The E test for antimicrobial susceptibility testing of Helicobacter pylori

Antimicrobial susceptibility testing of Helicobacter pylori by conventional means (disc diffusion) is of questionable value because the bacterium grows so slowly. We have determined the susceptibilities of 20 isolates of H. pylori to ampicillin, erythromycin, gentamicin, metronidazole, rifampicin and tetracycline by the E test, a novel technique for measuring MICs, and compared the results with those determined by standard agar dilution and disc diffusion methods. Eighty-one per cent of E test-determined MICs were within one two-fold dilution and 93% were within two two-fold dilutions of those determined by the reference agar dilution method. MICs determined by the E test were highly reproducible; replicate results were invariably within one two-fold dilution. MICs were not affected by the size of the inoculum, preincubation or incubation times. Regression lines for disc diffusion could not be determined because of the lack of resistant strains. However, metronidazole-resistant strains (MIC > 32 mg/L) were easily distinguishable from susceptible strains (MIC < 1 mg/L). In conclusion, the E test is a reliable method for determining the antimicrobial susceptibility of H. pylori.