Methicillin-resistant Staphylococcus aureus with reduced susceptibility to vancomycin in Canada

This study defines the characteristics of decreasing vancomycin susceptibility in multiple isolates of methicillin-resistant Staphylococcus aureus (MRSA) recovered from a hospitalized patient in Canada over a period of 6 months. The MICs of the isolates increased during therapy with vancomycin. The patient fractured her right hip while in the United States. She was started on treatment with vancomycin. The MICs of successive isolates increased from ≤1 to 4 mg/L over 6 months. Then, an isolate tested at 8 mg/L initially and 4 mg/L with confirmatory E-test (AB BIODISK, Solna, Sweden). One month later, MRSA was still present in her wound, and therapy was changed to linezolid with rifampin. Subsequent cultures were negative for MRSA. Susceptibility testing was performed on the BD Phoenix (Becton Dickinson Diagnostic Systems, Sparks, MD), Dade Microscan (Dade Behring Microscan, Sacramento, CA), Pasco MIC (Becton Dickinson, Sparks, MD), Vitek 2 (bioMerieux, St. Louis, MO), and Sensititre (Trek Diagnostic Systems, Cleveland, OH) systems, and by E-test. Molecular typing (pulsed-field gel electrophoresis [PFGE]) was used to verify the relatedness of the isolates. Transmission electron microscopy (TEM) was used to assess the cell wall thickness of isolates with differing MICs. Population analysis was performed to assess for vancomycin hetero-resistance. MICs of 4 mg/L were only obtained with BD Phoenix, E-test, and broth microdilution. All isolates were identical by PFGE. The most resistant isolate had a thicker cell wall on TEM. Vancomycin hetero-resistance was observed in the resistant isolates. This is the first strain of MRSA with reduced susceptibility to vancomycin reported in Canada. The breakpoints for vancomycin susceptibility have been revised in light of such observations.