Review Article

Small Intestinal Bacterial Overgrowth

Prevalence, Clinical Features, Current and Developing Diagnostic Tests, and Treatment

E. Grace; C. Shaw; K. Whelan; H. J. N. Andreyev


Aliment Pharmacol Ther. 2013;38(7):674-688. 

In This Article

Abstract and Introduction


Background The symptoms and signs of small intestinal bacterial overgrowth (SIBO) are often identical to a variety of diseases and can lead to diagnostic confusion.

Aims To review the diagnostic options for SIBO and present new investigative options for the condition.

Methods A literature search was performed on MEDLINE, EMBASE and Web of Science for English articles and abstracts. Search terms included free text words and combinations of the following terms 'small intestinal bacterial overgrowth', 'small bowel bacterial overgrowth', 'diagnostic tests', 'treatment', 'antibiotics', 'probiotics', 'metabonomics', 'proton nuclear magnetic resonance spectroscopy', 'electronic nose' and 'field asymmetric ion mobility spectrometry'.

Results All of the available methods to test for SIBO have inherent limitations and no 'gold-standard' diagnostic test for the condition exists. Accurate diagnosis of SIBO requires identification of bacterial species growing inappropriately within the small intestine and symptom response to antibiotics. Proton nuclear magnetic resonance spectroscopy, electronic nose technology and/or field asymmetric ion mobility spectrometry may represent better investigative options for the condition.

Conclusions Novel diagnostic options are needed to supplement or replace available tests.


In the healthy human host, there are intrinsic mechanisms that control the number and composition of the microbiota in different regions of the gastrointestinal (GI) tract. Gastric acid destroys many bacteria before they leave the stomach. Once in the small intestine, biliary and pancreatic secretions limit bacterial growth; antegrade peristalsis in the small intestine reduces luminal growth potential; the intestinal mucus layer traps bacteria and the ileo-caecal valve inhibits retrograde translocation of bacteria from the colon into the ileum. Clinical conditions associated with small intestinal bacterial overgrowth (SIBO) are shown in Table 1.

There is no consensus as to a definition for SIBO. As a result, its true prevalence and relationship with other clinical disorders remain uncertain. The most commonly cited definition is quantitative: 105 or more colony-forming units per millilitre (CFU/mL) of bacteria grown from a small intestinal aspirate.[1] However, many patients with a wide range of GI conditions and symptoms have increased bacterial counts in the small intestine compared with healthy controls and older age also correlates with rising counts of small intestinal strict anaerobes, although total bacterial counts generally remain below 105 CFU/mL.[2]

Some authors suggest that the presence of upper respiratory bacteria in SIBO is clinically significant, but the presence of these organisms is not clearly associated with abnormal GI symptoms.[3] To date, it is only intestinal overgrowth with microbiota that commonly colonise the colon (mainly Gram-negative, strict anaerobes and Enterococci) that is clearly linked to a pathological state characterised by abnormal GI symptoms. If these bacteria are eradicated by antibiotics, then the symptoms resolve.[48,49]

There are three common approaches towards diagnosing the condition: the first is the traditional approach of classifying it in quantitative terms in a microbiological context; the second is the breath testing technique using carbohydrates (e.g. glucose and lactulose); the third uses the symptomatic response to a trial of antibiotics. Two or three of these techniques are often combined for a more robust approach.