Comparison of E-test with agar dilution methods in testing susceptibility of N. gonorrhoeae to azithromycin

BACKGROUND AND OBJECTIVES: The antimicrobial susceptibility pattern of Neisseria gonorrhoeae varies from one country to another and may also change with time. To monitor these variations and changes, it is desirable to have a method that is simple and reproducible. This study was undertaken to determine the in vitro susceptibility of N. gonorrhoeae to azithromycin and to assess the reliability of results obtained using E-test methodology for determination of the minimum inhibitory concentration (MIC) of azithromycin.

STUDY DESIGN: The MICs for 135 clinical isolates of N. gonorrhoeae were determined by a modified Kirby-Bauer method recommended by the National Committee for Clinical Laboratory Standards against penicillin, cefuroxime, ceftriaxone, norfloxacin, tetracycline, kanamycin, spectinomycin, and azithromycin. The MIC of azithromycin was determined by both the E-test and agar dilution method. All tests were done simultaneously.

RESULTS: The MIC of azithromycin to all 135 isolates ranged from 0.078 to 0.25 microgram/ml with the agar dilution method and from 0.016 to 0.50 microgram/ml with the E-test. The MIC50 and MIC90 of azithromycin were 0.064 microgram/ml and 0.125 microgram/ml, respectively, by the agar dilution method, whereas they are slightly higher by the E-test method. Seventy-six of the isolates were beta-lactamase producers and 69 were high-level tetracycline-resistant N. gonorrhoeae. There was no difference in the MIC50 and MIC90 of azithromycin in these groups of isolates. The percentage agreement within the acceptable +/-1 log2 dilution difference between MICs obtained by E-test and those obtained by the agar dilution method was 97.8%.

CONCLUSIONS: Azithromycin has a very good in vitro antigonococcal activity, and the E-test is a reliable method to determine the MIC of azithromycin against N. gonorrhoeae.