What Causes Small Intestinal Bacterial Overgrowth (SIBO)
SIBO develops when protective mechanisms that normally prevent bacterial colonization of the small intestine fail, allowing colonic-type bacteria to proliferate in the proximal small bowel. 1, 2
Primary Mechanisms Leading to SIBO
Gastric Acid Suppression
- Proton pump inhibitors (PPIs) are a well-established and common cause of SIBO because gastric acid secretion is one of the key endogenous mechanisms preventing bacterial overgrowth 1
- Even one month of omeprazole therapy is sufficient to reduce gastric acid enough to allow bacterial proliferation in the small intestine 1
- The American Gastroenterological Association recommends considering the risk of SIBO when prescribing PPIs 1
- Some studies suggest that PPI use might lead to symptomatic SIBO or at least increased numbers of bacteria, and following antibiotics they accelerate recurrence 3
Impaired Gut Motility
- Impaired migrating myoelectric complex (MMC) prevents clearance of intestinal debris, creating gut stasis that allows anaerobic bacteria to proliferate in stagnant bowel loops 2
- Altered motility has been implicated as a confounding factor in SIBO development 3
- However, gut motility typically normalizes within 2-4 weeks after acute viral gastroenteritis, making prolonged dysmotility from remote infections unlikely 1
Anatomic and Structural Abnormalities
- Bowel obstruction and diverticula create stagnant loops where bacteria can accumulate 4
- Structural alterations from prior surgery, particularly in Crohn's disease patients, are major risk factors 2
- SIBO is present in one-third of patients with Crohn's disease, with prior surgery and fibrostenosing disease as major risk factors 2
Impaired Immune Function
- Immunosuppression allows bacterial overgrowth to occur unchecked 4
- Impairment of systemic and local immunity contributes to SIBO development 5
Disease-Specific Risk Factors
Chronic Pancreatitis
- Up to 92% of patients with chronic pancreatitis and exocrine insufficiency develop SIBO due to reduced bacteriostatic pancreatic secretions 2
Prior Antibiotic Use
- A history of antibiotic use can predispose to bacterial overgrowth by disrupting normal gut microbiota 1
- Prolonged antibiotic use is linked to gut microbiome dysbiosis that can facilitate SIBO 4
Inflammatory Bowel Disease
- SIBO is present in one-third of Crohn's disease patients, particularly those with prior surgery and fibrostenosing disease 2
Important Clinical Context for Recurrent SIBO
Why SIBO Recurs After Rifaximin Treatment
- SIBO can recur in up to 14% of patients without surgical history and more frequently in those with pancreatic exocrine insufficiency and diabetes 1
- Recurrence typically occurs because the underlying predisposing factor (such as PPI use, impaired motility, or anatomic abnormality) remains unaddressed 1
- For patients with recurrent SIBO, rotating antibiotics with 1-2 week periods without antibiotics before repeating can be effective 6, 1
- Addressing underlying causes such as impaired gut motility is crucial for preventing recurrence 6
Critical Pitfall to Avoid
- Do not restart PPIs after SIBO treatment unless absolutely necessary; consider H2-blockers as alternatives if acid suppression is required 1
- The European Society of Gastrointestinal Motility recommends discontinuing omeprazole immediately to remove the SIBO predisposing factor 1
- Do not assume persistent motility dysfunction from a remote viral illness when acid suppression is the more likely culprit 1
Pathophysiologic Consequences
- Bacterial fermentation of unabsorbed carbohydrates produces hydrogen and methane gas, causing bloating and flatulence 2
- Bacterial overgrowth deconjugates bile salts and degrades pancreatic enzymes, preventing normal fat digestion and absorption, leading to steatorrhea 2
- Bacterial consumption of vitamin B12 and bile salt deconjugation causes B12 malabsorption 1