Small Intestinal Bacterial Overgrowth (SIBO)

Small intestinal bacterial overgrowth (SIBO), also known as bacterial overgrowth or small bowel bacterial overgrowth syndrome (SBBOS), is a condition characterised by excessive bacterial growth in the small intestine.

Unlike the colon, which is rich in bacteria, the small intestine normally contains fewer than 100,000 organisms per millilitre. Patients with SIBO may experience symptoms such as nausea, bloating, vomiting, diarrhoea, malnutrition, weight loss, and malabsorption.

The ileocecal valve prevents reflux of bacteria from the colon into the small bowel. Resection of the valve can lead to bacterial overgrowth.

Signs and Symptoms

Symptoms commonly associated with SIBO include bloating, diarrhoea, constipation, and abdominal pain or discomfort. Steatorrhea, a type of diarrhoea where fats are not properly absorbed and spill into the stool, may be seen in more severe cases. Other symptoms include nausea, flatus, constipation, abdominal distension, fatigue, and weakness. SIBO can lead to increased permeability of the small intestine, weight loss, and malnutrition in children, affecting their growth.

Deficiency of vitamin B12 can occur in bacterial overgrowth

Longstanding bacterial overgrowth can result in complications due to malabsorption of nutrients, such as elevated folate levels, vitamin B12 deficiency, and other nutritional deficiencies. Anaemia may also occur due to malabsorption of iron and vitamin B12, leading to microcytic and megaloblastic anaemia, respectively.

Related Conditions

There have been proposed links between SIBO and other disorders like irritable bowel syndrome (IBS). Some studies suggest that up to 80% of patients with IBS may have SIBO, as detected by the hydrogen breath test. However, there is a lack of consensus on this link, with some authors arguing that abnormal breath test results in IBS patients do not necessarily indicate SIBO.

Cause

Certain risk factors predispose individuals to developing SIBO. These include motility disorders, anatomical changes leading to stasis, immune system disorders, interference with proteolytic enzyme, gastric acid, or bile production, and conditions that cause colon bacteria to enter the small bowel. Impairment of the migrating motor complex (MMC) is strongly associated with SIBO. Other causes include diseases like scleroderma, diabetes, pancreatitis, and use of proton pump inhibitors.

E. coli, shown in this electron micrograph, is commonly isolated in patients with bacterial overgrowth

Diagnosis

Diagnosis can be made through various methods, with the gold standard being the aspiration of more than 105 bacteria per millilitre from the small bowel. Breath tests are commonly used but have reliability issues with high false positive rates. The D-xylose absorption test can detect malabsorption by measuring D-xylose levels in urine and blood.

Aspiration of bacteria from the jejunum is the gold standard for diagnosis. A bacterial load of greater than 105 bacteria per millilitre is diagnostic for bacterial overgrowth

Biopsies of the small bowel can mimic celiac disease, showing partial villous atrophy.

Biopsies of the small bowel in bacterial overgrowth can mimic celiac disease, with partial villous atrophy

Treatment

Treatment focuses on identifying and correcting root causes, resolving nutritional deficiencies, and administering antibiotics. Probiotics may be used as first-line therapy, with antibiotics reserved for more severe cases. Antibiotics such as rifaximin show the best evidence of effectiveness. A cyclical antibiotic regimen may prevent tolerance. Prokinetic drugs can be used to extend the SIBO-free period after treatment.

Probiotics like Lactobacillus casei, Lactobacillus plantarum, and Lactobacillus acidophilus have shown effectiveness, while Lactobacillus fermentum and Saccharomyces boulardii have not. An elemental diet can also be highly effective, providing nutrition while depriving bacteria of a food source. FODMAP restriction diets may help alleviate symptoms by eliminating foods that feed the bacteria.

Self-assessment MCQs (single best answer)

What is Small Intestinal Bacterial Overgrowth (SIBO) primarily characterised by?

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Which symptom is NOT commonly associated with SIBO?

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What type of anaemia can result from vitamin B12 deficiency due to SIBO?

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What is the gold standard for diagnosing SIBO?

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Which bacterial species is commonly isolated in patients with SIBO?

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Which of the following is NOT a proposed link to SIBO?

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What condition can cause increased permeability of the small intestine, weight loss, and malnutrition in children with SIBO?

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Which method of treatment for SIBO is considered effective for more severe cases?

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Which probiotic has NOT shown effectiveness in treating SIBO?

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What is a symptom of steatorrhea seen in severe cases of SIBO?

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