Volume 17, Number 11—November 2011
Reduced Susceptibility to Vancomycin in Staphylococcus aureus
To the Editor: I read with interest the article by Aguado et al. (1). I congratulate the authors for their high-quality research and would like to make 2 brief comments.
My first point regards the mechanism by which methicillin-susceptible Staphylococcus aureus (MSSA) with reduced susceptibility to vancomycin would also acquire decreased susceptibility to β-lactams. The authors “hypothesize that certain structural modifications might also occur in the cell wall of strains with high vancomycin MIC, including a thicker cell wall as it has been described in MRSA [methicillin-resistant S. aureus].” In addition to cell wall thickening, possible mechanisms could include reduction in autolysis (2,3) and in the cell wall content of penicillin binding protein 4 (PBP4) (3). A study of MSSA isolates has shown a reduction in autolysis and in the bactericidal activity of oxacillin after development of intermediate vancomycin susceptibility (2). Lower content of PBP4 and decreased autolysis were reported in MRSA isolates after reduced susceptibility to vancomycin developed after exposure to this antimicrobial drug (3). Decreased PBP4 has been associated with reduced methicillin susceptibility in S. aureus (4).
Second, knowing whether the authors had the exact vancomycin MIC of the isolates by broth microdilution to compare with the Etest results would be interesting. In the article, they only state that “all 99 MSSA strains were susceptible to vancomycin (MIC <2 μg/mL) by the broth microdilution method.” Although difficult to compare (because Etest dilution progression is arithmetic and broth microdilution is performed with a geometric dilution), some authors have found substantial differences between the vancomycin MIC results given by these 2 methods (5). These differences could have major laboratory and clinical implications.
- Aguado JM, San-Juan R, Lalueza A, Sanz F, Rodríguez-Otero J, Gómez-Gonzalez C, High vancomycin MIC and complicated methicillin-susceptible Staphylococcus aureus bacteremia. Emerg Infect Dis. 2011;17:1099–102.
- Pillai SK, Wennersten C, Venkataraman L, Eliopoulos G, Moellering R, Karchmer A. Development of reduced vancomycin susceptibility in methicillin-susceptible Staphylococcus aureus. Clin Infect Dis. 2009;49:1169–74.
- Sieradzki K, Tomasz A. Alterations of cell wall structure and metabolism accompany reduced susceptibility to vancomycin in an isogenic series of clinical isolates of Staphylococcus aureus. J Bacteriol. 2003;185:7103–10.
- Wyke AW, Ward JB, Hayes MV, Curtis NA. A role in vivo for penicillin-binding protein-4 of Staphylococcus aureus. Eur J Biochem. 1981;119:389–93.
- Mason EO, Lamberth LB, Hammerman WA, Hulten KG, Versalovic J, Kaplan SL. Vancomycin MICs for Staphylococcus aureus vary by detection method and have subtly increased in a pediatric population since 2005. J Clin Microbiol. 2009;47:1628–30.