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Helicobacter pylori is a human-pathogenic bacterial species that is subdivided geographically, with different genotypes predominating in different parts of the world. Here we test and extend an earlier conclusion that metronidazole (Mtz) resistance is due to mutation in rdxA (HP0954), which encodes a nitroreductase that converts Mtz from prodrug to bactericidal agent. We found that (i) rdxA genes PCR amplified from 50 representative Mtz^r strains from previously unstudied populations in Asia, South Africa, Europe, and the Americas could, in each case, transform Mtz^sH. pylori to Mtz^r; (ii) Mtz^r mutant derivatives of a cultured Mtz^sstrain resulted from mutation in rdxA; and (iii) transformation of Mtz^s strains with rdxA-null alleles usually resulted in moderate level Mtz resistance (16 μg/ml). However, resistance to higher Mtz levels was common among clinical isolates, a result that implicates at least one additional gene …