putrefactive processes become quantitatively more important in the distal bowel, w here carbohydrate is more limiting.