Comparison of the effects of anaerobic and microaerophilic incubation on resistance of Helicobacter pylori to metronidazole

To assess the influence of incubation conditions on the resistance of Helicobacter pylori this study compared the effect of micro-aerophilic and anaerobic incubation followed by micro-aerophilic incubation on the measurement of metronidazole resistance of 102 H. pylori isolates, by both disk diffusion and Epsilometer (E)-tests. Anaerobic incubation for 24 h before micro-aerophilic incubation for 48 h consistently increased metronidazole activity in both assay methods. Although statistically significant, this was microbiologically less significant, as only 4 of 102 isolates gave discrepant readings (all four were resistant in micro-aerophilic conditions but susceptible in anaerobic/micro-aerophilic conditions). In all four cases variation was by a few millimeters in zone size (i.e., all were close to the cut-off point). There was 100% agreement between disk diffusion and E-test results. Of 104 observations (52 duplicate assays: 13 strains, two atmospheric conditions, two methods of determining resistance) there was 100% intra-observer and inter-observer agreement with regard to susceptibility and resistance status for both E-test and disk diffusion methods. Anaerobic incubation followed by micro-aerophilic incubation had little effect on the estimation of prevalence of metronidazole resistance and seemed to add little, if any, significant advantage over micro-aerophilic incubation alone.