Normal, healthy gut bacteria are found in both small intestine and colon. However, the concentration of bacteria in the colon is much higher (about 100,000 times higher) than in the small intestine. Also, the type of bacteria found in the small intestine differs from that found in the colon. SIBO (small intestinal bacterial overgrowth) or SBBOS (small bowel bacterial overgrowth syndrome) is a condition in which abnormally high numbers of bacteria are found in the small intestine, and these bacteria can resemble the type normally found in the colon.

It has been reported that up to 80% of individuals with irritable bowel syndrome (IBS) may be SIBO-positive as determined by hydrogen and methane breath test. Studies have also shown that symptoms of IBS are significantly ameliorated upon SIBO therapy. Taken together, these data suggests that SIBO may play a significant role in the pathogenesis of IBS.

Individuals with small intestinal bacterial overgrowth can experience a range of gastrointestinal symptoms that may include gas and bloating, flatus, diarrhea (including steatorrhea), constipation, abdominal pain and discomfort, nausea and vomiting, nutrient malabsorption, fatigue, and in severe cases, malnutrition and weight loss. Patients with prolonged, untreated SIBO may display a variety of nutrient deficiencies secondary to malabsorption. These include anemias, especially microcytic anemia, which results from the inability to absorb iron, vitamin B12, and other nutrients essential for erythropoiesis.

Risk factors for the development of SIBO include disorders of gastrointestinal motility, disorders of the immune system, and conditions that lead to bacterial reflux from the colon into the small intestine. Alterations in gastrointestinal motility can result from diseases such as celiac disease or scleroderma that cause a general and widespread attenuation of peristalsis and an increase bacterial concentration in the small bowel. Diverticulitis is also frequently associated with SIBO as it causes small pockets in the small intestine where bacteria can collect and proliferate. Anatomical aberrations that may occur after resection of the gastrointestinal tract can also create regions of stasis where bacteria can collect. Pancreatitis, use/overuse of immunosuppressants, and genetic immunodeficiencies have been associated with SIBO. Surgical removal of the ileocecal valve can allow bacteria from the large bowel to inappropriately reflux into the ileum, and promote distal SIBO. Finally, the use of proton pump inhibitors, frequently used to prevent acid reflux, has been linked to the development of small intestinal bacterial overgrowth.