Anaerobic bacteria predominate in the normal human fecal flora, out-numbering aerobes at least 100 to one. The two most prevalent organisms are Bacteroides fragilis and Bifidobacterium. Ileostomy flora is, on the other hand, chiefly aerobic and the total count is lower (10(8) per ml of fluid, compared to 10(10) per gram for feces). In normal people, small bowel bacterial counts are generally 10(5) per ml or less. The upper small bowel consists primarily of Gram-positive aerobes in small numbers. In the terminal ileum, counts are higher and aerobes and anaerobes are present in equal numbers. In the presence of acute obstruction and certain bowel stasis or other syndromes, the small bowel flora may become relatively profuse and fecal in type. The stomach normally has less than 10(3) organisms per ml but counts are higher in gastric samples with pH above 4.0. Intestinal bacteria are important in such processes as conversion of bilirubin to urobilinogen, supply of vitamin K to the host, defense against infection, bile acid deconjugation and conversion, infections related to the bowel, the malabsorption of blind loop and other bacterial overgrowth syndromes, and hepatic coma.