The use of certain bacteria—primarily in the form of probiotics—to support or treat Irritable Bowel Syndrome (IBS) is justified by scientific evidence, although the quality and consistency of that evidence vary. Multiple randomized controlled trials and systematic reviews have examined the role of probiotics, which are live microorganisms intended to confer health benefits, in alleviating IBS symptoms such as bloating, abdominal pain, and irregular bowel movements.

The most studied probiotic strains for IBS include Bifidobacterium infantis 35624, Lactobacillus plantarum 299v, and combinations of Lactobacillus and Bifidobacterium species. Several meta-analyses, including a 2016 review in the journal Alimentary Pharmacology & Therapeutics, have found that specific probiotic strains can provide modest but statistically significant improvements in overall IBS symptoms compared to placebo. However, the effect sizes are generally small, and not all individuals respond equally. Moreover, not all probiotic formulations are effective—benefits appear to be strain- and dose-specific.

Guidelines from organizations such as the American College of Gastroenterology suggest that probiotics may be considered as a treatment option for IBS, but they also emphasize the need for further research to determine which strains and dosages are most effective. In summary, the use of bacteria (probiotics) in IBS is supported by scientific evidence, though it is not universally effective and is best viewed as an adjunct to other therapies.