Gut microbiota is the largest organ of the human body. Although growth of bacteria more than 10 5 colony forming unit (CFU) per milliliter in culture of upper gut aspirate is used to diagnosis small intestinal bacterial overgrowth (SIBO), 10 3 CFU or more is being considered to suggest the diagnosis, particularly if colonic type bacteria are present in the upper gut. Although neither very sensitive nor specific, hydrogen breath tests are widely used to diagnose SIBO. Rifaximin is the best treatment for SIBO due to its broad spectrum, lack of systemic absorption, and safety profile.
Key points
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The role of small intestinal bacterial overgrowth (SIBO) in various intestinal disorders, such as irritable bowel syndrome, celiac and Crohn disease, tropical sprue, gastrointestinal hypomotility states, hypochlorhydria, immunodeficiency states, and structural abnormalities, is increasingly understood.
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Though a bacterial count equal to or greater than 10 5 colony forming unit (CFU)/mL is considered diagnostic of SIBO, some experts suggest that a colony count equal to or greater than 10 3 CFU/mL should also be considered SIBO, particularly if coliforms are present in the upper gut.
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Hydrogen breath tests are popular noninvasive tests using different substrates such as glucose (though highly specific but quite insensitive) and lactulose (early-peak criteria is quite nonspecific and double-peak criteria is quite insensitive) to diagnose SIBO.
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The search for better diagnostic test for SIBO continues.
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Rifaximin is a quite safe and effective agent for treatment and retreatment of SIBO.
Introduction
The gastrointestinal (GI) tract harbors an enormous number of microbes, including bacteria, virus, fungus, and archaea. Interestingly, the microbial cells in the gut (10 14 cells) outnumber that in the human body (10 13 ). Upper gut (stomach, duodenum, and jejunum) is relatively less populated with microbes (0–10 3 bacteria per mL of aspirate) due to the gastric acid barrier ( Fig. 1 ). The microbes in the upper gut are dominated by those that can resist gastric acid, such as Helicobacter pylori and lactobacilli. The distal small bowel and colon have more microbes (10 7 to 10 12 ). Coliforms (Gram-negative nonspore-forming bacilli that ferment lactose) are present exclusively in the distal gut. Gut microbiota performs a variety of functions, such as barrier effect preventing invasion by the pathogenic organisms; regulation of immune function and motility; digestion of food; production of short-chain fatty acids, which regulate colonic water and electrolytes transport and are nutrient to the colonocytes; production of vitamins; metabolism of the drugs; detoxification of toxic compounds; and maintenance of overall homeostasis in the GI tract. Hence, maintenance of normal gut flora (eubiosis) is essential for keeping an individual healthy. Abnormality in the gut flora (dysbiosis), including quantitative increase in small bowel bacteria (ie, small intestinal bacterial overgrowth [SIBO]), qualitative alteration in relative proportion of friendly bacteria and harmful ones, or change in location of bacteria (eg, colonic type bacteria in the small intestine), is known to be associated with several diseases. Definition, etiologic factors, symptomatology, pathophysiology, diagnosis, and treatment of SIBO in relation to various intestinal disorders associated with it are reviewed in this article.
