Evaluating vancomycin susceptibility in Staphylococcus aureus.

Abstract

Evaluating vancomycin susceptibility in Staphylococcus aureus Marcelo J Mimica, Alessandra Navarini Department of Pathology, Division of Microbiology, Santa Casa de São Paulo School of Medicine, São Paulo, BrazilWe read the report by Phillips et al1 with great interest and would like to discuss it in comparison with our previous published data on the subject.2,3We have also studied a number of Staphylococcus aureus clinical isolates (n=125), comparing different vancomycin susceptibility tests, including microdilution, Etest® (bio-Mérieux, Marcy-l’Étoile, France), and brain heart infusion vancomycin screening plates. We found only one isolate with reduced susceptibility with a minimum inhibitory concentration (MIC) =4 mg/L when tested with Etest and 2 mg/L when tested with microdilution.2,3 Our results showed a tendency of higher lethality when higher MICs were present, even within the susceptible range,3 as some previous studies have shown.4,5Concordant to Phillips et al1 and other authors,6,7 we also reported a poor correlation between different tests. Comparing Etest and microdilution (approximating an Etest MIC value between two twofold dilutions up to the highest value), 58% of the isolates had similar MICs, whereas 38% had an MIC by Etest one dilution higher than microdilution. One isolate had an Etest MIC twofold higher and four isolates an Etest MIC onefold lower than microdilution.2However, in our study, a brain heart infusion screening plate with 2.0 mg/L of vancomycin showed a sensitivity of 100% to detect isolates with an MIC ≥2.0 by Etest and 91% to detect an MIC ≥2.0 by microdilution, making this test an interestingoption for initial screening of S. aureus isolates for reduced vancomycin susceptibility. Specificities were 63% and 38%, respectively, which would still make necessary the further testing with an MIC method, but in a much smaller number of isolates.2 This approach would be suitable for a large number of laboratories throughout the world where the routine MIC testing of all S. aureus isolates is not feasible.View original paper by Phillips et al.