Potential involvement of several nitroreductases in metronidazole resistance in Helicobacter pylori

Susceptibility of Helicobacter pylori to the antibiotic metronidazole has been attributed to the activity of an oxygen-insensitive NADPH-dependent nitroreductase (RdxA), with resistance to this antimicrobial arising from null mutations in rdxA. To obtain a better understanding of the factors involved in resistance, nitroreductase and metronidazole reduction activities were investigated in matched pairs of clinical and laboratory-derived sensitive and resistant H. pylori strains. Significant differences in enzyme activities were observed between sensitive and resistant strains, suggesting that metronidazole susceptibility in H. pylori was associated with more than one enzyme activity. To establish the mutations occurring in rdxA, the genes from seventeen bacterial strains, including matched pairs were sequenced. To assess whether metronidazole was responsible for inducing random mutations in this gene, the complete nucleotide sequence of gene hp0630, encoding an NAD(P)H-quinone reductase which also has NADPH-dependent nitroreductase activity, was determined in the same strains. All resistant strains showed nonsense, missense, or frameshift mutations randomly throughout rdxA. In contrast, no mutations were observed in hp0630. The results confirmed the presence of rdxA null mutations in resistant strains and suggested that other factors involved in the metabolism of metronidazole contributed to the resistant phenotype.