Small Intestinal Bacterial Overgrowth and Irritable Bowel Syndrome: A Bridge between Functional Organic Dichotomy

Abstract

The pathogenesis of irritable bowel syndrome (IBS), once thought to be largely psychogenic in origin, is now understood to be multifactorial. One of the reasons for this paradigm shift is the realization that gut dysbiosis, including small intestinal bacterial overgrowth (SIBO), causes IBS symptoms. Between 4% and 78% of patients with IBS and 1% and 40% of controls have SIBO; such wide variations in prevalence might result from population differences, IBS diagnostic criteria, and, most importantly, methods to diagnose SIBO. Although quantitative jejunal aspirate culture is considered the gold standard for the diagnosis of SIBO, noninvasive hydrogen breath tests have been popular. Although the glucose hydrogen breath test is highly specific, its sensitivity is low; in contrast, the early-peak criteria in the lactulose hydrogen breath test are highly nonspecific. Female gender, older age, diarrhea-predominant IBS, bloating and flatulence, proton pump inhibitor and narcotic intake, and low hemoglobin are associated with SIBO among IBS patients. Several therapeutic trials targeting gut microbes using antibiotics and probiotics have further demonstrated that not all symptoms in patients with IBS originate in the brain but rather in the gut, providing support for the micro-organic basis of IBS. A recent proof-of-concept study showing the high frequency of symptom improvement in patients with IBS with SIBO further supports this hypothesis.

Keywords: Bacterial overgrowth; Breath tests; Dysbiosis; Gastrointestinal microbiota; Probiotics; Rifaximin.

Fig. 1

Schematic diagram showing the frequency of small intestinal bacterial overgrowth (SIBO) using quantitative jejunal aspirate culture, glucose and lactulose hydrogen breath tests (GHBT and LHBT, respectively) among patients with irritable bowel syndrome (IBS), gut defense mechanisms that prevent the development of SIBO, factors associated with SIBO among patients with IBS, and mechanisms of IBS symptom development. As shown in the figure, the frequency of SIBO in IBS patients using LHBT (early-peak criteria) is higher than that by using upper gut aspirate culture and GHBT (LHBT [45%]; upper gut aspirate culture [23%] and GHBT [26%]). Moreover, SIBO is more frequent in healthy controls using LHBT due to false positive test results (LHBT [21%], upper gut aspirate culture [1%] and GHBT [5%]). GI, gastrointestinal; IBS-D, irritable bowel syndrome, diarrhea-predominan; PPI, proton pump inhibitor; CHO, carbohydrate.