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JAC Advance Access originally published online on February 18, 2009
Journal of Antimicrobial Chemotherapy 2009 63(4):699-703; doi:10.1093/jac/dkp019
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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

Effects of temperature on the detection of methicillin resistance in Staphylococcus aureus using cefoxitin disc diffusion testing with Iso-Sensitest agar

R. Skov1,*, R. Smyth2, A. Yusof3, A. Karlsson3, K. Mills3, N. Frimodt-Moller1 and G. Kahlmeter2,4

1 National Center for Antimicrobials and Infection Control, Statens Serum Institut, Copenhagen, Denmark 2 Department of Clinical Microbiology, Central Hospital, Växjö, Sweden 3 AB BIODISK Research Laboratories, Solna, Sweden 4 Department of Medical Sciences, Uppsala University, Uppsala, Sweden

Received 23 November 2008; returned 12 December 2008; revised 9 January 2009; accepted 10 January 2009

* Corresponding author. Tel: +45-3268-8348; Fax: +45-3268-3873; E-mail: rsk{at}ssi.dk

Objectives: Cefoxitin is today the substance of choice for the phenotypic detection of methicillin-resistant Staphylococcus aureus (MRSA). We investigated the influence of incubation temperature in the standard range, i.e. 35–37°C, and time, i.e. 18–20 h, versus a full 24 h.

Methods: Cefoxitin disc testing was examined at incubation temperatures of 35 and 36°C and times of 18–20 and 24 h, respectively, for 94 mecA-negative and 49 mecA-positive S. aureus on Iso-Sensitest agar using a semi-confluent inoculum.

Results: Cefoxitin inhibition zones on Iso-Sensitest agar were larger at temperatures above 35°C; two isolates (4%, 95% confidence interval=0.5–14%) incubated at 36°C were falsely categorized as susceptible to methicillin. Incubation time across 18–24 h did not impact results.

Conclusions: Detection of methicillin resistance in S. aureus using the cefoxitin disc method with a semi-confluent inoculum on Iso-Sensitest agar is influenced by incubation temperature, and the temperature should not exceed 35°C for the reliable detection of MRSA.

Keywords: susceptibility testing , MRSA , S. aureus


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