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.

[Decreasing Susceptibility of MRSA to Daptomycin During the Course of Treatment].

Methicillin-resistant Staphylococcus aureus (MRSA) with decreased susceptibility to daptomycin (DAP) were isolated from 4 patients receiving DAP from November 2013 to May 2014. These patients were treated with DAP for more than 7 days in all the cases. The pulsed-field gel electrophoresis (PFGE) patterns for MRSA isolates recovered from each patient pre- and post-DAP therapy were identical. Sequencing of mprF detected 2 amino acid substitutions (T345I or L826F) in 2 of the isolates. These results suggest that in vivo MRSA was resistant to DAP during DAP therapy. Furthermore, the MICs for DAP can vary by±1 dilution depending on the susceptibility test. When testing DAP susceptibility, there is a need to monitor reproducibility using different susceptibility tests, including the CLSI method.