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Small intestinal bacterial overgrowth: Clinical manifestations and diagnosis

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


Small intestinal bacterial overgrowth (SIBO) is a condition in which the small bowel is colonized by excessive aerobic and anaerobic microbes that are normally present in the colon.

This topic will review the etiology, pathogenesis, clinical manifestations, and diagnosis of SIBO. The management of SIBO is discussed in detail, separately. (See "Small intestinal bacterial overgrowth: Etiology and pathogenesis" and "Small intestinal bacterial overgrowth: Management".)


The population-based prevalence of SIBO is unclear. The incidence of SIBO increases with age.


Several disorders predispose to SIBO by altering mucosal defenses (table 1). Intestinal motility disorders and chronic pancreatitis are estimated to account for approximately 90 percent of cases of SIBO [1-3]. Patients may have more than one predisposing cause. As an example, the high prevalence of SIBO in older adults may be due to small intestinal dysmotility and gastric hypochlorhydria.

Motility disorders — The migrating motor complex (MMC) and in particular, phase III of the MMC clears the small bowel of debris [4]. A lack of interdigestive phase III activity in patients with irritable bowel syndrome, narcotic use, intestinal pseudo-obstruction, and diabetes predisposes them to SIBO. Small bowel motility is also affected in acute or chronic radiation enteritis, amyloidosis, and scleroderma [5-9]. (See "Pathophysiology of irritable bowel syndrome", section on 'Bacterial overgrowth' and "Gastrointestinal manifestations of systemic sclerosis (scleroderma)", section on 'Small intestinal involvement'.)

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Literature review current through: Nov 2017. | This topic last updated: Jul 25, 2017.
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