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Basic Information ⬇

AUTHOR: Fred F. Ferri, MD

Definition

Small bowel bacterial overgrowth (SIBO) is the presence of excessive native and/or nonnative bacteria in the small intestine (bacterial count >105/ml per jejunal aspirate) causing chronic diarrhea and malabsorption.

Synonyms

Bacterial overgrowth syndrome

SIBO

ICD-10CM CODES
K90.4Malabsorption due to intolerance, NEC
K90.89Other intestinal malabsorption
Epidemiology & Demographics
Prevalence

The prevalence of SIBO is varied based on the population studied and the diagnostic tests used. It has shown to be prevalent in up to 12.5% to 20% of the healthy population using glucose and lactulose breath test.

Predominant Sex & Age

SIBO affects predominantly the elderly population or those with recent upper GI (UGI) surgery, including bariatric surgery. The elderly population has decreased gastric secretion and hypomotility due to age-associated decline as well as increased use of motility-altering medications.

Risk Factors

  • Advanced age is a known risk factor as there is thought to be an age-associated decline in GI motility.
  • Patients with irritable bowel syndrome have a higher prevalence of SIBO compared to the general population. Initial studies have shown up to 65% to 80% of irritable bowel syndrome (IBS) patients with confirmed SIBO with an abnormal lactulose breath test.
  • Other risk factors include UGI tract surgery, inflammatory bowel disease, chronic pancreatitis, immunodeficiency, liver disease, and obesity.
Physical Findings & Clinical Presentation

  • Patients will present with nonspecific findings, which include abdominal distention, bloating, and/or pain. Other common symptoms include diarrhea and subsequent weight loss and weakness. Pathophysiology of symptoms and clinical consequences of SIBO are summarized in Table 1.
  • The severity of symptoms reflects the extent of bacterial overgrowth.
  • Severe malabsorption can present as symptoms secondary to vitamin deficiencies. Fat-soluble vitamin deficiencies can present as night blindness (vitamin A), osteomalacia and hypocalcemia (vitamin D), or prolonged bleeding (vitamin K). Bacterial overgrowth can affect vitamin B12 absorption in the ileum, leading to neuropathies with sensory ataxia.

TABLE 1 Pathophysiology of Symptoms and Clinical Consequences in Small Bowel Bacterial Overgrowth

ProcessMechanisms of ActionClinical Consequences
Mucosal injury induced by bacteria and/or their toxins or productsLoss of brush-border enzymesCarbohydrate maldigestion
Injury to epithelial barrier leading to enhanced intestinal permeabilityProtein-losing enteropathy; bacterial translocation and portal and systemic endotoxemia
Inflammatory response generating inflammatory cytokinesLiver injury and inflammation, systemic inflammatory responses
Luminal competition with the host for nutrientsConsumption of dietary proteinHypoproteinemia, edema
Consumption of vitamin B12B12 deficiency, megaloblastic anemia, neurologic symptoms
Consumption of thiamineThiamine deficiency
Consumption of nicotinamideNicotinamide deficiency
Bacterial metabolismFermentation of unabsorbed carbohydratesBloating, distension, flatulence
Diarrhea due to the effects of deconjugated bile acids in the colon; depletion of the bile acid pool leading to fat and fat-soluble vitamin malabsorption
Deconjugation of primary bile acids
Synthesis of vitamin KInterference with dosing of vitamin K antagonists (e.g., warfarin)
Synthesis of folateHigh serum folate levels
Synthesis of D-lactic acidD-lactic acidosis
Synthesis of alcoholLiver injury
Synthesis of acetaldehydeLiver injury

From Feldman M et al: Sleisenger and Fordtran’s gastrointestinal and liver disease, ed 10, Philadelphia, 2016, Elsevier.

Etiology

Disorders that disrupt protective mechanisms against bacterial burden predispose patients to SIBO. Box 1 summarizes diseases and disorders linked to SIBO based on pathophysiology.

  • Patients with structural or anatomic abnormalities are at greater risk. These include patients with small bowel diverticula, small intestinal strictures, surgical blind loops, ileocecal resections, or gastric resections (increasing common cause of SIBO).
  • Motility disorders predispose to SIBO because of the ineffective clearance of bacteria from the proximal bowel into the colon. Examples of this include gastroparesis and small bowel dysmotility, both suggestive of poorly controlled diabetes. Long-standing celiac disease can also interfere with small bowel motility.
  • It is thought that recent antibiotic use as well as antacid medication can alter the normal bacterial flora in the small intestine, contributing to SIBO.

BOX 1 Diseases and Disorders Linked to Small Bowel Bacterial Overgrowth (SIBO) Based on Pathophysiology

Dysmotility

Acromegaly:

  • Amyloidosis
  • Chronic opiate use
  • Diabetic autonomic neuropathy
  • Gastroparesis
  • Hypothyroidism
  • Idiopathic intestinal pseudo-obstruction
  • Long-standing use of motility-suppressing drugs
  • Myotonic muscular dystrophy
  • Systemic sclerosis/scleroderma
Altered Anatomy

Blind loops:

  • Gastrocolic or jejunocolic fistula
  • Ileocecal valve resection
  • Small intestinal diverticulosis
  • Strictures (Crohn disease, radiation, surgery)
  • Surgically induced alterations in anatomy (Billroth II gastrectomy, end-to-side anastomosis)
Hypochlorhydria

Long-term acid suppression (?)

