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Journal of Clinical Microbiology, April 2000, p. 1444-1448, Vol. 38, No. 4
0095-1137/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Antipneumococcal Activity of Telithromycin by Agar Dilution, Microdilution, E Test, and Disk Diffusion Methodologies

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

Department of Pathology, Hershey Medical Center, Hershey, Pennsylvania 17033,1 and Department of Pathology, Case Western Reserve University, Cleveland, Ohio 441062

Received 13 October 1999/Returned for modification 26 November 1999/Accepted 3 January 2000

Agar dilution and microdilution (both in air) and E test and disk diffusion (both in air and CO2) were used to test the activity of telithromycin against 110 erythromycin-susceptible and 106 erythromycin-resistant pneumococci. The MICs at which 50 and 90% of strains are inhibited (MIC50s and MIC90s, respectively) for erythromycin-susceptible strains varied between 0.008 and 0.016 µg/ml and 0.016 and 0.03 µg/ml when the samples were incubated in air. By comparison, telithromycin MIC50s and MIC90s for erythromycin-resistant strains were in air 0.03 to 0.125 and 0.125 to 0.5 µg/ml, respectively. When agar dilution was used as the reference method, essential agreement was found for 112 of 216 strains (51.9%) for microdilution, 168 of 216 (77.8%) for E test in air, and 132 of 216 (61.1%) for E test in CO2. With the exception of four strains tested by E test in CO2, all organisms were susceptible to a proposed telithromycin susceptibility breakpoint of <= 1 µg/ml. By disk diffusion with 15-µg telithromycin disks, all strains but one had zones of inhibition >= 19 mm in diameter when incubated in CO2, while all strains had zone diameters of >= 22 mm when incubated in air. Zone diameters in air were generally 4 to 5 mm larger than in CO2. By all methods, MICs and zones of all erythromycin-resistant strains occurred in clusters separated from those seen with erythromycin-susceptible strains. The results for macrolide-resistant strains with erm and mef resistance determinants were similar. The results show that (i) telithromycin is very active against erythromycin-susceptible and -resistant strains irrespective of macrolide resistance mechanism; (ii) susceptibility to telithromycin can be reliably tested by the agar, microdilution, E test, and disk diffusion methods; and (iii) incubation in CO2 led to smaller zones by disk diffusion and higher MICs by E test, but at a susceptible MIC breakpoint of <= 1 µg/ml and a susceptible zone diameter cutoff of >= 19 mm in CO2, 215 of 216 strains were found to be susceptible to telithromycin.

* 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, April 2000, p. 1444-1448, Vol. 38, No. 4
0095-1137/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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