Mouse-Colonizing Helicobacter pylori SS1 Is Unusually Susceptible to Metronidazole Due to Two Complementary Reductase Activities

In most strains of Helicobacter pylori, mutational inactivation of the rdxA (HP0954) gene, which encodes a nitroreductase that converts metronidazole (MTZ) from a harmless prodrug to a mutagenic and bacteriocidal product, is sufficient to make this pathogen resistant to clinically significant levels of MTZ. Here we report that SS1, a strain with the special ability to colonize mice, is unusual in being susceptible to very low concentrations of MTZ (0.5 μg/ml) and in being especially difficult to mutate to MTZ resistance (Mtz(r)). These phenotypic traits were traced to expression in this strain of the normally quiescent H. pylori frxA gene (HP0642, an rdxA paralog) along with rdxA. Transformation tests using rdxA::cam and frxA::kan insertion mutant DNAs, with selection solely for the chloramphenicol and kanamycin resistance markers, and sequence analyses of frxA in spontaneous Mtz(r) derivatives of rdxA null mutant strains each showed that the development of Mtz(r) in SS1 required inactivation of both rdxA and frxA. Inactivation of either gene alone left SS1 susceptible to MTZ, although it was readily mutable from an MTZ-susceptible to an Mtz(r) phenotype. Reverse transcriptase PCR tests showed that frxA mRNA was at least 10-fold more abundant in SS1 than in reference strain 26695. It is proposed that these reductases play primarily nutritional roles during bacterial growth.