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Vancomycin for Staph Infections: Bactericidal Effect Essential

With MRSA infections, vancomycin treatment can fail even when diagnostic lab reports indicate susceptibility.

Although vancomycin remains the standard of care for treating serious methicillin-resistant Staphylococcus aureus (MRSA) infections, several studies have suggested that overall, it is less clinically effective than ß-lactam antibiotics (see Journal Watch Infectious Diseases Jul 28 2003). Now, to better define the limitations of vancomycin therapy, investigators have tested in vitro bactericidal activity of vancomycin (16 µg/mL) in 30 strains of MRSA that are considered to be susceptible to this agent. The isolates, collected from 1998 through 2001, came from patients enrolled -- generally because of vancomycin treatment failure -- in multicenter prospective trials of alternative antibiotic therapy for MRSA bacteremia.

The strains varied widely in susceptibility, with 72-hour killing that ranged from 0.17 to 8.16 log₁₀ colony-forming units per milliliter. Mean killing was 6.26 log₁₀ CFU/mL for the 7 strains associated with vancomycin treatment success and 4.88 log₁₀ CFU/mL for the 23 strains associated with treatment failure (P=0.07). Clinical efficacy was 55.6% for strains with MICs 0.5 µg/mL but only 9.5% for strains with MICs of 1 or 2 µg/mL (P=0.01).

Comment: This study is very small, but the results provide objective support for the clinical observations on limitations of vancomycin for serious MRSA infections, particularly for strains with relatively high MICs. Unfortunately, the in vitro bactericidal assay used in this study cannot be readily replicated in clinical laboratories. Additionally, as noted by the authors, most isolates came from patients with vancomycin treatment failure, so extrapolating from these results to the interpretation of standard laboratory MIC determinations is inappropriate.

— Richard T. Ellison III, MD

Published in Journal Watch Infectious Diseases July 9, 2004

Citation(s):

Sakoulas G et al. Relationship of MIC and bactericidal activity to efficacy of vancomycin for treatment of methicillin-resistant Staphylococcus aureus bacteremia. J Clin Microbiol 2004 Jun; 42:2398-402.

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