Derivatives of a Vancomycin-ResistantStaphylococcus aureusStrain Isolated at Hershey Medical Center

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Abstract
Antimicrobial susceptibilities and genetic relatedness of the vancomycin-resistantStaphylococcus aureusstrain (VRSA) isolated at Hershey, Pa. (VRSA Hershey), and its vancomycin-susceptible and high-level-resistant derivatives were studied and compared to 32 methicillin-resistantS. aureusstrains (MRSA) isolated from patients and medical staff in contact with the VRSA patient. Derivatives of VRSA were obtained by subculturing six VRSA colonies from the original culture with or without vancomycin. Ten days of drug-free subculture caused the loss ofvanAin two vancomycin-susceptible derivatives for which vancomycin MICs were 1 to 4 μg/ml. Multistep selection of three VRSA clones with vancomycin for 10 days increased vancomycin MICs from 32 to 1,024 to 2,048 μg/ml. MICs of teicoplanin, dalbavancin, and oritavancin were also increased from 4, 0.5, and 0.12 to 64, 1, and 32 μg/ml, respectively. Pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing analysis indicated that VRSA Hershey was thevanA-acquired variety of a common MRSA clone in our hospital with sequence type 5 (ST5). Three of five vancomycin-intermediateS. aureusstrains tested from geographically different areas were also ST5, and the Michigan VRSA was ST371, a one-allele variant of ST5. Derivatives of VRSA Hershey had differences in PFGE profiles and the size of SmaI fragment that carries thevanAgene cluster, indicating instability of this cluster in VRSA Hershey. However induction with vancomycin increased glycopeptide MICs and stabilized the resistance.