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Antimicrobial Agents and Chemotherapy, August 2009, p. 3447-3452, Vol. 53, No. 8
0066-4804/09/$08.00+0 doi:10.1128/AAC.01365-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
Prospective Comparison of the Clinical Impacts of Heterogeneous Vancomycin-Intermediate Methicillin-Resistant Staphylococcus aureus (MRSA) and Vancomycin-Susceptible MRSA
,
K. C. Horne,1
B. P. Howden,1,2,3
E. A. Grabsch,2
M. Graham,1
P. B. Ward,2
S. Xie,2
B. C. Mayall,2
P. D. R. Johnson,1,5 and
M. L. Grayson1,2,4,5*
Infectious Diseases,1 Microbiology Departments, Austin Health, Heidelberg,2 Department of Microbiology,3 Department of Epidemiology and Preventive Medicine, Monash University, Melbourne,4 Department of Medicine, University of Melbourne, Melbourne, Victoria, Australia5
Received 10 October 2008/ Returned for modification 15 March 2009/ Accepted 1 June 2009
Although methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) strains with reduced susceptibility to vancomycin (RVS-MRSA; including vancomycin-intermediate S. aureus [VISA] and heterogeneous VISA [hVISA]) have been linked with vancomycin treatment failure, it is unclear whether they are more pathogenic than vancomycin-susceptible MRSA (VS-MRSA). We prospectively assessed patients with clinical MRSA isolates during a 10-month period to determine clinical status (infection versus colonization) and therapeutic outcome before correlating these findings with the results of detailed in vitro assessment of vancomycin susceptibility, including population analysis profile (PAP) testing. hVISA and VISA were defined by standard PAP criteria (area-under-the-curve ratio compared to that of the reference hVISA strain Mu3 [
0.9]) and routine CLSI criteria (vancomycin MIC, 4 to 8 µg/ml), respectively. Among the 117 patients assessed, 58 had RVS-MRSA isolates (56 hVISA and 2 VISA) and 59 had VS-MRSA isolates; the patient demographics and comorbidities were similar. RVS-MRSA was associated with a lower rate of infection than VS-MRSA (29/58 versus 46/59; P = 0.003), including a lower rate of bacteremia (3/58 versus 20/59, respectively; P < 0.001). The cure rates in RVS-MRSA and VS-MRSA groups were not statistically different (16/26 versus 31/42; P = 0.43), but the post hoc assessment of treatment regimes and study size made detailed conclusions difficult. The results of the macro method Etest correlated well with the PAP results (sensitivity, 98.3%, and specificity, 91.5%), but broth microdilution and our preliminary RVS-MRSA detection method correlated poorly. All isolates were susceptible to linezolid and daptomycin. These data suggest that detailed prospective laboratory identification of RVS-MRSA isolates may be of limited value and that, instead, such in vitro investigation should be reserved for isolates from patients who are failing appropriate anti-MRSA therapy.
* Corresponding author. Mailing address: Infectious Diseases Department, Austin Hospital, Austin Health, P.O. Box 5555 Studley Rd., Heidelberg, VIC, Australia 3084. Phone: (613) 9496 6676. Fax: (613) 9496 6677. E-mail: Lindsay.Grayson{at}austin.org.au
Published ahead of print on 8 June 2009.
Supplemental material for this article may be found at http://aac.asm.org/.
Antimicrobial Agents and Chemotherapy, August 2009, p. 3447-3452, Vol. 53, No. 8
0066-4804/09/$08.00+0 doi:10.1128/AAC.01365-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
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