The most prevalent community-associated methicillin-resistant Staphylococcus aureus (C-MRSA) strains in Taiwan, ST59 clones, carry staphylococcal cassette chromosome mec (SCCmec) type V and, to a lesser extent, type IV. These strains show wide variation in sensitivity to oxacillin, but the reasons for this variation are unknown. Here we compared the sequence of the mecA gene from different SCCmec type clinical strains and found they contain different mecA promoter mutations. Analysis of mecA promoter activity by reporter-gene fusions showed that single base substitutions in the promoter have a strong influence on mecA transcription. The different mecA variants including promoter sequences were expressed in the methicillin-sensitive Staphylococcus aureus (MSSA) strain C195 (ST59 background). PBP2a production among the parental and promoter mutant mecA genes showed a close correlation with mecA transcription levels. Furthermore, the quantity of PBP2a also closely correlated with the level of oxacillin resistance in the C195 background. Our data suggest that mecA promoter mutations play an important role in determining the level of oxacillin resistance. The mecA promoter mutation G-25A (25 bases upstream of the mecA translation start site) was found to be associated with high oxacillin minimum inhibitory concentration (MIC) (256 mug/ml), G-7T conferred a moderate oxacillin MIC (32-64 mug/ml), C-33T showed a low oxacillin MIC (4-8 mug/ml), and A-38G reversed the effect of the C-33T mutation, restoring the oxacillin resistance level in the double mutant A-38G/C-33T. These observations may explain why C-MRSA strains in Taiwan carrying SCCmec type IV or V have such enormous variation in oxacillin MIC.
Date:
2014-02
Relation:
Antimicrobial Agents and Chemotherapy. 2014 Feb;58(2):1047-1054.
