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Basic concepts / Intestinal microflora
Intestinal microflora
Intestinal bacteria are also a crucial component of the enterohepatic circulation: metabolites that are conjugated in the liver (conjugation increases the solubility of the metabolite in bile, but the conjugated compounds are poorly absorbed by the intestinal mucosa) and excreted in the bile are deconjugated in the intestine by bacterial enzymes (such as ß-glucuronidase, sulfatase, and various glycosidases), then absorbed through the mucosa, and returned to the liver via the portal circulation. Many endogenous compounds undergo enterohepatic circulation, including bilirubin, bile acids, cholesterol, estrogens, and vitamin D metabolites.
The intestinal microflora synthesizes vitamin K (a cofactor that is necessary in the production of prothrombin and other important blood clotting factors), biotin, vitamin B12, folic acid, and thiamine.
The intestinal flora also plays a role in fibre digestion, fermentation of indigestible carbohydrates (dietary fibre) and split of these carbohydrates into short-chain fatty acids, such as acetate, propionate, and butyrate. The major source of such fermentable carbohydrates in the human colon is vegetal cell wall polysaccharides, such as pectin, cellulose, and hemicellulose. The acids produced from these fibre substrates by fermenting bacteria would be an important energy source for the host.
Like other complex ecosystems, the intestinal microflora is relatively stable over time, maintaining roughly constant numbers and types of bacteria in each area of the bowel. This stability of normal flora both discourages infection by exogenous pathogens and prevents overgrowth of potentially pathogenic members.
The intestinal microflora may prevent infection by interfering with pathogens.]
New organisms entering the system in contaminated food or water generally are suppressed by the established flora, by producing its own antibacterial substances (e.g., bacteriocins and fatty acids). Then, the microflora itself stabilizes the normal population and prevents implantation of pathogens. However, sometimes, antibiotics that kill part of the intestinal flora sometimes upset its balance and may open the door to infection or pathologic overgrowth.
In the upper intestine, Lactobacilli and Enterococci predominate, while in the lower ileum and cecum, the flora is mainly faecal. Bacteria occur in the lumen, overlie the epithelial cells, but do not normally penetrate the bowel wall.
Many hundreds of different types of bacteria, varying widely in physiology and biochemistry, exist in a multitude of different microhabitats in the lumen of the large gut, the mucin layer and on mucosal surfaces. Both microbiota and host obtain clear benefits from association(2).
In the adult colon, 96-99% of the resident bacterial flora consists of anaerobes:
· Bacteroides species, especially B. Fragilis;
· Fusobacterium species;
· Anaerobic Lactobacilli, Bifidobacterium;
· Clostridia;
· Anaerobic gram-positive cocci (Peptostreptococcus species).
Only 1-4% represents facultative aerobes.
The intestinal flora includes low populations of potentially pathogenic organisms that cause disease if they overgrow. For example, Clostridium difficile overgrowth causes severe inflammation of the colon with diarrhoea (pseudomembranous colitis). Administration of antibiotics initiates the process by disturbing the intestinal flora. In human beings, antimicrobial drugs taken orally have the potential to temporarily suppress the drug-susceptible components of the faecal flora, and favour both infections by exogenous pathogens and overgrowth by endogenous pathogens.
Major functions of the gut microflora include metabolic activities that result in salvage of energy and absorbable nutrients, trophic effects on the intestinal epithelium, and protection of the host against invasion by harmful microbes.]
Indeed, not only the gut microflora is an essential constituent of the intestine defence barrier, in addition, there would appear to be a role for gut microflora in the control and maintenance of intestinal epithelial homeostasis(1).
1. Gaskins HR . Immunological aspects of host/microbiota interactions at the intestinal epithelium. In Mackie R. I., White B. A., and Isaacson R. E. (eds.). Gastrointestinal microbiology . New York: International Thomson Publishing 1997 (2): 537-587.
2.Macfarlane G.T., Macfarlane S. Human Colonic Microbiata: Ecology, Physiology and Metabolic Potential of Intestinal Bacteria. Scand J Gastroenterol 1997; 32 Suppl 222; 3-9.
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