We know a lot about the symptoms of Irritable Bowel Syndrome (also known as Spastic Colon or IBS) — pain, cramps and diarrhea — but little or nothing about what causes it. In the absence of a known cause, IBS has long been linked to psychological or emotional stress. Of course, the same was once true of stomach ulcers, most of which we now know are caused by infection.

According to a new study, bacterial infection may also be the hidden cause of IBS. This would be good news for the estimated one out of five American adults who suffer from IBS, because bacterial infections can be easily treated with antibiotics.

The study, done by researchers at Cedars-Sinai Medical Center in Los Angeles, found a strong association between Irritable Bowel Syndrome (IBS) and a condition called SIBO, in which there is an excess of bacteria in the small intestine.

"Our research shows that the majority of patients we evaluated with IBS had excessive bacteria in the small intestine and that treatment with antibiotics greatly reduced the gastrointestinal symptoms of this condition," said Dr. Mark Pimentel, the principal investigator of the study, which was published in the December 2000 issue of The American Journal of Gastroenterology.

Pimentel and his team looked at 202 patients who were given a breath test that detects SIBO. They found that SIBO was present in a surprising 78 percent of the patients tested.

The next step was to determine whether eliminating the SIBO would improve the symptoms of IBS. The answer was a qualified yes. Those who tested positive for SIBO were given antibiotics for 10 days. Then they were given a second breath test. The investigators found no SIBO in more than half of these patients and some, usually lower levels, of SIBO in the rest.

"On the whole, we found a significant reduction in the symptoms of IBS even in those patients in which SIBO was only partially eradicated," said Dr. Pimentel.

Among the patients in which no SIBO was detected after antibiotic treatment, about half reported no symptoms of IBS whatsoever, while IBS symptoms were significantly reduced in the remaining half.

"This is the first study to demonstrate that complete eradication of SIBO with commonly prescribed antibiotics substantially improves the symptoms of IBS, especially those associated with bloating, diarrhea and abdominal pain," said Dr. Henry C. Lin, a coauthor of the study.

Asked to examine these findings, TheDoctor's Gary Gray, M.D., Professor of Medicine, Emeritus (Gastroenterology), Stanford University School of Medicine, called them "very provocative. It would be wonderful if we could treat patients with IBS by eliminating bacterial overgrowth along with the symptoms of IBS. Certainly, bacterial overgrowth may produce symptoms similar to those in IBS, but the high proportion of IBS patients with SIBO was unexpected."

Dr. Gray added, however, that "the findings will need to be corroborated by additional -- and, hopefully, larger-scale studies, for several reasons. For one, we do not know what percentage of people without IBS would test positive for SIBO using the breath test. Furthermore, even in this small study, nearly half of the IBS patients did not experience any improvement in symptoms such as diarrhea and abdominal pain even after their SIBO was eradicated."

The bottom line, according to Dr. Gray, is that we cannot say, based on this study alone, that IBS should be treated with antibiotics.

In a follow-up effort, the Cedars-Sinai research team announced that they plan to conduct further studies using longer courses of antibiotics as well as non-antibiotic therapies on IBS sufferers who test positive for SIBO.

Reviewed by Gary M. Gray, M.D.