Does sub-culturing of positive MRSA blood cultures affect vancomycin MICs?

Introduction. Empirical vancomycin (VAN) treatment failure for methicillin-resistant Staphylococcus aureus (MRSA) bacteraemia, with significantly higher mortality, has been reported for MRSA strains with reduced VAN susceptibility. Aim. Our goal was to study the effect of sub-culture on VAN minimum inhibitory concentration (MIC) values compared to direct susceptibility of MRSA-positive blood cultures. Methodology. Using 19 MRSA-positive blood cultures and 19 seeded MRSA-positive blood cultures, we compared the VAN MICs from direct susceptibility testing of MRSA-positive blood cultures and MRSA sub-cultured from positive blood cultures. Results. In comparing direct VAN MICs from MRSA-positive blood cultures and standard agar dilution, nearly half of the MICs from agar dilution were lower, with one sample decreasing from 1.5 to 0.75 µg ml−1. Furthermore, in seeded blood cultures, 80 % or more showed lower values from standard agar dilution compared to direct VAN MICs. Conclusion. Our results reveal a trend towards lower MICs after positive blood culture isolates are sub-cultured. Some clinical failures among MRSA infections treated with VAN may result from this phenomenon.