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.

A meta‐analysis on small intestinal bacterial overgrowth in patients with different subtypes of irritable bowel syndrome

“…These were considered to be the gold standard for making the diagnosis of IBS at the time of publication, but now, these criteria have been superseded by Rome IV criteria . Excessive gas productions leading to bloating because of SIBO is seen as the dominant framework for understanding the link between IBS and SIBO by some researchers . As Rome I and Manning's treated flatulence and bloating as one of the cardinal symptoms of IBS, which was subsequently dropped from Rome III and IV, it is reasonable to assume that these finding may not apply to patients diagnosed with Rome IV criteria.…”

“…. One of the critical peripheral factors implicated in the pathogenesis of IBS is gut dysbiosis including SIBO . Symptoms of SIBO, such as abdominal pain, discomfort, bloating, flatulence, and loose motions, are somewhat similar to those of IBS .…”

“…As diarrhea is a common manifestation of SIBO, patients with diarrhea‐predominant IBS are expected to have SIBO more often . Moreover, slow gut transit in patients with constipation‐predominant IBS may also lead to a false negative test for SIBO .…”

Marshall and Warren Lecture 2019: A paradigm shift in pathophysiological basis of irritable bowel syndrome and its implication on treatment

“…Using real‐time polymerase chain reaction and next‐generation sequencing, several studies, mostly using fecal sample, showed gut microbiota dysbiosis among patients with IBS compared with healthy controls 24, 31 . In a recent meta‐analysis of 23 case–control studies on 1340 participants, patients with IBS had lower levels of good bacteria of genera Lactobacillus and Bifidobacterium and marginally higher levels of bad bugs of genera Enterobacter compared with the controls 32 …”

Impact of intestinal parasites on microbiota and cobalamin gene sequences: a pilot study

“…These microorganisms may promote intestinal homeostasis and may protect against inflammatory diseases [54][55][56][57]. Vitamin B12 is absorbed in the small intestines [58][59][60] while the majority of microbiota reside in the colon [58], although, the small intestine is not sterile and does contain a robust microbiota that influences the absorption of vitamins [61][62][63]. Specific bacteria produce vitamin B12 [64], and the children infected with G. duodenalis with DNA levels above 1 fg/µl may be unable to synthesize the required amounts of vitamin B12 for nutritional benefit.…”