Small Intestinal Bacterial Overgrowth

Small intestinal bacterial overgrowth (SIBO) is defined as excessive bacterial colonization of the small intestine (≥10^5 CFU/mL on jejunal aspirate culture, or positive breath test). Normally, the small intestine has relatively few bacteria (<10^4 CFU/mL) maintained by: gastric acid (kills ingested bacteria), intestinal motility (migrating motor complex clears bacteria between meals), ileocecal valve (prevents colonic bacterial reflux), mucosal immunity (IgA secretion), bile acids (bacteriostatic), and pancreatic enzyme secretion. SIBO develops when these protective mechanisms fail. Risk factors include: achlorhydria/PPI use (reduced gastric acid barrier), intestinal dysmotility (diabetes, scleroderma, opioids, hypothyroidism — impaired MMC), anatomic abnormalities (small bowel diverticula, strictures, blind loops from surgery, absent ileocecal valve), and immunodeficiency (IgA deficiency, HIV). The overgrown bacteria deconjugate bile acids (causing fat malabsorption and steatorrhea), consume vitamin B12 (causing deficiency), produce folate (levels may be elevated), ferment carbohydrates (producing hydrogen and methane gas causing bloating, distension, and flatulence), and damage the small bowel mucosa (causing villous blunting and increased permeability). Methane-predominant SIBO is associated with constipation rather than diarrhea.