Evaluation of vancomycin and daptomycin MIC trends for methicillin resistant Staphylococcus aureus blood isolates over an 11 year period

İngilizce (OpenAlex)

Sir, There are many reports concerning an increase in vancomycin MICs for Staphylococcus aureus isolates over time, which has been described as MIC creep.1–4 Considering the limitations and effectiveness of vancomycin, alternative agents are needed for the treatment of S. aureus infections. Daptomycin is one of the new drugs that is active against various Gram-positive bacteria.5–7 The aim of this study was to determine the in vitro activity of vancomycin and daptomycin against methicillin-resistant S. aureus (MRSA) strains, to investigate the agreement between the vancomycin Etest and the broth microdilution method (BMD) and to evaluate the possible MIC creep of MRSA blood isolates in a Turkish university hospital from 1999 to 2009. A total of 299 non-duplicate MRSA blood isolates identified at Hacettepe University Adult Hospital Laboratory were included in the study. All isolates were stored at −80°C and subcultured twice prior to testing. All testing for each isolate was performed on the same day, and utilized the same bacterial suspension with a turbidity equivalent to that of a 0.5 McFarland standard. S. aureus ATCC 29213 was used as a quality control strain with each experiment. Vancomycin BMD was performed according to CLSI guidelines using cation-adjusted Mueller–Hinton broth (BBL, Becton Dickinson, USA).8 The Etest was performed according to the manufacturer's manual, using vancomycin and daptomycin Etest strips (bioMérieux, France) on Mueller–Hinton agar (BBL, Becton Dickinson, USA). All tests were read at 24 h after incubation at 35°C. The Etest MIC was rounded to the next highest concentration of BMD for comparison with the CLSI MIC value where necessary. Rates of resistance and susceptibility were determined for vancomycin and daptomycin according to the MIC breakpoints proposed by CLSI guidelines.9 For determining the percentage agreement between BMD and the standard Etest, discrepancies between MIC values of no more than one dilution (±1 dilution) were calculated. Vancomycin and daptomycin MIC breakpoints proposed for the BMD method were applied to and used for the Etest in order to determine the categorical agreement between the two methods. For the assessment of categorical agreement, minor error was defined as susceptible (S) or resistant (R) by one method and intermediate (I) by the other. Major error was defined as R according to the Etest and S according to BMD, and very major error was defined as S according to the Etest and R according to BMD.10 The MIC trends over the 11 years were assessed using Kruskal–Wallis analysis, and a P value of <0.05 was considered significant.