What is Small Intestine Bacterial Overgrowth (SIBO)?

Small Intestinal Bacterial Overgrowth (SIBO)

Small Intestinal Bacterial Overgrowth (SIBO) is a condition characterized by excessive bacterial load in the small bowel, resulting in excessive fermentation, inflammation, and adverse clinical symptoms including malabsorption, abdominal discomfort, diarrhea, constipation, flatulence, and bloating. 1

Definition and Pathophysiology

SIBO occurs when there is:

• Increased bacterial concentration in the small intestine (traditionally defined as >10^5 colony-forming units/mL in jejunal aspirates) 2
• Altered composition of small intestinal microbiota
• Disruption of normal gut homeostasis

The condition develops due to failure of normal protective mechanisms:

• Impaired migrating motor complex (MMC) function
• Reduced gastric acid secretion
• Compromised intestinal immunity
• Loss of ileocecal valve function
• Structural abnormalities creating stasis

Clinical Manifestations

SIBO presents with various symptoms that can significantly impact quality of life:

• Gastrointestinal symptoms:

  • Bloating and abdominal distension
  • Abdominal pain (often postprandial)
  • Diarrhea or constipation
  • Excessive flatulence (often foul-smelling)
  • Nausea and vomiting (sometimes feculent)

• Malabsorption-related issues:

  • Steatorrhea (fat malabsorption)
  • Weight loss and malnutrition
  • Vitamin deficiencies (particularly fat-soluble vitamins A, D, E)
  • Vitamin B12 deficiency
  • Protein-losing enteropathy 1

Risk Factors

SIBO is associated with numerous conditions that disrupt normal gut physiology:

• Structural abnormalities:

  • Prior abdominal surgery
  • Strictures or adhesions
  • Diverticula
  • Loss of ileocecal valve

• Motility disorders:

  • Chronic intestinal pseudo-obstruction
  • Scleroderma
  • Diabetic enteropathy

• Other conditions:

  • Chronic pancreatitis (occurs in up to 92% of patients with pancreatic exocrine insufficiency) 1, 3
  • Inflammatory bowel disease (up to 30% in Crohn's disease) 1
  • Celiac disease
  • Diabetes mellitus
  • Heavy smoking 1

Diagnostic Approaches

The diagnosis of SIBO remains challenging due to lack of standardization:

• Small bowel aspirate culture (traditional gold standard):

  • Qualitative assessment via upper GI endoscopy
  • Positive when bacterial count exceeds 10^5 CFU/mL 3, 2

• Breath testing:

  • Combined hydrogen and methane breath testing is more accurate than hydrogen testing alone
  • Options include glucose or lactulose breath tests
  • Limitations include variations in performance and interpretation 1, 3

• Clinical diagnosis:

  • Based on symptoms that worsen after carbohydrate-rich meals
  • Temporary improvement with antibiotics
  • Chronic symptoms resistant to conventional treatments 3

• Empiric treatment:

  • May be considered in high-risk patients with chronic watery diarrhea, malnutrition, weight loss, or underlying conditions causing small bowel dysmotility 3

Treatment Strategies

Antibiotic Therapy

• First-line treatment: Rifaximin 550 mg twice daily for 1-2 weeks (60-80% effectiveness in proven SIBO) 3
• Alternative antibiotics:
  • Doxycycline
  • Ciprofloxacin
  • Amoxicillin-clavulanic acid
  • Cefoxitin
  • Metronidazole (less effective) 3, 4

Dietary Modifications

• Low-FODMAP diet to reduce fermentable carbohydrates
• Avoiding gas-producing foods
• Consuming polyphenol-rich foods to support healthy microbiome 3

Prokinetics and Motility Agents

• May help prevent recurrence by improving intestinal motility
• Options include prucalopride, erythromycin, and tegaserod

Probiotics and Prebiotics

• May help restore normal gut flora
• Options include Weizmannia coagulans, Alkalihalobacillus clausii, Lacticaseibacillus rhamnosus, Limosilactobacillus reuteri, and Saccharomyces boulardii 3, 5

Prevention of Recurrence

SIBO frequently recurs after successful treatment. Strategies to prevent recurrence include:

• Identifying and modifying predisposing factors
• Reducing or discontinuing medications that worsen motility (anticholinergics, opioids, long-term PPIs)
• Managing underlying conditions causing dysmotility 3
• Considering cyclical antibiotic therapy in high-risk patients

Special Considerations

• In patients with motility disorders, dilated segments of small bowel, or blind loops, occasional antibiotic treatment may be beneficial 3
• SIBO can significantly impact quality of life and lead to serious nutritional deficiencies if left untreated
• The condition often coexists with other gastrointestinal disorders, complicating diagnosis and management

SIBO represents a significant dysbiosis in gut microbiota that alters gut neuromotor-sensory function and barrier function, potentially influencing systemic immune responses through the brain-gut axis 3. Early recognition and appropriate management are essential to prevent complications and improve quality of life.