VALIDATION OF A MODIFIED KIRBY-BAUER DISK DIFFUSION METHOD FOR METRONIDAZOLE SUSCEPTIBILITY TESTING OF HELICOBACTER-PYLORI

Triple antimicrobial therapy that includes metronidazole has been recommended as a first-line therapy for Helicobacter pylori because if has the highest eradication rates. However, resistance in H. pylori to metronidazole has been reported worldwide and its presence may reduce the efficacy of triple therapy. Various methods for testing H. pylori against metronidazole have been used including agar dilution, disk diffusion and the Etest but there has been little standardization of methods. One hundred isolates of H. pylori from different patients were tested for susceptibility to metronidazole by agar dilution, Etest and disk diffusion (5 mu g disk). The agar dilution results confirmed the MIC susceptibility breakpoint to be less than or equal to 8 mu g/ml. Using this breakpoint there was close agreement (98%) between Etest and agar dilution results. For susceptible strains, MICs by E-test were generally one twofold dilution lower. Using the error-rate bounded method, agreement between disk diffusion zone diameter and MIC was 98% for agar dilution with breakpoints of greater than or equal to 12 mm and less than or equal to 8 mu g/ml and 100% for Etest with breakpoints of greater than or equal to 12 mm and less than or equal to 8 mu g/ml. The Etest discriminated better than agar dilution between susceptible and resistant strains and was simple to perform. The disk diffusion test is a reliable and cheaper alternative to the Etest with susceptibility being a zone diameter greater than or equal to 12 mm with a 5 mu g disk. The prevalence of metronidazole resistance in this study was 40% by Etest.