Background: Strains of methicillin-resistant Staphylococcus aureus (MRSA) with decreased susceptibility to vancomycin (Van) have become a concern because of the increased risk of clinical failure.
Methods: In this study, we obtained all clinical isolates of MRSA cultured over a 1 month period (April 2007) from the microbiology laboratory in a 500-bed community/teaching hospital in Lansing, Michigan. Each isolate was analyzed for Van susceptibility and selected other agents. All isolates with Van minimal inhibitory concentrations (MICs) of 2.0 μg/mL were further analyzed by transmission electron microscopy for cell wall thickness, polymerase chain reaction for accessory gene regulator (agr) subtypes, and δ-hemolysin expression.
Results: We found that 52 (65%) of the 80 strains had Van MICs greater than 1.0 μg/mL. None were resistant or heteroresistant to Van or other agents tested such as linezolid (MIC90, 1.5 μg/mL), daptomycin (MIC90, 0.75 μg/mL), and tigecycline (MIC90, 0.19 μg/mL). The mean cell wall thickness of 9 strains with Van MICs of 2.0 μg/mL was similar to an S. aureus American Type Culture Collection (ATCC, no. 29213) strain with a Van MIC of 0.5 μg/mL. Analysis of the agr subtypes found that 5 isolates (56%) had agr II polymorphism, and all 9 strains had diminished or absent δ-hemolysin expression.
Conclusions: Most MRSA strains in this study had Van MICs greater than 1.0 μg/mL. Those MRSA strains with Van MICs of 2.0 μg/mL did not have thickened cell walls but had decreased δ-hemolysin expression, which is associated with diminished bactericidal activity of Van.