Comparison of Agar Dilution, Microdilution, E-Test, and Disk Diffusion Methods for Testing Activity of Cefditoren against Streptococcus pneumoniae

This study evaluated the susceptibility of pneumococci to cefditoren by agar dilution and microdilution methods (both in air) and by E-test (AB Biodisk, Solna, Sweden) and disk diffusion methods (both in CO(2)). By the three MIC tests, the MICs at which 50 and 90% of isolates were inhibited (MIC(50)s and MIC(90)s) were, respectively, as follows (in micrograms per milliliter): for the 65 penicillin-susceptible strains tested, 0.016 and 0.03 (by agar dilution), 0.016 and 0.03 (by microdilution), and 0.016 and 0.03 (by E test); for the 68 penicillin-intermediate strains tested, 0.125 and 0.5 (by agar dilution), 0.125 and 0.5 (by microdilution), and 0.25 and 0.5 (by E test); and for the 67 penicillin-resistant strains tested, 1.0 and 1.0 (by agar dilution), 0.5 and 1.0 (by microdilution), and 1.0 and 1.0 (by E test). With tentative cefditoren breakpoints (in micrograms per milliliter) of ≤2.0 (susceptible), 4.0 (intermediate), and ≥8.0 (resistant), all strains were susceptible to cefditoren by agar, microdilution, and E-test results; with breakpoints of ≤1.0, 2.0, and ≥4.0 μg/ml, 97% of strains were cefditoren susceptible by agar dilution results, 98% were susceptible by microdilution results, and 99% were susceptible by E-test results. When microdilution and E-test results were compared to those from the reference agar dilution method, 191 (95.5%) and 183 (91.5%) of strains gave essential agreement (±1 log(2) dilution); 8 (2.7%) minor discrepancies were found for both methods with a breakpoint of ≤1.0 μg/ml, and no discrepancies were found with a breakpoint of ≤2.0 μg/ml. Disk test results (breakpoint, ≤1.0 μg/ml) produced 2 major and 30 minor errors, with corresponding zone diameters (in millimeters) of ≥20 (susceptible), 17 to 19 (intermediate), and ≤16 (resistant); a ≤2.0-μg/ml breakpoint yielded zone diameters of ≥16 mm (susceptible). All three methods for testing the MIC of cefditoren showed excellent correlation.