Bacterial overgrowth

The gut contains many resident bacteria with anaerobic bacteria, e.g. Bacteroides, bifidobacteria, being 100-1000 times more abundant than aerobic (facultative anaerobes), e.g. Escherichia, Enterobacter, Enterococcus. This gut microflora has major functions: -metabolic, e.g. fermentation of non-digestible dietary residues into short chain fatty acids as an energy source in the colon. -initiate vitamin K production. -control epithelial cell proliferation -involved in the development and maintenance of the immune system. -protect the gut mucosa from colonization by pathogenic bacteria.

The upper part of the small intestine is almost sterile, containing only a few organisms derived fromthe mouth. Gastric acid kills most organisms and intestinal motility keeps the jejunum empty. The normal terminal ileum contains faecal-type organisms, mainly Escherichia coli and anaerobes and the colon has abundant bacteria

Bacterial overgrowth is normally only found associated with a structural abnormality of the small intestine, although it can occur occasionally in the elderly without such abnormality. E. coli and/or Bacteroides, both in concentrations greater than 106/mL are found as part of a mixed flora. These bacteria are capable of deconjugating and dehydroxylating bile salts, so that unconjugated and dehydroxylated bile salts can be detected in aspirates by chromatography. The clinical features are chiefly diarrhoea and steatorrhoea. Steatorrhoea occurs as a result of conjugated bile salt deficiency.

The bacteria are able to metabolize vitamin B12 and interfere with its binding to intrinsic factor, thereby leading to B12 deficiency. Conversely some bacteria produce folic acid giving a high serum folate.

Bacterial overgrowth has only minimal effects on other substances absorbed from the small intestine. The vitamin B12 deficiency is not so severe as to produce a neurological deficit. Confirmation of bacterial overgrowth is with the hydrogen breath test; aspiration studies are not routinely performed.

Although bacterial overgrowth may be responsible for the presenting symptoms, it must be remembered that many of the symptoms may be due to the underlying small bowel pathology.

Treatment

If possible, the underlying lesion should be corrected (e.g. a stricture should be resected). With multiple diverticula, grossly dilated bowel, or in Crohn's disease, this may not be possible and rotating courses of antibiotics are necessary, such as metronidazole, a tetracycline, or ciprofloxacin.