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Journal of Clinical Microbiology, November 2001, p. 3946-3951, Vol. 39, No. 11
0095-1137/01/$04.00+0   DOI: 10.1128/JCM.39.11.3946-3951.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Methicillin-Resistant Staphylococcus aureus: Comparison of Susceptibility Testing Methods and Analysis of mecA-Positive Susceptible Strains

George Sakoulas,1,* Howard S. Gold,1 Lata Venkataraman,2 Paola C. DeGirolami,2 George M. Eliopoulos,1 and Qinfang Qian2

Division of Infectious Diseases, Department of Medicine,1 and Division of Laboratory and Transfusion Medicine, Department of Pathology,2 Beth Israel Deaconess Medical Center, Boston, Massachusetts

Received 24 May 2001/Returned for modification 20 July 2001/Accepted 14 August 2001

Methicillin-resistant Staphylococcus aureus (MRSA) is responsible for an increasing number of serious nosocomial and community-acquired infections. Phenotypic heterogeneous drug resistance (heteroresistance) to antistaphylococcal beta-lactams affects the results of susceptibility testing. The present study compared the MRSA-Screen latex agglutination test (Denka Seiken Co., Ltd., Tokyo, Japan) for detection of PBP 2a with agar dilution, the VITEK-1 and VITEK-2 systems (bioMérieux, St. Louis, Mo.), and the oxacillin agar screen test for detection of MRSA, with PCR for the mecA gene used as the "gold standard" assay. Analysis of 107 methicillin-susceptible S. aureus (MSSA) isolates and 203 MRSA isolates revealed that the MRSA-Screen latex agglutination test is superior to any single phenotype-based susceptibility testing method, with a sensitivity of 100% and a specificity of 99.1%. Only one isolate that lacked mecA was weakly positive by the MRSA-Screen latex agglutination test. This isolate was phenotypically susceptible to oxacillin and did not contain the mecA gene by Southern blot hybridization. The oxacillin agar screen test, the VITEK-1 system, the VITEK-2 system, and agar dilution showed sensitivities of 99.0, 99.0, 99.5, and 99%, respectively, and specificities of 98.1, 100, 97.2, and 100%, respectively. The differences in sensitivity or specificity were not statistically significant. Oxacillin bactericidal assays showed that mecA- and PBP 2a-positive S. aureus isolates that are susceptible to antistaphylococcal beta-lactams by conventional methods are functionally resistant to oxacillin. We conclude that the accuracy of the MRSA-Screen latex agglutination method for detection of PBP 2a approaches the accuracy of PCR and is more accurate than any susceptibility testing method used alone for the detection of MRSA.

* Corresponding author. Mailing address: Division of Infectious Diseases, Beth Israel Deaconess Medical Center, Kennedy Building, 6th Floor, 330 Brookline Ave., Boston, MA 02215. Phone: (617) 632-0760. Fax: (617) 632-0766. E-mail: gsakoula{at}caregroup.harvard.edu.

Journal of Clinical Microbiology, November 2001, p. 3946-3951, Vol. 39, No. 11
0095-1137/01/$04.00+0   DOI: 10.1128/JCM.39.11.3946-3951.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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Copyright © 2001 by the American Society for Microbiology. All rights reserved.