Small intestinal bowel overgrowth (SIBO) is a condition in which there is upward proliferation of the colonic bacteria, so that they start to grow in the small intestine.

The usual definition is more than 105  colony-forming units of microflora per mL of aspirated fluid from the upper intestine.

This occurs only in a state of disease, because the healthy gut has a largely unchanged composition of microflora throughout life.

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Risk Factors for SIBO

The normal gut motility is designed to move food through the whole length of the intestine, with waves of peristalsis.

Any disorder which causes delay in gut motility may result in SIBO, including chronic diabetes, connective tissue disorders, viral infections, or gut ischemia.

Motility disorders which affect the small bowel may also underlie SIBO, possibly due to the fact that bacteria are not easily carried downwards from the small bowel into the colon, allowing them to grow more extensively than normal.

Anatomical abnormalities of the upper gut may predispose to it, but SIBO may occur even when the gastrointestinal tract is anatomically normal.

Reduction in gastric acid, and immunologic gut disturbances may also increase the risk of SIBO.

Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is another common disorder found in millions of Americans. The most frequent complaints are of abdominal pain, irregular bowel movements, and altered fecal forms (diarrhea or constipation at various times). These symptoms are quite common in SIBO as well, with most patients experiencing flatulence, bloating, loose motion and abdominal pain.

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The two conditions have been found to coexist, though the prevalence varies with geographical region and with the criteria used for the diagnosis of these disorders. For example, using the early peak criteria of certain breath tests may yield a significant percentage of false positives, while others may be too insensitive.

Functions of the Colonic Microflora

The functions of the colonic microflora are many and diverse. Species such as E. coli, Streptococcus, Staphylococcus and Klebsiella are found in most people. They use up unabsorbed dietary carbohydrates, producing short-chain fatty acids of biological value in the process.

These are a rich source of energy to the colonic mucosal cells. In addition, they produce several bioactive molecules such as vitamin K and folate. They also contribute to the immune vigor of the host.

Risk Factors for IBS-Associated SIBO

Those patients with IBS who are at greater risk of SIBO may have one or more of the following characteristics:

  • Female gender
  • Older age group, such as above the age of 55 years, because of a combination of slower gut movements, history of gut surgery, small bowel diverticulosis and the use of medications which reduce peristaltic frequency and intensity
  • Complain of bloating and flatulence
  • Have predominant diarrhea
  • Treatment with proton pump inhibitors which may predispose to SIBO
  • Treatment with narcotic analgesics which slow the gut movements
  • Anemia

Most patients with IBS who also have SIBO have predominantly Enterococcus or Gram-negative species on aspiration and culture. These may reflect different underlying pathologies, with low gastric acidity predisposing to Gram-positive microflora but intestinal hypomotility or anatomical abnormalities triggering coliform overgrowth.

Pathophysiology of the IBS-SIBO Connection

The qualitative and quantitative alteration in gut flora seen in SIBO are associated with bacterial fermentation of undigested carbohydrate residues in the diet, leading to the overproduction of hydrogen, methane, and carbon dioxide.

These are thought to be the cause of bloating and other gut-related symptoms in IBS.

For instance, methane causes a reduction in gut motility which is associated with constipation. However, SIBO more often causes diarrheal symptoms in IBS.

This is due to various byproducts of SIBO such as the production of enterotoxins, increased intestinal permeability, cobalamin deficiency, mucosal inflammation due to immune responses in the small intestine, and deconjugation of bile salts in the gut lumen.

Other abnormal digestive metabolites such as short-chain fatty acids in the small bowel are also implicated in the intestinal symptoms of IBS.

Other mechanisms include the increased levels of gut hormones. This occurs because of a greater number of enterochromaffin cells in the colonic mucosa, possibly due to increased immune activation in the gut, and increased visceral hypersensitivity as well as gut motility due to the immune chemicals produced in response to such activation.


Patients with IBS who also have SIBO are usually treated with antibiotics, prokinetics and probiotics as indicated. Dietary and lifestyle modifications are also an important part of treatment in many patients.


Further Reading

Last Updated: Mar 7, 2019

Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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