The concept of small intestinal bacterial overgrowth in relation to functional gastrointestinal disorders

The role of small intestinal bacterial overgrowth in the genesis of functional gut symptoms is controversial, but therapeutic benefits of antibiotics in controlled trials have highlighted the need to better evaluate and manage this state. Its definition is unclear due to limitations in assessing the microbiota in the small intestine and uncertainty regarding what is normal. Methodologies to detect bacterial overgrowth in clinical practice have limitations. The most commonly used is breath hydrogen testing after lactulose, but this cannot readily differentiate rapid orocecal transit from small intestinal bacterial overgrowth. Symptom generation might derive from fermentation of dietary carbohydrates with resultant luminal distension, from impairment of digestion and absorption with changes to the luminal composition, or by interaction with the enteric nervous system and immune system with subsequent changes in nociceptive thresholds and/or motility patterns. Therapy is usually directed toward reducing the bacterial load with antibiotics, but altering the functional properties of the microbiota by reducing or changing the supply of fermentative substrate or by the use of probiotics are promising alternatives. Controversy will continue until concepts are broadened, consensus in definition is reached, and evaluation of efficacy of candidate therapies is more rigorous.