Urease activity of wild-type H. pylori 26695, nixA::aphA3 mutant, HP1512::cat mutant, and nixA::aphA3 HP1512::cat double mutant strains. In addition to an HP1512::cat mutant, a nixA::aphA3 mutant and a nixA::aphA3 HP1512::cat double mutant were constructed. Urease activity was measured via the phenol-hypochlorite urease assay after approximately 18 h of growth in unsupplemented BBF. The urease activities of the HP1512::cat (n = 13) and nixA::aphA3 (n = 3) single mutants were significantly less than that of the wild type (n = 7) (P < 0.001 and P < 0.01, respectively). The urease activity of the H. pylori 26695 HP1512::cat nixA::aphA3 (n = 3) double mutant was significantly less (P < 0.001) than the wild type but not significantly different that that of each single mutant.