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JAC Advance Access published online on May 12, 2005
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dki156
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© The Author 2005. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oupjournals.org
Received February 10, 2005
Revised April 12, 2005
Accepted April 14, 2005

Brief report

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

Sam K. Bouchillon 1*, Jack L. Johnson 1, Daryl J. Hoban 1, Tim M. Stevens 1, and Brian M. Johnson 1

1 Laboratories International for Microbiology Studies, Inc., 2122 Palmer Drive, Schaumburg, IL 60173-3817, USA

* To whom correspondence should be addressed.
Sam K. Bouchillon, E-mail: sbouchillon{at}ihmainc.com


  Abstract

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.
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