  • Postsurgical
Immune Deficiency

Acquired immune deficiency syndrome (e.g., AIDS, severe malnutrition)

  • Inherited immune deficiencies
Multifactorial Causes

Advanced age:

  • Celiac disease
  • Chronic pancreatitis
  • Crohn disease
  • Cystic fibrosis
  • End-stage kidney disease
  • Intestinal failure
  • Liver disease
  • Radiation enteropathy
  • Tropical sprue
Unclear or Undefined Relationship to SIBO

Erosive esophagitis:

  • Interstitial cystitis
  • Irritable bowel syndrome
  • Parkinson disease
  • Restless legs syndrome
  • Rosacea
  • Severe obesity

From Feldman M et al: Sleisenger and Fordtran’s gastrointestinal and liver disease, ed 10, Philadelphia, 2016, Elsevier.

Diagnosis ⬆ ⬇

Differential Diagnosis

  • Celiac disease
  • Chronic pancreatitis
  • Inflammatory bowel disease
  • Irritable bowel syndrome
  • Tropical sprue
  • Whipple disease
  • Lactose intolerance
Workup

Diagnostic testing should include workup for diarrhea, anemia, and malabsorption. Although endoscopy with jejunal aspirate and culture was a diagnostic tool of choice, its role is limited because of low specificity. Hydrogen breath tests have their limitations as well, but they are noninvasive and easy to perform.

Laboratory Tests

  • Breath tests have become more commonplace in diagnosing SIBO. Typically fermenting bacteria reside in the colon. In SIBO, fermenting bacteria is present in the small intestine as well. A carbohydrate test dose (typically lactulose or glucose) is given, and its byproduct (hydrogen) is measured as it is excreted in the breath. In SIBO, exhaled hydrogen concentrations rise early.
  • Standard anemia workup is essential. CBC may suggest macrocytic anemia secondary to B12 deficiency.
  • Nutritional status should be evaluated with albumin levels.
  • Stool evaluation can aid in the diagnosis as well. An increase in fecal fat may be suggestive of SIBO. Stool WBC, culture, ova, and parasites should be ordered as well to rule out other infectious etiology.
Imaging Studies

  • Endoscopic evaluation of the small intestine can be useful in finding structural and motility causes of bacterial overgrowth such as diverticula and strictures. Small bowel biopsy may aid in the diagnosis of celiac disease as well.
  • Jejunal aspirate cultures via endoscopy are considered a standard of diagnosis. Aspirate cultures that exceed 105 organisms/ml suggest the presence of SIBO.
  • There are several limitations to jejunal aspirate cultures. Bacterial overgrowth is not uniform and may be in inaccessible areas to endoscopist and can easily be missed. Contamination from oropharyngeal flora can lead to false-positive tests. Also, endoscopy is an invasive test, and other methods of testing such as a breath test may be a more practical initial approach.

Treatment ⬆ ⬇

The goal is to treat the underlying cause and treat the bacterial overgrowth with antibiotic therapy.

Nonpharmacologic Therapy

Structural disorders such as strictures, fistula, and diverticula may require surgical intervention.

Acute General Rx

  • 7- to 10-day course of antibiotic therapy with rifaximin, amoxicillin-clavulanate, or metronidazole and ciprofloxacin has been shown to be beneficial.
  • Nutritional support with vitamin replacement and dietary modification (lactose-free diet).
Chronic Rx

  • Recurrence is common after antibiotic therapy. These patients may require subsequent courses of antibiotic therapy.
  • Avoid using antacid medication.
  • Avoid drugs that reduce GI motility (opioids).
  • Consider lactose-free diet if the response to antimicrobial agents is incomplete.
Disposition

Prognosis is dependent on underlying cause of SIBO. Although recurrence rate is high, antibiotic therapy remains the mainstay of therapy often requiring repeated courses if the underlying condition cannot be resolved.

Referral

  • Gastroenterology consultation for small bowel evaluation
  • Surgical consultation with an underlying structural disorder

Pearls & Considerations ⬆

Comments

  • SIBO is due to a disruption of protective mechanisms against bacterial burden.
  • Look for risk factors including UGI tract surgery, structural disorders, inflammatory bowel disease (IBD), IBS, and disorders decreasing GI motility.
  • Diagnosis can be made with hydrogen breath test or endoscopic jejunal aspirate culture.
  • Treatment is with antibiotics.
  • The combination of vitamin B12 deficiency (due to bacterial consumption) and an elevated serum folate level (due to bacterial production) is suggestive of SIBO.
  • SIBO can contribute to symptoms of IBS or IBD.
Related Content

Small Intestinal Bacterial Overgrowth (Patient Information)

Irritable Bowel Syndrome (Related Key Topic)

Malabsorption (Related Key Topic)