Serial Daptomycin Selection Generates Daptomycin-Nonsusceptible Staphylococcus aureus Strains with a Heterogeneous Vancomycin-Intermediate Phenotype

In order to better understand the mechanism of daptomycin resistance, we generated a daptomycin-nonsusceptible derivative strain, strain 10*3d1 (MIC = 3.0 mu g/ml), by in vitro exposure of methicillin-resistant Staphylococcus aureus strain N315 Delta IP (MIC = 0.5 mu g/ml) to daptomycin. We also obtained a daptomycin-susceptible phenotypic revertant strain, strain 10*3d1-10 (MIC = 1.0 mu g/ml), by passaging 10*3d1 in drug-free medium for 10 days. The resultant triple-isogenic strains were analyzed for their phenotypes and gene expression by microarray analysis. No significant differences in the membrane fluidities of 10*3d1 and 10*3d1-10 compared to the membrane fluidity of N315 Delta IP were observed. Resistant strain 10*3d1 had the highest membrane potential, followed by strains 10*3d1-10 and N315 Delta IP. The vancomycin and teicoplanin MICs also increased. Teichoic acid genes (tagA, tagG), mprF encoding lysyl-phosphatidylglycerol, and cls encoding cardiolipin synthase were downregulated in 10*3d1 and 10*3d1-10. The vraF and vraG genes, which encode ATP binding cassette transporter proteins, were upregulated in 10*3d1. The vraSR two-component regulatory system was upregulated, and electron microscopy revealed that the cell wall of 10*3d1 was significantly thicker than that of the parental strain. Taken together, daptomycin exposure selected a daptomycin-nonsusceptible strain with a phenotype similar to that of heterogeneous vancomycin-intermediate S. aureus and a transcription profile that partially overlapped that of heterogeneous vancomycin-intermediate S. aureus.