Small intestinal bacterial overgrowth

Pathophysiology - Gut Garden Gone Wild

• A breakdown in host defenses allows excessive bacterial proliferation (esp. anaerobes, coliforms) in the small bowel, an area that should normally have a low bacterial count.
• Primary Mechanisms:
  • ↓ Gastric Acid: Achlorhydria, often from chronic PPI use.
  • ↓ Intestinal Motility: Impaired peristaltic "housekeeping" waves (e.g., diabetes, scleroderma, opioids).
  • Anatomic Stasis: Surgical blind loops, strictures, or fistulas that create stagnant areas.

⭐ Chronic use of Proton Pump Inhibitors (PPIs) is a major iatrogenic risk factor by suppressing the natural gastric acid barrier against bacterial colonization.

Presentation & Risks - The Usual Suspects

• Clinical Picture: Chronic, non-specific GI symptoms.
  • Bloating, excessive flatulence, abdominal pain/cramping.
  • Watery diarrhea; may have steatorrhea (foul-smelling, floating stools).
  • Unintentional weight loss & signs of malabsorption.
• Key Risk Factors:
  • ↓ Motility: Diabetic gastroparesis, scleroderma, intestinal pseudo-obstruction.
  • Anatomic: Surgical blind loops (Roux-en-Y), strictures (Crohn's), fistulae, small intestinal diverticula.
  • ↓ Acid: Chronic PPI therapy.
  • Immune: IgA deficiency, CVID.

⭐ SIBO paradox: Bacteria consume Vitamin B12 leading to deficiency (megaloblastic anemia, neuropathy), but produce folate and Vitamin K, so levels may be normal or high.

Diagnosis - Breath & Beyond

• Gold Standard: Endoscopy with jejunal aspirate culture showing > 10³ CFU/mL (some use 10⁵ CFU/mL). Invasive, rarely the first step.
• Non-Invasive Testing (Preferred): Carbohydrate breath tests.
  • Glucose Breath Test (GBT): Detects proximal SIBO. Easier to interpret.
  • Lactulose Breath Test (LBT): Can detect more distal SIBO, but higher false-positive rate due to normal colonic fermentation.

⭐ False Positives: A rapid small bowel transit time can cause the lactulose substrate to reach the colon prematurely, leading to a false-positive result that mimics SIBO.

Management - Weeding the Overgrowth

Primary goal: Eradicate bacteria, manage underlying causes, and correct nutritional deficits.

• Antibiotic Therapy (“Weeding”):
  • Rifaximin: First-line agent. Dose: 550 mg TID for 14 days. Non-absorbable, acts locally.
  • Alternatives: For methane-predominant SIBO or rifaximin failure.
    • Neomycin (often with Rifaximin).
    • Metronidazole.
• Address Root Cause:
  • Treat motility disorders (e.g., gastroparesis).
  • Correct anatomical issues (e.g., surgical strictures).
  • Review medications (e.g., PPIs, opioids).
• Nutritional Support:
  • Supplement Vitamin B12, fat-soluble vitamins (A, D, E), and iron.
  • Low FODMAP diet for symptom control.

⭐ Recurrence is common. If symptoms return, consider cyclic antibiotic therapy or using prokinetics (e.g., low-dose erythromycin) to stimulate gut motility and prevent relapse.

High‑Yield Points - ⚡ Biggest Takeaways

• SIBO results from excessive bacteria in the small bowel, typically due to stasis from impaired motility (e.g., diabetes, scleroderma) or anatomic alterations.
• Key symptoms include bloating, flatulence, watery diarrhea, and abdominal pain; severe cases cause malabsorption and steatorrhea.
• Gold standard for diagnosis is a jejunal aspirate culture showing >10^5 CFU/mL, but carbohydrate breath tests (glucose, lactulose) are more common.
• Strongly associated with long-term PPI use, diabetes, and Roux-en-Y gastric bypass.
• Treatment is primarily with antibiotics like rifaximin or neomycin.