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

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 1mupirocin 4 mg/L) and with the disc diffusion method using 10 mg mupirocin Neo-Sensitabs (MUP-10) and mupirocin paper discs of 5, 20 and 200 mg (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 mg 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