The stomach and proximal small intestine contain relatively few microorganisms, mostly because of the presence of gastric acid, which makes those parts of your gastrointestinal tract an unpleasant home for bacteria. However, there is a medical condition known as small intestinal bacterial overgrowth (SIBO) or small bowel bacterial overgrowth (SBBO), which is defined as an increase in the number of bacteria in the upper GI tract.