Small Intestinal Bacterial Overgrowth and Irritable Bowel Syndrome: What Is the Association?

Yehuda Ringel, MD

September 18, 2006


What is the evidence for the role of small intestinal bacterial overgrowth in the etiology of irritable bowel syndrome?

Response from Yehuda Ringel, MD

Several studies over the last few years have suggested an association between abnormal hydrogen breath test findings and irritable bowel syndrome (IBS), and proposed small intestinal bacterial overgrowth (SIBO) as a potential etiologic factor in the disorder. In 2 early provocative studies, Pimentel and colleagues,[1,2] from Cedars-Sinai Medical Center in Los Angeles, California, reported that 78% to 84% of their IBS patients had abnormal lactulose hydrogen breath test results compared with 20% of patients in a control group (P < .01). Furthermore, neomycin treatment resulted in significant improvement in their patients' symptoms (eg, 35.3% bowel normalization in the treated group compared with 13.9% in the placebo group; P < .001), and up to 48% of eradicated subjects no longer met the Rome criteria for IBS. Although these studies were first criticized because of patient-selection bias and the low accuracy of lactulose breath testing in defining SIBO, several studies with glucose breath testing have also shown a higher prevalence of SIBO in patients with IBS. For example, a recent retrospective epidemiologic case-control study demonstrated positive glucose breath tests in 31% of patients with IBS vs only 4% in the control group (odds ratio [OR], 2.65; 95% confidence interval [CI], 3.5-33.7; P < .00001).[3] These results support Pimentel and colleagues' early findings of an association between IBS and SIBO. However, other studies have shown a much lower prevalence of SIBO in patients with IBS. In a recently reported retrospective study of patients who were referred for glucose hydrogen breath testing for SIBO, only 11% of 113 patients who met the Rome II criteria for IBS tested positive for SIBO, suggesting that IBS symptoms are often unrelated to SIBO.[4] On the basis of currently available data, the contributing role of SIBO in the pathophysiology of IBS remains controversial, and the large variation in the prevalence of SIBO in IBS (10% to 84%) indicates the problematic state of this research, particularly with regard to the accuracy of breath testing in detecting SIBO in patients with altered (particularly accelerated) gastrointestinal motility.

Further epidemiologic studies and placebo-controlled clinical trials aiming at eradicating SIBO are necessary to clarify the true impact of SIBO on IBS symptoms. With regard to the latter, several small treatment trials have been reported and demonstrated improvement in IBS symptoms with antibiotic (eg, neomycin and rifaximin) therapy.[2,5] However, the results of a larger multicenter study with rifaximin are awaited with anticipation.

From a clinical standpoint, until this issue is clarified, clinicians should consider SIBO in an IBS patient with typical symptoms (eg, bloating, distention, and diarrhea), as well as in patients with these symptoms who do not fulfill the diagnostic criteria for IBS.


  1. Pimentel M, Chow EJ, Lin HC. Eradication of small intestinal bacterial overgrowth reduces symptoms of irritable bowel syndrome. Am J Gastroenterol. 2000;95:3503-3506. Abstract

  2. Pimentel M, Chow EJ, Lin HC. Normalization of lactulose breath testing correlates with symptom improvement in irritable bowel syndrome. a double-blind, randomized, placebo-controlled study. Am J Gastroenterol. 2003;98:412-419. Abstract

  3. Lupascu A, Gabrielli M, Lauritano EC, et al. Hydrogen glucose breath test to detect small intestinal bacterial overgrowth: a prevalence case-control study in irritable bowel syndrome. Aliment Pharmacol Ther. 2005;22:1157-1160. Abstract

  4. Harris LA, Crowell MD, DiBaise JK, Olden K. Is small intestinal bacterial overgrowth (SIBO) really prevalent in irritable bowel syndrome (IBS)? Am J Gastroenterol. 2005;100:S336-S337.

  5. Pimentel M, Park S, Kong Y, et al. A 10-day course of rifaximin, a non-absorbable antibiotic, produces a durable improvement in all symptoms of irritable bowel syndrome: a double-blind randomized controlled study. Gastroenterology. 2006;130:A134.