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Small intestinal bacterial overgrowth: Etiology and pathogenesis

Mark Pimentel, MD, FRCP(C)
Section Editor
J Thomas Lamont, MD
Deputy Editor
Shilpa Grover, MD, MPH


Small intestinal bacterial overgrowth (SIBO) is a condition in which colonic bacteria are seen in excess in the small intestine. When present, this overabundance of organisms can result in intestinal symptoms and, in extreme cases, malabsorption.

This topic will review the etiology and pathogenesis of SIBO. The clinical manifestations, diagnosis, and treatment of SIBO are discussed separately. (See "Clinical manifestations and diagnosis of small intestinal bacterial overgrowth" and "Treatment of small intestinal bacterial overgrowth".)


Overview — The stomach and proximal small bowel normally contain relatively few bacteria due to the presence of gastric acidity and the effects of peristalsis. Lactobacilli, enterococci, gram-positive aerobes, or facultative anaerobes predominate in concentrations of 104 organisms/mL in the mid to distal jejunum [1]. The concentration of coliforms rarely exceeds 103 organisms/mL [2]. Bacteroides, the predominant organism in the colon, is rarely found in the proximal small bowel.

The microbiology of the terminal ileum represents an intermediary zone between the aerobic flora found in the stomach and proximal small bowel and the dense population of anaerobic organisms found in the colon. The concentration of organisms may be as high as 109/mL and includes enterobacteria and other coliforms. If the ileocecal valve is dysfunctional or surgically absent, the microbiology of the terminal ileum resembles that of the colon. (See "Spatial organization of intestinal microbiota in health and disease", section on 'Stomach and duodenum'.)

In the colon, the concentration of microorganisms may exceed 1012/mL. The organisms are predominantly anaerobes such as Bacteroides, Lactobacillus, Clostridium, and bifidobacteria, although multiple species coexist. (See "Spatial organization of intestinal microbiota in health and disease", section on 'Bacteria in the colon'.)


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Literature review current through: Sep 2016. | This topic last updated: Feb 16, 2016.
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