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Journal of Clinical Microbiology, November 2004, p. 5041-5046, Vol. 42, No. 11
0095-1137/04/$08.00+0     DOI: 10.1128/JCM.42.11.5041-5046.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Evaluation of the BD PHOENIX Automated Microbiology System for Detection of Methicillin Resistance in Coagulase-Negative Staphylococci

Matthias A. Horstkotte,1* Johannes K.-M. Knobloch,1 Holger Rohde,1 Sabine Dobinsky,1 and Dietrich Mack1,2

Institut für Infektionsmedizin, Zentrum für Klinisch-Theoretische Medizin, Universitätsklinikum Hamburg-Eppendorf, Hamburg, Germany,1 Chair of Medical Microbiology and Infectious Diseases, The Clinical School, University of Wales Swansea, Swansea, United Kingdom2

Received 14 April 2004/ Returned for modification 31 May 2004/ Accepted 24 July 2004

The new BD PHOENIX automated microbiology system (Becton Dickinson Diagnostic Systems, Sparks, Md.) is designed for automated rapid antimicrobial susceptibility testing and identification of clinically relevant bacteria. In our study, the accuracy and speed of the BD PHOENIX oxacillin MIC determination for detecting methicillin resistance was evaluated for 200 clinical isolates of coagulase-negative staphylococci (CoNS). Compared to mecA PCR, the BD PHOENIX system detected methicillin resistance with a sensitivity of 99.2%. According to the actual NCCLS oxacillin MIC breakpoint of ≥0.5 µg/ml, the specificity was only 64.9%, attributable to false-positive results in 26 mecA-negative strains, including 16 non-Staphylococcus epidermidis strains. Alternative oxacillin breakpoints of ≥1, ≥2, and ≥4 µg/ml resulted in increased specificities of 83.8, 94.6, and 100% and high sensitivities of 99.2, 99.2, and 96.7%, respectively. Similarly, NCCLS broth microdilution oxacillin MICs exhibited a sensitivity of 100% but a low degree of specificity. However, the previous oxacillin MIC breakpoint of ≥4 µg/ml performed with a sensitivity of 98.4% and a specificity of 98.7%. BD PHOENIX oxacillin MIC results were available after 9 h for 40.5% of the examined CoNS strains and were completed after 17 h. Our results revealed the high reliability of the BD PHOENIX system as a phenotypic method for detection of resistance to oxacillin in mecA-positive CoNS. However, for the improvement of specificity, reevaluation of the optimal oxacillin MIC breakpoint for CoNS appears to be necessary.

* Corresponding author. Mailing address: Institut für Infektionsmedizin, Universitätsklinikum Hamburg-Eppendorf, Martinistrasse 52, D-20246 Hamburg, Germany. Phone: 49 40/42803-3147. Fax: 49 40/42803-4881. E-mail: horstkotte{at}uke.uni-hamburg.de.

Journal of Clinical Microbiology, November 2004, p. 5041-5046, Vol. 42, No. 11
0095-1137/04/$08.00+0     DOI: 10.1128/JCM.42.11.5041-5046.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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