EVALUATION OF IN-VITRO SPECTRA OF ACTIVITY OF AZITHROMYCIN, CLARITHROMYCIN, AND ERYTHROMYCIN TESTED AGAINST STRAINS OF NEISSERIA-GONORRHOEAE BY REFERENCE AGAR DILUTION, DISK DIFFUSION, AND ETEST METHODS

The macrolide-azilide susceptibility testing (agar dilution, disk diff usion, Etest) criteria for 105 Neisseria gonorrhoeae strains were eval uated. In addition, the potencies of azithromycin, clarithromycin, and erythromycin were studied. The most active macrolide-azilide agent wa s azithromycin (MIC at which 90% of the isolates are inhibited [MIG(90 )], 0.5 mu g/ml) compared with clarithromycin (MIC(90), 1.5 to 2 mu g/ ml) and erythromycin (MIC(90), 2 to 4 mu g/ml). The Etest (AB Biodisk, Solna, Sweden) was observed to produce MIC results very similar to th ose of the reference agar dilution test (GC agar base), with 100% of t he results within 1 log(2) dilution step of the reference MICs. The di sk diffusion test zone diameters for all three drugs correlated at an acceptable level (r = -0.81 to -0.92) with the reference agar dilution MICs. Interpretive criteria for susceptibility were proposed for azit hromycin at a MIC of less than or equal to 2 mu g/ml and a disk diffus ion test zone of greater than or equal to 25 mm. No category for resis tance was proposed because of the paucity of strains for which MICs we re >2 mu g/ml. These tentative criteria should be further validated by correlations with clinical trial data for gonococcal strains (as they emerge) that have azithromycin MICs above the proposed susceptible ca tegory range.