Studies showing improvements in GI symptoms in patients with IBS treated with antibiotics such as rifaximin and neomycin also provide evidence for the role of altered gut flora in this condition.
According to the authors of “Rifaximin in Irritable Bowel Syndrome: Rationale, Evidence and Clinical Use,” published in Therapeutic Advances in Chronic Disease, small intestinal bacterial overgrowth (SIBO) can cause “excessive gas production and malabsorption with a variety of nonspecific symptoms, such as diarrhea, gas bloating, abdominal pain and constipation.”
They also note that there is some evidence that “a shift in the host—gut microbial relationship” seen in SIBO may contribute to the pathogenesis of irritable bowel syndrome (IBS).
Because there is also evidence of “a role for gut bacteria and antibiotics in the pathophysiology and treatment of IBS respectively,” the authors reviewed the current data on the association between SIBO and IBS, as well as the available data on the clinical use of the antibiotic rifaximin in the treatment of SIBO in patients with IBS.
There is mounting evidence that altered gut flora may play an important role in IBS. Some studies have suggested that up to 50% of patients with IBS also have SIBO. Other research on the association between gut bacteria and IBS suggests that variations in the type and quantity of gas produced by gut bacteria may explain the different types of IBS.
Another study of patients with IBS-D (diarrhea-dominant IBS), found “the altered composition of the intestinal microbiota is associated with a significant increase in detrimental bacterial groups and a decrease in beneficial ones, and a reduction in microbial richness.”
Because of the overlap in symptoms between SIBO and IBS, one method of determining whether (and the extent to which) SIBO is the cause of symptoms is to administer a lactulose breath test (LBT), follow up a positive test result with a course of antibiotics, and then administer a second breath test. A normal result on the second test would suggest that SIBO was the cause of at least some of the symptoms.
One study that used this approach to test the effectiveness of the antibiotic neomycin in improving symptoms in patients with a diagnosis of IBS found that not only is it common for patients with IBS to have a positive breath test, treatment with antibiotics resulted in symptom improvement and bowel normalization, providing more “evidence to suggest the gut microbial origin of IBS.”
Several studies have shown that the antibiotic rifaximin produces similar symptom improvements in patients with IBS. In one study, who had IBS without constipation were treated with rifaximin three times a day for two weeks, and then followed for 10 weeks and assessed for relief of IBS symptoms. The researchers found that treatment with rifaximin produced “significant relief of IBS symptoms,” which lasted for up to two weeks after treatment.
In another study, IBS patients who had a positive LBT were treated with rifaximin for two weeks also reported significant improvement in IBS symptoms, effects that lasted for up to three months following treatment.
According to the authors, these and other study results are part of the growing evidence “pointing towards the benefit of a short course of treatment with rifaximin in the global improvement of patients with IBS.” They also support the idea that IBS symptoms are at least in part caused by changes in the composition of intestinal bacteria.