Decreasing in Vitro Susceptibility of Clinical Staphylococcus aureus Isolates to Vancomycin at the New York Hospital: Quantitative Testing Redux

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Abstract
Recent descriptions of Staphylococcus aureus isolates with intermediate- and high-level vancomycin resistance [1, 2] have been accompanied by reports of the slow bactericidal activity of vancomycin and of vancomycin treatment failures in persons with infection due to methicillin-resistant S. aureus (MRSA) [1–3]. However, vancomycin-resistant isolates of S. aureus are exceedingly rare, and most reported cases of vancomycin treatment failure involve organisms that are fully susceptible to vancomycin [2, 3]. One recent study of vancomycin-susceptible MRSA isolates correlated the clinical efficacy of vancomycin with both the MIC for the target organism and the magnitude of bactericidal activity exerted by vancomycin. Among MRSA isolates with MICs ⩽2 μg/mL, a range of >7 log10 µg/mL in the bactericidal activity of vancomycin was observed [3]. These results emphasize the potential importance of the bactericidal activity of vancomycin and the relative imprecision of the MIC measurement as a quantitative surrogate of vancomycin susceptibility among S. aureus.