Subpopulations of Helicobacter pylori are responsible for discrepancies inthe outcome of nitroimidazole susceptibility testing

Metronidazole susceptibility testing by E test,vas compared to that by disk diffusion for 263 Helicobacter pylori isolates and to that by breakpoint a gar dilution for 90 H. pylori isolates. In 5% and 6% of the cases, respecti vely. results were discrepant. For each of 52 clinical isolates an E test w as performed on 10 separate colonies. Subpopulations of resistant and susce ptible bacteria were found in five cases, From three isolates, each colony was subcultured and tested up to 10 times. All but 1 of 292 tests showed th e same result. We conclude that the E test is reliable and that subpopulati ons are responsible for discordant results.