In vitro activity of metronidazole against Helicobacter pylori as determined by agar dilution and agar diffusion

Metronidazole activity against 25 clinical isolates of Helicobacter pylori was evaluated by agar dilution, epsilometer (E-test; AB Biodisk, Solna, Sweden), and disk diffusion methods after 3 and 5 days of incubation in a microaerophilic atmosphere. Agar dilution, performed in duplicate, provided reproducible results with MICs for 50% of the isolates of less than or equal to 0.12 microgram/ml after 3 and 5 days of incubation and MICs for 90% of the isolates of 2 and 4 micrograms/ml after 3 and 5 days of incubation, respectively. Reproducibility of MICs was slightly better after 5 days than after 3 days of incubation. MICs obtained with the E-test were higher, with 76 and 68% of isolates inhibited by less than or equal to 16 micrograms of metronidazole per ml after 3 and 5 days, respectively, in contrast with corresponding values of 92 and 88% for agar dilution. Zone diameters obtained with the commercially available 80-micrograms metronidazole elution disk were too large (greater than or equal to 41 mm) to allow discrimination between susceptible and resistant isolates, although resistant subpopulations were detected by the appearance of inner colonies in four isolates. In conclusion, the E-test was easy to perform and interpret, and it appeared to be more likely than agar dilution to detect metronidazole resistance in vitro in H. pylori.