Small Intestinal Bacterial Overgrowth (SIBO) is defined as an increase in the number of bacteria, and/or changes in the types of bacteria present in the small bowel. In most patients, SIBO is not caused by a single type of bacteria, but is an overgrowth of the various types of bacteria that should normally be found in the colon. Less commonly, SIBO results from an increase in the otherwise normal bacteria of the small bowel.