Irritable bowel syndrome (IBS) is a common diagnosis in gastroenterology with patients usually subgrouped under the Rome clinical criteria according to their bowel pattern. Although an individual may be classified as diarrhea-predominant IBS or constipation-predominant IBS, patients often resist this subgrouping by saying that they have both constipation and diarrhea. Regardless of their predominant bowel complaint, 92% of IBS patients share the symptom of bloating. Bloating secondary to abnormal bacterial fermentation is a feature of small intestinal bacterial overgrowth (SIBO). The role for an antibiotic-sensitive mechanism in IBS such as SIBO is supported by 2 double-blind, randomized, placebo-controlled trial showing: (1) a 75% improvement in global bowel symptoms when bacterial overgrowth was successfully treated with a nonabsorbable antibiotic and (2) a sustained improvement of symptoms for a period of 10 weeks after the treatment with a small bowel-targeting antibiotic. Although diarrhea is a well-known symptom in SIBO, constipation may also be a consequence of altered host-gut microbial relationship through the action of the bacterial gas methane on intestinal motility. In addition to bloating and bowel symptoms, a number of extraintestinal complaints are common in patients with IBS, including psychologic changes such as anxiety. Animal data would suggest that anxiety may also be a consequence of altered host-gut microbial relationship. In this review, we will discuss the role of gut bacteria in the gastrointestinal and extraintestinal presentations of IBS.
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