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JAC Advance Access originally published online on January 10, 2009
Journal of Antimicrobial Chemotherapy 2009 63(3):493-496; doi:10.1093/jac/dkn527
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© The Author 2009. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Original research

Comparison of three phenotypic techniques for detection of methicillin resistance in Staphylococcus spp. reveals a species-dependent performance

Michael A. John1,2, Julia Burden1, J. Ian Stuart1, Romina C. Reyes1, Robert Lannigan1,2, Sue Milburn1, Deb Diagre1, Bev Wilson1 and Zafar Hussain1,2,*

1 London Health Sciences Centre, London, Ontario, Canada 2 Department of Microbiology and Immunology, Schulich School of Medicine and Dentistry, University of Western Ontario, London, Ontario, Canada

Received 29 October 2008; returned 25 November 2008; revised 28 November 2008; accepted 7 December 2008

* Corresponding author. E3-316, Victoria Hospital, 800 Commissioner’s Road, London, Ontario, Canada N6C 6B5. Tel: +1-529-685-8149; Fax: +1-519-685-8203; E-mail: zafar.hussain{at}lhsc.on.ca

Objectives: To evaluate the usefulness of the cefoxitin screen in Vitek® 2 Gram-positive panels for recognizing methicillin-resistant strains of staphylococci.

Methods: Seven hundred and ninety-nine non-duplicate isolates of Staphylococcus aureus and coagulase-negative strains were included in the study. Methicillin resistance was measured using PCR for the mecA gene, the CLSI cefoxitin disc diffusion method, the Vitek® 2 cefoxitin screen and the Vitek® 2 oxacillin susceptibility test.

Results: Compared with the molecular detection of methicillin resistance the overall sensitivities and specificities of the phenotypic tests for cefoxitin disc diffusion were 94.9% and 97.0%, for Vitek® 2 cefoxitin screen were 94.6% and 93.5% and for Vitek® 2 oxacillin susceptibility test were 93.8% and 77.9%. The cephamycin tests (cefoxitin disc diffusion and Vitek® 2 screen) were not able to identify mecA-positive strains of Staphylococcus simulans. In addition, the performance of the Vitek® 2 system was poor against Staphylococcus cohnii subspecies, Staphylococcus hominis hominis and Staphylococcus saprophyticus.

Conclusions: Overall, the performance of the Vitek® 2 system for differentiating mecA-positive staphylococci was comparable to PCR and the CLSI disc diffusion method; however, performance was species-dependent. Thus, before accepting the results produced by Vitek® 2, species identification may be required.

Keywords: susceptibility screen , cefoxitin screen , detection of methicillin susceptibility in staphylococci


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