What is Small Intestine Bacterial Overgrowth (SIBO)?

Small Intestinal Bacterial Overgrowth (SIBO)

Small intestinal bacterial overgrowth (SIBO) is a condition characterized by excessive growth of bacteria in the small intestine, leading to malabsorption, bloating, abdominal pain, diarrhea, and other gastrointestinal symptoms that significantly impact quality of life. 1

Definition and Pathophysiology

SIBO occurs when the normal protective mechanisms that limit bacterial growth in the small intestine fail, resulting in:

• Bacterial counts exceeding 10^5 colony-forming units per milliliter in the small intestine 1
• Proliferation of colonic-type bacteria in the small bowel 2
• Disruption of the migrating myoelectric complex (MMC), which normally clears debris from the small bowel 2

Several endogenous mechanisms normally prevent bacterial overgrowth:

• Gastric acid secretion
• Intestinal motility
• Intact ileocecal valve
• Intestinal immunoglobulin secretion
• Bacteriostatic properties of pancreatic and biliary secretions 2

Risk Factors

Patients at increased risk for SIBO include those with:

• Intestinal dysmotility disorders 2
• Prior surgical history (especially those affecting the ileocecal valve) 1
• Stricturing or fistulizing Crohn's disease 1
• Pancreatic exocrine insufficiency 1
• Diabetes with associated gut dysmotility 1
• Heavy smokers 1
• Prolonged use of medications that affect motility (anticholinergics, opioids, PPIs) 1

Clinical Manifestations

SIBO presents with various symptoms including:

• Bloating and abdominal distension
• Abdominal pain
• Diarrhea or constipation
• Flatulence (often foul-smelling)
• Malabsorption leading to:
  • Steatorrhea
  • Weight loss
  • Fat-soluble vitamin deficiencies (A, D, E)
  • Vitamin B12 deficiency
  • Protein-losing enteropathy 2, 1

Unique metabolic consequences can include:

• D-lactic acidosis (high anion gap acidosis)
• Elevated ammonia levels 2
• Subtotal villous atrophy on intestinal biopsy 2

Diagnosis

There is currently no gold standard for diagnosing SIBO 2. Diagnostic approaches include:

1. Breath Testing:

   • Hydrogen and methane-based breath tests using glucose or lactulose
   • Limitations include variable sensitivity and specificity 1
   • Caution with lactulose hydrogen breath test as it may measure transit time rather than SIBO 2

2. Small Bowel Aspirates:

   • Culture of jejunal aspirates (≥10^5 CFU/ml is diagnostic)
   • Considered more definitive but invasive 1, 3

3. Clinical Diagnosis:

   • Symptoms worsening after carbohydrate-rich meals
   • Temporary improvement with antibiotics
   • Chronic symptoms resistant to conventional treatments 1

4. Empiric Treatment:

   • Consider in high-risk patients with chronic watery diarrhea, malnutrition, or weight loss 1

Management

1. Antibiotic Therapy

• First-line: Rifaximin 550 mg three times daily for 14 days 1
• Alternative options:
  • Metronidazole
  • Ciprofloxacin
  • Amoxicillin-clavulanic acid
  • Cephalosporins
  • Tetracyclines 1
• Rotation strategy: Consider rotating antibiotics in 2-6 week cycles to prevent resistance 1

2. Dietary Management

• Low-FODMAP diet to reduce fermentable carbohydrates 1
• Avoid gas-producing foods
• Consume polyphenol-rich foods to support healthy microbiome
• Practical tips:
  • Eat slowly and chew thoroughly
  • Smaller, more frequent meals
  • Separate liquids from solids (drink 15 minutes before or 30 minutes after meals) 1

3. Adjunctive Therapies

• Probiotics: Specific strains may help restore microbial balance:

  • Weizmannia coagulans
  • Alkalihalobacillus clausii
  • Lacticaseibacillus rhamnosus
  • Limosilactobacillus reuteri
  • Saccharomyces boulardii 1

• Prokinetics: Improve intestinal motility and prevent SIBO recurrence

  • Prucalopride
  • Metoclopramide
  • Can reduce recurrence rates by 30-50% 1

4. Address Underlying Causes

• Treat primary conditions causing dysmotility
• Avoid medications that worsen gastric motility (anticholinergics, opioids, long-term PPIs) 1

Monitoring and Follow-up

• Monitor nutritional status
• Assess for micronutrient deficiencies (iron, B12, fat-soluble vitamins)
• Consider DEXA scanning in malnourished patients 1
• Establish regular toileting schedule
• Ensure adequate fluid, bulk, and fiber intake 1

Clinical Pitfalls to Avoid

1. Misdiagnosis: SIBO symptoms overlap with many functional GI disorders, particularly IBS 2
2. Overreliance on breath testing: Breath tests have variable sensitivity and specificity 2
3. Inadequate treatment duration: Short courses of antibiotics may lead to recurrence
4. Failure to address underlying causes: Treating symptoms without addressing the root cause leads to chronic recurrence 1
5. Neglecting nutritional deficiencies: Monitor and supplement vitamins and minerals as needed 2