Impact of psm-mec in the mobile genetic element on the clinical characteristics and outcome of SCCmec-II methicillin-resistant Staphylococcus aureus bacteraemia in Japan

Over-expression of alpha-phenol-soluble modulins (PSMs) results in high virulence of community-associated methicillin-resistant Staphylococcus aureus (MRSA). The psm-mec gene, located in the mobile genetic element SCCmec-II, suppresses PSMs production. Fifty-two patients with MRSA bacteraemia were enrolled. MRSA isolates were evaluated with regard to the psm-mec gene sequence, bacterial virulence, and the minimum inhibitory concentration (MIC) of vancomycin and teicoplanin. Fifty-one MRSA isolates were classified as SCCmec-II, and 10 had one point mutation in the psm-mec promoter. We compared clinical characteristics and outcomes between mutant MRSA and wild-type MRSA. Production of PSM3 in mutant MRSA was significantly increased, but biofilm formation was suppressed. Wild-type MRSA caused more catheter-related bloodstream infections (30/41 vs. 3/10, p0.0028), whereas mutant MRSA formed more deep abscesses (4/10 vs. 3/41, p0.035). Bacteraemia caused by mutant MRSA was associated with reduced 30-day mortality (1/10 vs. 13/41, p0.25), although this difference was not significant. The MIC90 of teicoplanin was higher for wild-type MRSA (1.5mg/L vs. 1mg/L), but the MIC of vancomycin was not different between the two groups. The 30-day mortality of MRSA with a high MIC of teicoplanin (1.5mg/L) was higher than that of strains with a lower MIC (0.75mg/L) (6/10 vs. 6/33, p0.017). Mutation of the psm-mec promoter contributes to virulence of SCCmec-II MRSA, and the product of psm-mec may determine the clinical characteristics of bacteraemia caused by SCCmec-II MRSA, but it does not affect mortality.