Methicillin-Resistant Staphylococcus aureus: Microbiologic Characteristics, Antimicrobial Susceptibilities, and Assessment of Virulence of an Epidemic Strain

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Abstract
An epidemic strain of methicillin-resistant Staphylococcus aureus (MRSA) isolated from patients at the University of Virginia Hospital in Charlottesville (MRSA-Va) was characterized, and virulence properties were compared with those of three clinically significant strains of methicillin-sensitive S. aureus. Unlike most known strains of MRSA, MRSA-Va was sensitive to tetracycline and streptomycin and exhibited highlevel homogeneous methicillin resistance (minimal inhibitory concentration, >128 μg/ml). The expression of resistance was not influenced by incubation temperature. MRSA-Va contained significantly more catalase (P < 0.05) than methicillin-sensitive strains but about the same amount of protein A. Phagocytosis and killing of MRSA-Va by normal polymorphonuclear leukocytes were not significantly different from that of the Wood-46 strain of S. aureus. In mouse virulence studies using both intraperitoneal and intravenous modes of infection, 50% lethal doses for MRSA-Vawere comparable with those of the three methicillin-sensitive strains. This epidemic strain of MRSA appears to be fully virulent.