Journal Article

Impact of carbon dioxide on the susceptibility of key respiratory tract pathogens to telithromycin and azithromycin

Sam K. Bouchillon, Jack L. Johnson, Daryl J. Hoban, Tim M. Stevens and Brian M. Johnson

in Journal of Antimicrobial Chemotherapy

Published on behalf of British Society for Antimicrobial Chemotherapy

Volume 56, issue 1, pages 224-227
Published in print July 2005 | ISSN: 0305-7453
Published online April 2005 | e-ISSN: 1460-2091 | DOI:
Impact of carbon dioxide on the susceptibility of key respiratory tract pathogens to telithromycin and azithromycin

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Objectives: To determine the quantitative differences in telithromycin and azithromycin MIC values against Streptococcus pneumoniae, Haemophilus influenzae and Streptococcus pyogenes obtained using two recommended and commonly used methodologies: CLSI reference standard broth microdilution in ambient air and Etest® concentration gradient in CO2.

Methods: Four hundred clinical isolates (S. pneumoniae, n=200; H. influenzae, n=100; S. pyogenes, n=100) were evaluated in seven independent laboratories. Telithromycin and azithromycin MICs were determined using CLSI broth microdilution panels incubated in ambient air and Etest® strips incubated in CO2. Standard quality control reference strains—S. pneumoniae ATCC 49619 (n=10) and H. influenzae ATCC 49247 (n=10)—were also tested.

Results: Telithromycin and azithromycin Etest® MICs in CO2 were elevated for all organisms when compared with values obtained using broth microdilution in ambient air. Telithromycin geometric mean MIC values increased in CO2 by 2.05, 1.00 and 1.78 log2 dilutions for S. pneumoniae, H. influenzae and S. pyogenes, respectively. The corresponding values for azithromycin were 2.54, 1.21 and 3.0 log2 dilutions, respectively.

Conclusions: Telithromycin MICs measured using Etest® in CO2 are consistently elevated compared with those generated by CLSI broth microdilution measured in ambient air. These findings indicate that Etest® should not be routinely used for the determination of telithromycin MICs against S. pneumoniae, H. influenzae and S. pyogenes, unless appropriate corrective factors are applied before reporting MICs or applying interpretive susceptibilities. Based on results from this study, Etest® MIC breakpoints and quality control ranges are proposed.

Keywords: ketolides; S. pneumoniae; H. influenzae; S. pyogenes; susceptibility testing

Journal Article.  1985 words. 

Subjects: Medical Oncology ; Critical Care

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