What is SIBO (Small Intestinal Bacterial Overgrowth)?
SIBO is a pathological condition characterized by an abnormal increase in bacterial population in the small intestine, which normally maintains relatively low bacterial counts, leading to symptoms including bloating, abdominal pain, gas, diarrhea, and malabsorption. 1, 2
Definition and Core Pathophysiology
SIBO represents excessive bacterial proliferation specifically in the small intestine, distinct from gastric bacterial overgrowth (which occurs when acid barriers are compromised) and colonic dysbiosis (which involves qualitative changes in bacterial composition rather than quantitative overgrowth). 1
The condition develops when protective mechanisms fail, including disruption of the migrating motor complex (MMC), reduced gastric acid secretion, incompetent ileocecal valve, impaired pancreatic/biliary secretions, or defective intestinal immunity. 3, 1
Bacterial overgrowth deconjugates bile salts and degrades pancreatic enzymes, causing malabsorption of fat-soluble vitamins (A, D, E, K), vitamin B12, and other nutrients. 4
Clinical Manifestations
Primary gastrointestinal symptoms include bloating, abdominal distension, pain, gas, and diarrhea. 5, 2
Systemic manifestations result from malabsorption and bacterial metabolite production, including profound fatigue, muscle weakness and pain, cognitive dysfunction, and weight loss exceeding 10% of body weight in severe cases. 4
Bacterial production of toxic metabolites such as D-lactic acid and ammonia contributes to systemic symptoms including fatigue, malaise, and high anion gap acidosis. 4
Prolonged untreated SIBO causes cumulative damage with demonstrable physiological changes including skeletal and cardiac muscle weakness, poor concentration, prolonged sleeping, and low body temperature. 4
Common Underlying Causes
Motility Disorders:
- Neuropathies affecting the MMC cause non-propulsive contractions and gut stasis, creating ideal conditions for bacterial proliferation. 3
- Diabetes with autonomic neuropathy and chronic intestinal pseudo-obstruction are significant causes. 3
Anatomical Alterations:
- Resection of the ileocecal valve allows reflux of colonic bacteria into the small intestine. 3
- Prior surgeries altering normal intestinal anatomy predispose to SIBO. 3
Pharmacological Causes:
- Prolonged proton pump inhibitor (PPI) use reduces the gastric acid barrier. 6, 3
- Medications affecting motility including vincristine, anticolinergics, and clozapine. 3
Other Conditions:
- Pancreatic insufficiency reduces bacteriostatic pancreatic secretions. 3
- Radiation therapy damages intestinal motility. 6, 3
- SIBO complicates up to 92% of chronic pancreatitis cases with exocrine insufficiency. 3
Diagnostic Approach
Testing is recommended over empirical treatment to establish diagnosis, aid antibiotic stewardship, and identify coexisting disorders common in this patient population. 6
Hydrogen combined with methane breath testing is more effective than hydrogen testing alone for identifying SIBO. 6
Glucose or lactulose breath tests are helpful when clearly positive, though not always accurate. 6
Quantitative small bowel aspiration via endoscopy is time-consuming but can confirm diagnosis; qualitative assessment is easier and requires prior agreement with microbiology services on processing and reporting. 6
Critical pitfall: Breath testing abnormalities often reflect rapid small intestinal transit with colonic fermentation rather than true small intestinal overgrowth, as demonstrated when radiotracer reaches the cecum before hydrogen levels meet abnormal thresholds. 6, 1
Treatment Approach
First-Line Antibiotic Therapy:
- Rifaximin 550 mg twice daily for 1-2 weeks is the most investigated and effective treatment, with 60-80% efficacy in proven SIBO. 6, 4
- Alternative equally effective antibiotics include doxycycline, ciprofloxacin, amoxicillin-clavulanic acid, and cefoxitin. 6
- Metronidazole is less effective. 6
- Methane-producing organisms are particularly difficult to eradicate and often require more aggressive or combination therapy. 4
Management Strategy Based on Underlying Cause:
- Reversible causes (e.g., immunosuppression during chemotherapy) typically require only one antibiotic course. 6
- Recurrent SIBO requires low-dose long-term antibiotics, cyclical antibiotics, or recurrent short courses. 6
- Non-absorbed antibiotics are preferable to reduce systemic resistance risk. 6
Adjunctive Measures:
- Monitor and replace fat-soluble vitamins (A, D, E, K) and vitamin B12, as deficiencies directly cause muscle weakness, pain, and neurological symptoms. 4
- Address underlying causes including motility disorders, anatomical issues, and medication adjustments (particularly PPIs). 3, 1
- Dietary modifications including low FODMAP diets and elemental diets can complement antibiotic therapy. 2, 7
Important Clinical Considerations
SIBO etiology is frequently multifactorial with more than one mechanism involved. 3
In patients with severe chronic dismotility, SIBO is practically inevitable and can cause cachexia even without evident diarrhea. 3
The condition significantly impacts quality of life, and comprehensive treatment combining pharmacological therapy, dietary intervention, and strategies targeting gut microbiota produces sustained improvement. 8
Long-term follow-up is essential due to high recurrence rates. 2