Comparative evaluation of two methods for testing metronidazole susceptibility of Helicobacter pylori in routine practice.

Applicabilities of modified disk diffusion method (MDDM) and screening agar method (SAM) were evaluated in order to detect metronidazole resistance in vitro among 96 Helicobacter pylori strains. SAM was based on 8-mg/liter metronidazole as breakpoint, and MDDM was performed using disks containing 5 micrograms of the drug. Of 96 strains tested, 48 (50%) exhibited inhibitory zones of 26 mm or less by MDDM, and 44 (45.8%) were found resistant to 8 mg/liter metronidazole. Colonies growing within the inhibitory zones by MDDM were detected in six (6.2%) of all strains tested, and the subcultures revealed resistance by both methods. Overall agreement between the results of both tests was found in 90 (93.8%) of the 96 isolates. There is a possibility of great discrepancies between the two methods for testing H. pylori against metronidazole, so the results must be interpreted with caution.