ReviewThe concept of small intestinal bacterial overgrowth in relation to functional gastrointestinal disorders
Section snippets
Small intestinal bacterial overgrowth—a flawed concept?
The concentration of bacteria in the jejunum and ileum is usually quoted as 104–108 colony-forming units (CFU)/mL, which contrasts to 109–1012 CFU/mL in the large bowel. SIBO has been traditionally defined according to the number and often type of culturable bacteria in duodenal or jejunal aspirates; for example, 105 CFU/mL of “colonic-type” bacteria has been commonly used [1], [2], [3], [4], [5]. Such a definition, however, presents four important conceptual difficulties.
First, because much of
How to detect SIBO
Multiple techniques have been used to detect SIBO and have recently been systematically reviewed [10]. The accurate detection in clinical practice of SIBO has been difficult and no test can be considered truly validated due to the lack of a gold standard. Culture-based techniques have convincingly demonstrated excessive density of bacteria in the proximal small intestine in association with motility disturbances or anatomical abnormalities in which stasis is the common denominator [12].
SIBO—normal or abnormal?
One of the conceptual stumbling blocks around SIBO has been the consideration by most authors that SIBO is abnormal. There would be little argument that a positive glucose breath test in a patient with anatomical abnormalities of the proximal small intestine indicates the presence of an abnormal population of bacteria. However, the same might not apply to a patient with normal anatomy and functional gut symptoms. Likewise, a test that is positive for SIBO in a moderate proportion of healthy
Mechanisms by which SIBO may be associated with functional gut symptoms
While the association of SIBO with functional gut symptoms remains controversial, there is evidence that SIBO potentially may be involved in triggering symptoms. This might occur via two mechanisms. The first is by the impairment of digestion and/or absorption. SIBO may lead to deconjugation of bile salts with subsequent fat malabsorption, leading to fatty acid diarrhea [24]. It has also been demonstrated by breath hydrogen studies that malabsorption of fructose and possibly lactose is
Therapeutic approaches to SIBO
In light of the potential mechanisms by which SIBO might cause symptoms, therapeutic approaches theoretically include reducing the bacterial load and/or changing the functional properties of the microbiota.
Conclusions
Managing functional gut symptoms continues to be a clinical challenge, and better targeting of therapy toward factors that are pathogenically related to the genesis of symptoms is needed. The concept of SIBO has provided an exciting new potential therapeutic target. The presence of SIBO is probably best assessed by examining the time of first rise in breath hydrogen after lactulose, but the finding of ERBHAL is not specific for, only suggestive of, SIBO. ERBHAL itself is a normal phenomenon,
Acknowledgments
JSB was in receipt of a Sir Robert Menzies Memorial Research Scholarship in the Allied Health Sciences and received a Postdoctoral Publication Award from Monash University.
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Both authors contributed equally to the text.