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Journal of Clinical Microbiology, September 2007, p. 2917-2922, Vol. 45, No. 9
0095-1137/07/$08.00+0     doi:10.1128/JCM.00913-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Accuracy of Six Antimicrobial Susceptibility Methods for Testing Linezolid against Staphylococci and Enterococci

Centers for Disease Control and Prevention, Atlanta, Georgia 30333,¹ Rollins School of Public Heath, Emory University, Atlanta, Georgia 30322,² Micromyx, Kalamazoo, Michigan 49008³

Received 2 May 2007/ Returned for modification 18 June 2007/ Accepted 9 July 2007

A challenge panel of enterococci (n = 50) and staphylococci (n = 50), including 17 and 15 isolates that were nonsusceptible to linezolid, respectively, were tested with the Clinical and Laboratory Standards Institute broth microdilution and disk diffusion reference methods. In addition, all 100 isolates were tested in parallel by Etest (AB Biodisk, Solna, Sweden), MicroScan WalkAway (Dade, West Sacramento, CA), BD Phoenix (BD Diagnostic Systems, Sparks, MD), VITEK (bioMérieux, Durham, NC), and VITEK 2 (bioMérieux) by using the manufacturers’ protocols. Compared to the results of the broth microdilution method for detecting linezolid-nonsusceptible staphylococci and enterococci, MicroScan results showed the highest category agreement (96.0%). The overall categorical agreement levels for VITEK 2, Etest, Phoenix, disk diffusion, and VITEK were 93.0%, 90.0%, 89.6%, 88.0%, and 85.9%, respectively. The essential agreement levels (results within ±1 doubling dilution of the MIC determined by the reference method) for MicroScan, Phoenix, VITEK 2, Etest, and VITEK were 99.0%, 95.8%, 92.0%, 92.0%, and 85.9%, respectively. The very major error rates for staphylococci were the highest for VITEK (35.7%), Etest (40.0%), and disk diffusion (53.3%), although the total number of resistant isolates tested was small. The very major error rate for enterococci with VITEK was 20.0%. Three systems (MicroScan, VITEK, and VITEK 2) provided no interpretations of nonsusceptible results for staphylococci. These data, from a challenge panel of isolates, illustrate that the recent emergence of linezolid-nonsusceptible staphylococci and enterococci is providing a challenge for many susceptibility testing systems.

Published ahead of print on 18 July 2007.

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