Oxacillin-susceptible methicillin-resistant Staphylococcus aureus (OS-MRSA), a hidden resistant mechanism among clinically significant isolates in the Wessex region/UK

Purpose: Methicillin-resistant Staphylococcus aureus (MRSA) is defined as S. aureus genetically having the mecA or mecC genes or phenotypically showing minimum inhibitory concentration (MIC) of oxacillin higher than 2 mg/L. However, recently, cefoxitin/oxacillin-susceptible mecA-positive S. aureus (OS-MRSA) has been reported worldwide. Little is known about the prevalence and virulence of these strains among clinically significant isolates in the UK. The aims were to (1) investigate the prevalence of OS-MRSA in seven major hospitals in the Wessex region/UK from a cohort of 500 clinically significant phenotypically identified MSSA isolates, (2) genetically characterise OS-MRSA strains by pulsed-field gel electrophoresis (PFGE) and compare these to common UK epidemic strains; and (3) to determine Panton-Valentine leukocidin (PVL; lukFS) gene carriage rates among these isolates.

Results: OS-MRSA was found in six isolates (1.2 %) of phenotypically identified and reported MSSA isolates by conventional methods. PFGE showed OS-MRSA strains to be genetically diverse and distinct from the common UK epidemic strains EMRSA-15 and EMRSA-16. None of these OS-MRSA stains carried the genes encoding PVL; however, overall positivity rate for PVL was 4.4 %, much higher than the nationally reported rates of 2 % in the UK.

Conclusion: There are still many unknowns regarding phenotypic and/or genetic characterization of the emerging OS-MRSA isolates in the UK and worldwide. Data regarding their epidemiology and optimal therapy for infection are limited and need further investigation not only in the UK, but also worldwide, as it is likely to have an impact on the empirical treatment of S. aureus infections.