Irritable bowel syndrome and small intestinal bacterial overgrowth: meaningful association or unnecessary hype

Abstract

Irritable bowel syndrome (IBS) is a common condition characterized by abdominal pain or discomfort, bloating, and altered stool form and passage. Small intestinal bacterial overgrowth (SIBO) is a condition in which there is overgrowth of bacteria in small bowel in excess of 10⁵ colony forming units per milliliter on culture of the upper gut aspirate. Frequency of SIBO varied from 4%-78% among patients with IBS and from 1%-40% among controls. Higher frequency in some studies might be due to fallacious criteria [post-lactulose breath-hydrogen rise 20 PPM above basal within 90 min (early-peak)]. Glucose hydrogen breath test (GHBT) has a low sensitivity to diagnose SIBO. Hence, studies based on GHBT might have under-estimated frequency of SIBO. Therefore, it is important to analyze these studies carefully to evaluate whether the reported association between IBS and SIBO is over or under-projected. This review evaluates studies on association between SIBO and IBS, discordance between different studies, their strength and weakness including methodological issues and evidence on therapeutic manipulation of gut flora on symptoms of IBS.

Keywords: Dysbiosis; Functional bowel disease; Glucose hydrogen breath test; Gut flora; Lactulose hydrogen breath test.

Figure 1

Outline of principle of method and interpretation of glucose and lactulose hydrogen breath tests. On the left panel, method and result of glucose hydrogen breath test (GHBT) is shown. A1: GHBT is negative for SIBO as there is no peak in hydrogen production; A2: GHBT shows presence of SIBO. On the right panel, method and result of lactulose hydrogen breath test (LHBT) is shown. B1: LHBT is negative for SIBO as there is no peak in hydrogen production; B2: LHBT shows an early peak (within 90 min of lactulose ingestion); B3: LHBT shows double peaks in hydrogen, the earlier one from small bowel due to SIBO and the later one from the colon. Please note that Quintron machine of the left gives values of hydrogen, methane and CO₂ (for correction) and the Bedfont machine of right side estimates hydrogen only. It is also important to note that in the graphs hydrogen breath test do not show methane levels. SIBO: Small intestinal bacterial overgrowth; GHBT: Glucose hydrogen breath test; LHBT: Lactulose hydrogen breath Test; PPM: Parts per million.

Figure 2

Outline of method of culturing bacteria from the upper gut aspirate and counting the colonies by serial dilution technique. DW: Distilled water.

Figure 3

Bile acid breath test involves the administration of the bile acid ¹⁴C glycocholic acid, and the detection of ¹⁴CO₂, which would be elevated in small intestinal bacterial overgrowth.