Does bacteria play a role in IBS? If so, is it helpful or harmful? What about antibiotics, or probiotics? Bacteria are present in the normal gut (intestines) and in large numbers the lower parts of the intestine. These “normal” bacteria have important functions in life. A variety of factors may disturb the mutually beneficial relationship between the gut bacteria and its host – and disease may result. The possibility that gut bacteria could have a role in irritable bowel syndrome (IBS) may surprise some; there is indeed, now quite substantial evidence to support the idea that disturbances in the bacteria that populate the intestine may have a role in at least some patients with IBS. This article looks at recent findings about possible influences of bacteria in IBS, which may be either negative or positive, and at various treatment approaches.