STANDARDIZATION OF DISK DIFFUSION TEST AND ITS CLINICAL-SIGNIFICANCE FOR SUSCEPTIBILITY TESTING OF METRONIDAZOLE AGAINST HELICOBACTER-PYLORI

Susceptibilities of 121 clinical Helicobacter pylori strains to metron idazole were determined by both a 5-mu g metronidazole disk diffusion test and a plate dilution method in duplicate and after different peri ods of incubation. The distribution of MICs of metronidazole against H . pylori among the strains was found to be bimodal. The diameters of i nhibitory zones obtained by the disk diffusion test and the MICs obtai ned by the plate dilution method correlated well, especially after 4 d ays of incubation (r = 0.77). An inhibitory zone diameter of 20 mm was found to correspond to a MIC of 8 mu g/ml and is recommended as a sui table zone for differentiating susceptibility and resistance with a 5- mu g metronidazole disk. Three interpretive categories of susceptibili ty results were defined; strains with inhibitory zone diameters of mor e than 26 mm were defined as susceptible (MIC, < 4 mu g/ml), strains w ith zone diameters of 20 to 26 mm were deemed intermediate (MIC, 4 to 8 mu g/ml), and those with zone diameters of less than 20 mm were deem ed resistant (MIC, > 8 mu g/ml). Furthermore, 76 H. pylori-positive pa tients with duodenal ulcers or nonulcer dyspepsia were treated,vith a I week of triple therapy (colloidal bismuth subcitrate, metronidazole, and tetracycline). H. pylori strains were isolated before treatment f rom antral biopsies from those patients, and the metronidazole suscept ibilities of the strains were determined by the disk diffusion test. H . pylori status was evaluated again 4 weeks after completion of treatm ent. The eradication rates for susceptible, intermediate, and resistan t strains were 95.9% (47 of 49), 62.5% (5 of 8), and 52.6% (10 of 19), respectively. It is concluded that the 5-mu g disk diffusion test is easy to perform and gives final results similar to those of the plate dilution method. The three interpretive categories of susceptibility m ay be of benefit for clinical choice of chemotherapy in eradicating H. pylori.