Bacterial overgrowth of the small intestine

Small intestinal bacterial overgrowth can be defined as the presence of excessive bacteria in the small intestine which can interfere with digestion and absorption. Predisposing causes include sustained hypochlorhydria induced by proton pump inhibitors, small intestinal dysmotility and stasis due to anatomical or motor abnormalities, and reduced antibacterial activity as seen in immunological deficiency and chronic pancreatitis. Presentation is predominantly from consequences of malabsorption, including gastrointestinal symptoms (e.g. diarrhoea or steatorrhoea) and features of specific nutrient malabsorption (e.g. osteoporosis, anaemia, neuropathy, and night blindness). Definitive diagnosis is difficult, requiring a properly collected and appropriately cultured aspirate from the proximal small intestine revealing a total concentration of a mixed growth of bacteria generally greater than 105 organisms/ml. Alternative investigations frequently used include glucose/lactulose breath tests or either the 13C- or 14C-xylose breath test, with elevated levels of 13CO2 or 14CO2 found in the breath. There may be low levels of cobalamin (metabolized by Gram-negative anaerobes), increased serum folate (synthesized by overgrowth flora), and increased urinary indicans (intraluminal product of bacterial tryptophan metabolism). Aside from treatment of any nutritional deficiencies, specific treatment is with an antimicrobial that is effective against both aerobic and anaerobic enteric bacteria (e.g. doxycycline, amoxicillin–clavulanic acid, rifaximin, or ciprofloxacin), which can be administered in rotation to reduce antibiotic resistance. Where possible and appropriate, correction of any underlying cause should also be performed.