E Ecology of the GI Tract

3. The human stomach and most of the small intestine (duodenum and jejunum) contain few bacteria and lack anaerobes. In contrast, the distal small intestine (ileum) has a significant bacterial flora containing aerobes and anaerobes. The colon contains still far higher numbers of bacteria, the vast majority of which are anaerobes. Normally gut bacteria are not pathogenic (disease causing) and are referred to as commensals, with the normal bacterial flora in the gut being termed the �commensal flora�. The human stomach and most of the small intestine (duodenum and jejunum) contain few bacteria and lack anaerobes. In contrast, the distal small intestine (ileum) has a significant bacterial flora containing aerobes and anaerobes. The colon contains still far higher numbers of bacteria, the vast majority of which are anaerobes. Normally gut bacteria are not pathogenic (disease causing) and are referred to as commensals, with the normal bacterial flora in the gut being termed the �commensal flora�.

5. The bacteria in the colon vary from individual to individual and within a given individual tend to be relatively constant. This has led to the notion that if one knew the entire bacterial flora in a given individual, it would be almost as good a marker of that person as a fingerprint.The bacteria in the colon vary from individual to individual and within a given individual tend to be relatively constant. This has led to the notion that if one knew the entire bacterial flora in a given individual, it would be almost as good a marker of that person as a fingerprint.

6. A number of factors control the bacterial flora in the intestinal tract. Particularly important is the presence of acid in the stomach, innate immune factors (e.g., antimicrobial peptides and mediators), secretory immunoglobulin, and normal peristalsis and motility which keeps the small intestine relatively bacteria-free.A number of factors control the bacterial flora in the intestinal tract. Particularly important is the presence of acid in the stomach, innate immune factors (e.g., antimicrobial peptides and mediators), secretory immunoglobulin, and normal peristalsis and motility which keeps the small intestine relatively bacteria-free.

8. The presence of the commensal flora markedly affects susceptibility of the host to infection with enteropathogenic bacteria in the intestinal tract. The LD50 is the number of bacteria it takes to kill 50% of the mice. It takes far few bacteria when the bacteria are injected into the peritoneal cavity or placed in the intestine of germ free mice compared to mice with a normal commensal bacterial flora.The presence of the commensal flora markedly affects susceptibility of the host to infection with enteropathogenic bacteria in the intestinal tract. The LD50 is the number of bacteria it takes to kill 50% of the mice. It takes far few bacteria when the bacteria are injected into the peritoneal cavity or placed in the intestine of germ free mice compared to mice with a normal commensal bacterial flora.

9. Antibiotics change the bacterial flora, which can be associated with colitis. The colitis is caused by a bacteria (C. difficile) that can colonize the intestine whose flora has been altered by the antibiotic. C. difficile produce a toxin that causes the colitis and that can kill mice. This table shows that normal mice not treated with antibiotics lack this bacteria and survive normally. After antibiotic treatment, mice whose intestine is replenished with normal cecal (proximal colon) commensal bacteria have fewer C. difficile and mostly survive whereas those whose intestinal tract is replenished with killed bacteria or no bacteria (broth control) mostly die.Antibiotics change the bacterial flora, which can be associated with colitis. The colitis is caused by a bacteria (C. difficile) that can colonize the intestine whose flora has been altered by the antibiotic. C. difficile produce a toxin that causes the colitis and that can kill mice. This table shows that normal mice not treated with antibiotics lack this bacteria and survive normally. After antibiotic treatment, mice whose intestine is replenished with normal cecal (proximal colon) commensal bacteria have fewer C. difficile and mostly survive whereas those whose intestinal tract is replenished with killed bacteria or no bacteria (broth control) mostly die.

11. The commensal bacterial microflora produces enzymes that alter host produced molecules. For example, bacterial urease can convert urea in the intestine to ammonia.The commensal bacterial microflora produces enzymes that alter host produced molecules. For example, bacterial urease can convert urea in the intestine to ammonia.

12. Bacterial enzymes also can act on host polysaccharides, proteins, amino acids and fatty acids to generate various by products.Bacterial enzymes also can act on host polysaccharides, proteins, amino acids and fatty acids to generate various by products.

14. Short chain fatty acids which are a source of energy for colon epithelial cells, and intestinal gas are the major byproducts of the bacterial breakdown of undigested polysaccharides and cellulose that reach the colon.Short chain fatty acids which are a source of energy for colon epithelial cells, and intestinal gas are the major byproducts of the bacterial breakdown of undigested polysaccharides and cellulose that reach the colon.

16. When there is an increased bacterial microflora in the small intestine, the bacteria can lead to a number of disorders in the host. For example, bacteria can convert polysaccharides to gas leading to a feeling of distension and increased flatulence. Bacteria can also take up Vitamin B12 leading to a deficiency of that vitamin and an ensuing anemia.When there is an increased bacterial microflora in the small intestine, the bacteria can lead to a number of disorders in the host. For example, bacteria can convert polysaccharides to gas leading to a feeling of distension and increased flatulence. Bacteria can also take up Vitamin B12 leading to a deficiency of that vitamin and an ensuing anemia.

17. The majority of gas produced and expelled from the colon is nitrogen, with smaller amounts of hydrogen and carbon dioxide. The amount of methane produced is quite variable among different individuals. Oxygen is present at only low amounts and most of the gases associated with the odor of flatus are present in only trace amounts.The majority of gas produced and expelled from the colon is nitrogen, with smaller amounts of hydrogen and carbon dioxide. The amount of methane produced is quite variable among different individuals. Oxygen is present at only low amounts and most of the gases associated with the odor of flatus are present in only trace amounts.

18. Lactulose is not normally broken down in the small intestine and enters the colon where it is metabolized by colonic bacteria. Following an oral dose of lactulose, its breakdown in the colon into gas can be monitored by measuring breath or luminal hydrogen production.Lactulose is not normally broken down in the small intestine and enters the colon where it is metabolized by colonic bacteria. Following an oral dose of lactulose, its breakdown in the colon into gas can be monitored by measuring breath or luminal hydrogen production.