Chapter Twelve The Intestinal Microbiota in the Irritable Bowel Syndrome

The irritable bowel syndrome (IBS) is a chronic abdominal symptom complex occurring in a bowel devoid of discernible relevant pathology. There is growing interest in the role of the intestinal microbiota as a basis for the intestinal and possibly behavioral manifestations of this condition. Molecular-based microbial profiling has revealed compositional changes in the microbiota of at least a subset of IBS patients but the data are often conflicting and no microbial signature for this condition has yet been defined. Animal studies in which a previously stable intestinal microbiota is perturbed, by antibiotics or dietary change, results in alterations in intestinal function reminiscent of that seen in IBS patients. These include visceral sensitivity to painful stimuli, altered motility and intestinal barrier function as well as immune activation, and low-grade inflammation. More recent studies have shown that perturbation of the microbial composition of the gut alters brain chemistry and behavior. In a step toward establishing a causal link between an altar microbiota and gut-brain manifestations of IBS, colonization of germ-free mice with microbiota from IBS patients results in an IBS-like phenotype, including alterations and behavior if the donor exhibited psychiatric comorbidity, such as high levels of anxiety. This model provides an opportunity for exploring the mechanisms underlying host-microbe interactions relevant to the pathogenesis of IBS and for developing novel therapeutic targets.