Methicillin-resistant Staphylococcus aureus: microbiologic characteristics, antimicrobial susceptibilities, and assessment of virulence of an epidemic strain.

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 high-level homogeneous methicillin resistance (minimal inhibitory concentration, greater than 128 microgram/ml). The expression of resistance was not influenced by incubation temperature. MRSA-Va contained significantly more catalase (P less than 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-Va were comparable with those of the three methicillin-sensitive strains. This epidemic strain of MRSA appears to be fully virulent.