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Thirty-seven gnotobiotic piglets from seven litters were infected with either Helicobacter pylori N6 or urease-negative H. pylori N6ureG::Km which contains an insertion in the ureG gene and produces inactive urease. To produce achlorhydria, piglets were treated throughout the experiment with omeprazole (5 mg intravenously every 12 h) and ranitidine (75 mg orally every 6 h). Treatment resulted in elevation of gastric pH to 7.0 +/- 1.1 throughout the experiment. Control piglets were not treated and remained normochlorhydric. Strain N6 colonized well in both normal and achlorhydric piglets. All 10 piglets were colonized, and colonization ranged from 4.4 +/- 1.5 log10 CFU/g of gastric mucosa in normochlorhydric piglets sacrificed after 2 days to 6.9 +/- 0.5 log10 CFU/g in normochlorhydric piglets sacrificed after 5 days. Strain N6ureG::Km did not colonize any of seven normochlorhydric piglets and was recovered only in low numbers (< 100 CFU/g) from four of nine achlorhydric piglets. In the second experiment, piglets were coinoculated with both strains N6 and N6ureG::Km. Coinoculation did not affect colonization by urease-positive N6. Urease-deficient N6ureG::Km was unable to colonize even in the presence of urease-positive bacteria. These results confirm that urease enzymatic activity (and not urease protein) is essential for colonization, that this effect is independent of diffusible products of urea metabolism, and that gastric pH protection is not a major role of urease in promoting colonization by H. pylori.