Antimicrobial susceptibility testing of Helicobacter pylori - Comparison of E-test, broth microdilution, and disk diffusion for ampicillin, clarithromycin, and metronidazole

The optimal method for the determination of the minimum inhibitory concentration (MIC) of antimicrobials against Helicobacter pylori has not been established. The epsilometer agar diffusion gradient test (E-Test; AB Biodisk, Solna, Sweden) was compared with broth microdilution, the reference method, and disk diffusion for the antimicrobial susceptibility testing of 122 clinical isolates of H. pylori to ampicillin, clarithromycin, and metronidazole. Isolates were considered to be resistant when the MIC value was >8 mu g/ml for either ampicillin or metronidazole and >2 mu g/ml for clarithromycin. For an individual isolate, the MICs for ampicillin and clarithromycin determined by broth microdilution and the E-test were highly reproducible, with replicate results being within +/-1 log(2) dilution. The correlation between the MICs deter mined by E-test and broth microdilution was excellent for both ampicillin and clarithromycin (90.1% and 88.5% were within +/-log(2) dilution, and 98.3% and 96.7% of the values were within +/-2 log(2) dilution, respectively). In no instance did the interpretation of ''sensitive'' or ''resistant'' differ. Conversely, only 70.5% of the E-test results for metronidazole were within +/-1 log(2) dilution of the broth microdilution results. In addition, 15 (12.3%) of the H. pylori isolates interpreted as resistant by the E-test were sensitive by the broth microdilution method. All discrepancies occurred when the E-test MIC values fell between 8 and 32 mu g/ml. The results of the ampicillin and clarithromycin disk diffusion assay correlated 100% with the results of the broth microdilution. However, these data suggest that when the E-test MIC results for metronidazole yield values between 8 and 32 mu g/ml, the MIC should be reevaluated by another method.