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Journal of Clinical Microbiology, October 1999, p. 3296-3299, Vol. 37, No. 10
0095-1137/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

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

Linda M. Kelly,1 Michael R. Jacobs,2 and Peter C. Appelbaum1,*

Departments of Pathology (Clinical Microbiology), Hershey Medical Center, Hershey, Pennsylvania 17033,1 and Case Western Reserve University, Cleveland, Ohio 441062

Received 29 April 1999/Returned for modification 15 June 1999/Accepted 25 June 1999

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 CO2). By the three MIC tests, the MICs at which 50 and 90% of isolates were inhibited (MIC50s and MIC90s) 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 log2 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.


* Corresponding author. Mailing address: Department of Pathology, Hershey Medical Center, 500 University Dr., Hershey, PA 17033. Phone: (717) 531-5113. Fax: (717) 531-7953. E-mail: pappelbaum{at}psghs.edu.


Journal of Clinical Microbiology, October 1999, p. 3296-3299, Vol. 37, No. 10
0095-1137/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.



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