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JAC Advance Access originally published online on September 1, 2008
Journal of Antimicrobial Chemotherapy 2008 62(5):1018-1023; doi:10.1093/jac/dkn345
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© The Author 2008. 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

Evaluation of disc diffusion methods and Vitek 2 automated system for testing susceptibility to mupirocin in Staphylococcus aureus

Xavier Malaviolle*, Claire Nonhoff, Olivier Denis, Sylvianne Rottiers and Marc J. Struelens

Laboratoire de Référence MRSA-Staphylocoques, Department of Microbiology, Hôpital Erasme, Université Libre de Bruxelles, Brussels, Belgium

Received 22 May 2008; returned 26 June 2008; revised 4 August 2008; accepted 4 August 2008

* Corresponding author. Tel: +32-2-555-45-18; Fax: +32-2-555-31-10; E-mail: xmalavio{at}ulb.ac.be

Objectives: To compare the performance of the automated Vitek 2 system, the disc diffusion method and a home-made mupirocin screen agar (MSA) to detect mupirocin resistance in methicillin-resistant Staphylococcus aureus (MRSA) isolates.

Methods: A total of 125 MRSA isolates were tested. The level of mupirocin resistance was determined by agar dilution and Etest techniques (gold standard), by the Vitek 2 system, on MSA (Mueller–Hinton + mupirocin 4 mg/L) and with the disc diffusion method using 10 µg mupirocin Neo-Sensitabs (MUP-10) and mupirocin paper discs of 5, 20 and 200 µg (MUP-5, MUP-20 and MUP-200). High-level mupirocin resistance (HLMR) was confirmed by PCR for the mupA gene.

Results: Thirty-two MRSA isolates showed HLMR (MIC ≥512 mg/L) and harboured the mupA gene, 39 strains showed low-level mupirocin resistance (LLMR) (8–32 mg/L) without the mupA gene and 54 were susceptible without the mupA gene. The sensitivity and the specificity of the Vitek 2 system and the screening medium (MSA) for the detection of mupirocin resistance was 100%. The diffusion method using 5 and 10 µg discs demonstrated a sensitivity of 100% and a specificity of 98.1% and 100%, respectively. Using interpretative criteria of 6 and 17 mm, the MUP-20 disc showed the best classification concordance with reference methods.

Conclusions: The diffusion method using low-content discs or the Vitek 2 microdilution system showed excellent agreement with MICs and PCR results to separate mupirocin-susceptible from -resistant MRSA strains. Disc diffusion with MUP-20 or combined use of low and high mupirocin content discs enabled the classification of susceptibility categories (susceptible, LLMR and HLMR) but required overnight incubation compared with 12 h for the Vitek 2 system.

Keywords: MRSA , mupirocin resistance , low-level resistance , topical decolonization therapy


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