Researchers have estimated that the human body contains 10 to the power of 14 cells, only 10% of which are not bacteria and belong to the human body proper. The mammalian intestinal tract represents a complex, dynamic and diverse ecosystem of interacting aerobic and anaerobic, nonpathologic bacteria. This complex yet stable colony includes more than 400 separate species.

Bacterial Overgrowth

Bacterial overgrowth is the term used when there are excessive amounts of bacteria inhabiting the small intestine. Those disorders that alter small bowel motility appear to predispose individuals to the greatest extent. These include small bowel diverticula, surgically created blind loops, strictures, pseudo-obstruction, scleroderma, diabetic neuropathy, resection of small bowel including the ileocecal valve, cirrhosis, malnutrition, and abdominal radiation. Bacteriologic analysis of the microflora includes aerobic and anaerobic bacteria. Bacterial concentrations can range from 10 to the power of 7-9 (and rarely to 11) cfu/mL.

Additional host factors that allow for bacterial overgrowth include defective gastric acid secretion and defective local immunity. The use of acid blockade significantly affects the mean gastric bacterial count, such that as the pH rises above 4, the bacterial count increased from 0 to 10 to the power of 6.4 and the mean number of bacterial species increased from 0.5 to 4.3.

Clinical manifestations of bacterial overgrowth include diarrhea, steatorrhea, vitamin B12 deficiency, protein malnutrition, weight loss, and impaired sugar absorption.There is also evidence that functional disorders such as irritable bowel syndrome may be caused by bacterial overgrowth.These effects are mediated via increased deconjugation of bile salts, volatile fatty acids, alcohols, volatile amines, and hydroxyl fatty acids. These products can result in increasing intraluminal osmolarity and subsequent diarrhea. Malabsorption appears more common when colonization includes anaerobes. Some speculate it is the deconjugation of bile acids, specifically by Bacteroides strains, that favors the growth of anaerobes. B12 deficiency is thought to be due to uptake of the vitamin by the bacteria; indeed, ingested B12 in these patients is found in the feces bound to bacterial cell wall components. Amino acid absorption is also impaired in overgrowth, with increased fecal nitrogen.22d-Lactic acidosis has also been linked to bacterial overgrowth and the inability of humans to rapidly metabolize d-lactate.