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Phenotypic and Genotypic Analyses of Methicillin-Resistant Staphylococcus aureus Isolates from a Community Hospital

Tran, MaryAnn P. MD*; Stein, Gary E. PharmD*; Common, Ralph BS; Dybas, Leslie MS*; Kalra, Apoorv MD*; Havlichek, Daniel H. MD*

Infectious Diseases in Clinical Practice: November 2009 - Volume 17 - Issue 6 - p 378-381
doi: 10.1097/IPC.0b013e3181ab2c75
Original Articles

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.

From the Departments of *Medicine, and †Physiology, Michigan State University, East Lansing, MI.

Reprints: Gary E. Stein, PharmD, Michigan State University, Department of Medicine, B320 Life Sciences, East Lansing, MI 48824. E-mail:

This study was funded by the Sparrow Hospital/Michigan State University Infectious Diseases Fellowship Program (Dr Tran). Additional support was provided by Pfizer, Inc, Cubist Pharmaceuticals, and Wyeth Pharmaceuticals.

© 2009 Lippincott Williams & Wilkins, Inc.