Small intestinal bacterial overgrowth can result from alterations in intestinal anatomy or gastrointestinal motility, or lack of gastric acid secretion. This condition can lead to vitamin deficiencies, fat malabsorption, and undernutrition. Diagnosis is by breath test or quantitative culture of intestinal fluid aspirate. Treatment is with oral antibiotics.
Small intestinal bacterial overgrowth (SIBO) is a malabsorption disorder.
Under normal conditions, the proximal small bowel contains < 105 bacteria/mL, mainly gram-positive aerobic bacteria. This low bacterial count is maintained by normal peristalsis, normal gastric acid secretion, mucus, secretory IgA, and an intact ileocecal valve.
Etiology of SIBO
Anatomic alterations of the stomach and/or small intestine promote stasis of intestinal contents, leading to bacterial overgrowth. Conditions that cause or require anatomic alterations include small-bowel diverticulosis, surgical blind loops, postgastrectomy states (especially in the afferent loop of a Billroth II), strictures, or partial obstruction.
Intestinal motility disorders associated with diabetic neuropathy, systemic sclerosis, amyloidosis, hypothyroidism, and idiopathic intestinal pseudo-obstruction can also impair bacterial clearance. Achlorhydria and idiopathic changes in intestinal motility may cause bacterial overgrowth in older people.
Pathophysiology of SIBO
The most common species in SIBO include streptococci, Bacteroides, Escherichia, , Klebsiella, and Aeromonas.
The excess bacteria consume nutrients, including carbohydrates and vitamin B12, leading to caloric deprivation and vitamin B12 deficiency. However, because the bacteria produce folate, this deficiency is rare. The bacteria deconjugate bile salts, causing failure of micelle formation and subsequent fat malabsorption. Severe bacterial overgrowth also damages the intestinal mucosa.
Fat malabsorption and mucosal damage can cause diarrhea.
Symptoms and Signs of SIBO
The most common symptom of SIBO is bloating.
The other symptoms are abdominal discomfort, diarrhea, and excess flatulence. Some patients have significant diarrhea or steatorrhea. (See also the American Journal of Gastroenterology's 2020 guidelines for small intestinal bacterial overgrowth.)
Diagnosis of SIBO
Breath tests
Quantitative culture of intestinal aspirate
Imaging studies
Some clinicians advocate response to empiric antibiotic therapy as a diagnostic test. However, because bacterial overgrowth can mimic other malabsorptive disorders (eg, Crohn disease) and adverse effects of the antibiotics can worsen symptoms of diarrhea, establishing a definitive etiology is preferred.
(See also the American Journal of Gastroenterology's 2020 guidelines for small intestinal bacterial overgrowth.)
Breath testing,
A positive breath test is defined as a > 20-ppm increase of hydrogen from baseline in the first 90 minutes or a > 10-ppm increase of methane at any point during the test (see the American Journal of Gastroenterology's 2020 guidelines for small intestinal bacterial overgrowth).
The standard for diagnosis of SIBO is quantitative culture of intestinal fluid aspirate showing a bacterial count > 103 colony-forming units/mL. This method, however, requires endoscopy.
If the anatomic alterations are not due to previous surgery, an upper gastrointestinal series with small-bowel follow-through should be done to identify predisposing anatomic lesions. Alternatively, CT enterography or magnetic resonance enterography can be done.
Treatment of SIBO
Oral antibiotics (various)
Dietary modification
Treatment of bacterial overgrowth syndrome is with 10 to 14 days of oral antibiotics that cover both aerobic and anaerobic enteric bacteria. Empiric regimens include use of one or two of the following:
Antibiotic treatment can be cyclic, if symptoms tend to recur, and changed based on culture and sensitivity. Changing antibiotic treatment may be difficult, however, due to coexistence of multiple bacteria.
Because bacteria metabolize primarily carbohydrates in the intestinal lumen rather than fats, a diet high in fat and low in carbohydrates and fiber is beneficial.
Underlying conditions and nutritional deficiencies (eg, vitamin B12) should be corrected.
Key Points
Anatomic alterations in stomach or intestines or intestinal motility disorders lead to gastrointestinal stasis and thus bacterial overgrowth.
Bacteria deconjugate bile salts, causing fat malabsorption.
Oral antibiotics are used, and a high-fat, low-carbohydrate diet is followed.
More Information
The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
American Journal of Gastroenterology: Clinical guidelines for small intestinal bacterial overgrowth (2020)
