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Comparative evaluation of VITEK® 2 and three commercial gradient strip assays for daptomycin susceptibility testing
Robert E Weber1, Franziska Layer1, Ingo Klare1
1German Reference Centre for Staphylococci and Enterococci, Division Nosocomial Pathogens and Antibiotic Resistances, Department of Infectious Diseases, Robert Koch Institute, Wernigerode Branch, Wernigerode, Germany.
Objectives:
MRSA remains a major cause of severe nosocomial infections and the increased use of vancomycin and daptomycin for MRSA treatment over the last decade has led to the isolation of MRSA strains with decreased daptomycin susceptibility. In addition, a growing number of MSSA isolates with reduced susceptibility to daptomycin have been described lately. Surveillance of the emergence of such a daptomycin-non-susceptible MSSA population requires prompt and reliable daptomycin susceptibility testing. Therefore, this work aimed to evaluate the ability of commonly used methods to detect daptomycin resistance in clinical microbiological laboratories.
Methods:
We used commercially available manual and automated test systems, including VITEK® 2 and three gradient strip assays, in comparison with broth microdilution, to detect daptomycin resistance in a representative Staphylococcus aureus strain collection.
Results:
We found high inter-assay concordance as well as congruence with the reference method. This is demonstrated by essential agreement between commercial test systems and reference broth microdilution ranging from 98.1% to 100% and by categorical agreement from 98.2% to 99.1%. Thus, all systems used were able to detect daptomycin non-susceptibility in MRSA and MSSA isolates.
Conclusions:
Our data indicate that routine laboratories are at limited risk of overlooking further daptomycin resistance development, as long as commercially available test systems are used according to the manufacturer's recommendations. However, laboratories must be aware of an increasing number of daptomycin-non-susceptible MSSA isolates, including those exhibiting elevated MICs of glycopeptides.
