Normal and pathological microbial flora of the upper gastrointestinal tract.

The resting gastric juice of normal adults in north-western Europe usually contains less than 10(3) micro-organisms/ml. Gastric sterility in such individuals is based on acidity. During meals or after an H2-receptor antagonist, gastric pH levels favour microbial growth but gastric emptying and recurrence of acid conditions resterilizes the stomach. If gastric pH remains above 4, colonization takes place. Studies suggest that gastric pH is normally high, with resultant bacterial colonization, in most of Asia, Africa, south and central America and, possibly, southern and eastern Europe. The biochemical activity of the gastric flora is also pH-dependent. Mucosal colonization occurs only if gastric pH is high. The small intestine is normally sterile because bacteria are killed by bile or pancreatic enzymes. Overgrowth can occur if there are areas inaccessible to these and may have severe metabolic consequences.