Bacterial Overgrowth Syndrome

Small intestinal bacterial overgrowth (SIBO) is excessive proliferation of colonic-type bacteria within the small intestine, normally kept low by gastric acid, intestinal motility, and the ileocecal valve.

Predisposing factors include anatomical abnormalities (surgical blind loops, strictures, diverticula), motility disorders (diabetic autonomic neuropathy, scleroderma, opioid use), achlorhydria (PPI use, atrophic gastritis, prior gastric surgery), and immunodeficiency.

Bloating, abdominal distension and discomfort (often worse after meals), flatulence, diarrhea (from bacterial deconjugation of bile acids impairing fat absorption, causing steatorrhea in more significant cases), and, with chronic disease, malabsorption leading to weight loss and micronutrient deficiency, particularly vitamin B12 deficiency (paradoxically, folate can be normal or elevated, since colonic-type bacteria synthesize folate while consuming B12, a useful distinguishing biochemical pattern from simple dietary B12 deficiency).

• Hydrogen/methane breath testing (glucose or lactulose substrate) is the standard non-invasive test, an early rise in breath hydrogen or methane after substrate ingestion supports the diagnosis, though sensitivity and specificity are imperfect
• Small bowel aspirate and culture (quantitative bacterial count) is more invasive but more specific where available
• B12, folate, and fat-soluble vitamin levels assess for malabsorptive consequences
• Investigation for and correction of the underlying predisposing anatomical or motility abnormality

• Antibiotic treatment: rifaximin 550 mg PO tds for 14 days is preferred given minimal systemic absorption and good efficacy against the typically anaerobic/coliform overgrowth flora; alternatives include a 10 to 14 day course of a combination or single systemic agent (e.g. ciprofloxacin, co-amoxiclav, or metronidazole) per local practice, particularly if methane-predominant (associated with Methanobrevibacter, sometimes treated with rifaximin plus neomycin)
• Recurrence is common, particularly where the underlying predisposing factor (motility disorder, anatomical abnormality) cannot be corrected, and cyclical or recurrent antibiotic courses are often needed
• Address the underlying cause where feasible (surgical correction of a blind loop, optimizing diabetic control, reviewing chronic PPI use)
• Nutritional support and replacement of deficiencies (B12 IM as per standard protocol, fat-soluble vitamins as needed)
• Dietary modification (low FODMAP approaches) may reduce symptom burden as an adjunct, though does not treat the underlying overgrowth itself

Referral: gastroenterology for breath testing, recurrent disease, or where an underlying anatomical/motility cause requires further workup.