In healthy persons, the stomach, duodenum, and upper small bowel are sterile, or the number of organisms never surpasses 10⁵ colony-forming units per milliliter. The organisms commonly found are lactobacilli, streptococci, Haemophilus influenzae, Haemophilus parainfluenzae, Veillonella, and Propionibacterium acnes. Such mechanisms as gastric acidity, secretions of the intestine and pancreas, immunoglobulins, and, especially, intestinal peristalsis, aid in maintaining a low bacterial count. The distal ileum contains as many as 10⁹ colony-forming units per milliliter, including gram-negative bacilli and anaerobes. The ileocecal valve is important in preventing the growth of an anaerobic, colonic-type flora in the distal small bowel. The impairment of any of these mechanisms may result in bacterial overgrowth in the small bowel (Box 360.1).

Frequently, bacterial overgrowth in the small bowel leads to malabsorption of carbohydrates because of intraluminal use by bacteria. Presenting symptoms are abdominal distention as a result of formation of gas, excessive eructation, halitosis, vomiting, and diarrhea. Diarrhea can result from bacterial degradation of brush border disaccharidases and from a decrease in small-bowel villous height and a consequent decrease in the transport of monosaccharides.

Hypoproteinemia, which usually occurs in patients with bacterial overgrowth in the small bowel, can be the result of protein loss due to mucosal injury caused by the bacteria or of altered dietary protein absorption due to luminal bacterial degradation.