The Role of Gut Bacteria in IBS

Gut bacteria may play a role in irritable bowel syndrome (IBS). If you suffer from IBS, you may sometimes think that there is a war going on inside of your body. Well, the latest IBS research suggests that you might be on to something.

Female scientist examining a petri dish
Photo: Rafe Swan/Cultura/Getty Images

Your intestinal system is filled with billions of bacteria of all different kinds; altogether these bacteria are called the gut flora. In a state of optimal health, all of these bacteria play nicely together. Unfortunately, there are times when the balance of the gut flora is disturbed, a state known as intestinal dysbiosis, resulting in unpleasant gastrointestinal symptoms. This could happen for a variety of reasons, such as experiencing a bout of gastroenteritis (stomach flu) or as an aftereffect of a round of antibiotics. In the world of research, there are some new clues that an ongoing disturbance in the gut flora could be contributing to the discomfort you know as IBS. These clues come from four inter-related areas:

Post-Infectious IBS

The evidence is beginning to mount which indicates that IBS develops in some individuals following an acute bacterial infection in the digestive system. Studies of individuals who experience such an infection have found that approximately 25% will continue to experience unpleasant GI symptoms six months after the initial illness. More disturbing is the finding that one out of every 10 individuals who experience a severe GI infection will end up in the ongoing disorder known as IBS. In these cases, there is the identification of a clear link to an acute bout of digestive illness, are classified as post-infectious IBS (IBS-PI).

Lab research offers some concrete clues regarding IBS-PI. Using a procedure in which the tissue of the lining of the rectum is biopsied, investigators have found more inflammatory and serotonin-related cells in the rectal tissue of the individuals who developed IBS. This provides further evidence of the role of inflammation and the brain-gut connection in the maintenance of IBS symptoms. 


The role of bad bacteria in IBS is well-established. Probiotics are known as “friendly” bacteria because they are thought to be helpful to the health of your digestive system—and there is an increasing body of evidence linking probiotics to improved IBS symptoms.

There isn't yet enough solid research to establish a firm connection between probiotics and improved IBS symptoms and so gastroenterologists do not yet endorse probiotic supplementation for the condition. The American Gastroenterology Association's 2020 clinical practice guidelines recommends probiotics for IBS only in clinical trials.

According to some reports one particular type of probiotic, Bifidobacterium infantis has been clinically shown to reduce IBS symptoms. It is thought that taking a probiotics supplement helps return the bacteria within the gut flora to a more optimal state of balance.

Small Intestinal Bacterial Overgrowth (SIBO)

Small intestinal bacterial overgrowth (SIBO) is a condition in which there is an abnormally high number of bacteria in the small intestine. A new and somewhat controversial theory seeks to identify SIBO as a primary cause of IBS. Proponents of the SIBO theory believe that SIBO accounts for the symptom of bloating, the changes in motility that result in diarrhea and constipation, and the visceral hypersensitivity seen in IBS patients.

SIBO is generally diagnosed using a test that measures the amount of hydrogen in the breath following the ingestion of drinks containing lactulose. Lactulose is a sugar that is not absorbed by our bodies, so it is fermented by the bacteria within the intestinal system. If the amount of breath hydrogen is high a short time after drinking the lactulose solution, it is believed to reflect an abnormally high level of bacteria within the small intestine.

The controversy lies in terms of conflicting reports as to the accuracy of the hydrogen breath test, as well as conflicting reports as to how many IBS patients produce an abnormally high test result. As of right now, the conclusion within the field of IBS research is that SIBO may be relevant for a certain subset of IBS patients. 


Another area of research which indicates that gut bacteria play a part in IBS stems from the SIBO theory and the successful use of certain antibiotics as a treatment for IBS. Two particular antibiotics are used, Rifaximin and Neomycin, with Rifaximin showing a slight edge in terms of effectiveness. These antibiotics have been chosen because they are not absorbed in the stomach, and therefore are thought to be able to attack any bacteria lurking within the small intestine. Studies have shown that these antibiotics result in significant symptom improvement and may also be associated with positive changes in the hydrogen breath test. The downsides to the use of antibiotics have to do with their high cost as well as concern that they contribute to the development of more resistant forms of bacteria. Antibiotics would only be prescribed to individuals in which the hydrogen breath test indicates the presence of bacterial overgrowth in the small intestine. 

3 Sources
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