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Journal of Clinical Microbiology, October 2007, p. 3263-3269, Vol. 45, No. 10
0095-1137/07/$08.00+0     doi:10.1128/JCM.00836-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Investigation of Reduced Susceptibility to Glycopeptides among Methicillin-Resistant Staphylococcus aureus Isolates from Patients in Ireland and Evaluation of Agar Screening Methods for Detection of Heterogeneously Glycopeptide-Intermediate S. aureus{triangledown}

Margaret M. Fitzgibbon, Angela S. Rossney,* and Brian O'Connell

National MRSA Reference Laboratory, St James's Hospital, James's St., Dublin 8, Ireland, and Department of Clinical Microbiology, University of Dublin, Trinity College, Dublin 1, Ireland

Received 19 April 2007/ Returned for modification 5 June 2007/ Accepted 1 August 2007

Methicillin-resistant Staphylococcus aureus (MRSA) isolates (n = 3,189) from 2,990 patients were investigated by agar screening and by the Etest macromethod for reduced susceptibility to glycopeptide. No vancomycin-resistant S. aureus or glycopeptide-intermediate S. aureus (GISA) isolates were detected, but 178 isolates were confirmed as hetero-GISA (hGISA) by vancomycin population analysis profile (vPAP)-area under the curve (AUC) ratio determination and/or teicoplanin PAP (tPAP) methods. Of 139 isolates detected using the recommended Etest macromethod cutoff values of ≥8 mg/liter for both vancomycin and teicoplanin or ≥12 mg/liter for teicoplanin alone, 73 were confirmed as hGISA by vPAP-AUC, 95 were confirmed as hGISA by tPAP, and 108 were confirmed as hGISA by both methods. An Etest macromethod cutoff value of 8 mg/liter for teicoplanin alone detected a further 70 hGISA (17 were confirmed by vPAP-AUC and 70 were confirmed by tPAP). Agar screening utilizing brain heart infusion (BHI) agar containing 6 mg of vancomycin/liter (BHIV6) and Mueller-Hinton (MH) agar containing 8 mg of teicoplanin/liter (MHT8) failed to detect hGISA. MH agar containing 5 mg of teicoplanin/liter (MHT5) and BHI containing 5 mg of teicoplanin/liter (BHIT5) were evaluated using 10-µl volumes of three inoculum concentrations (with densities equivalent to 0.5 and 2.0 McFarland turbidity standards and stationary-phase BHI broth subcultures [MHT50.5, MHT52.0, MHT5S, BHIT50.5, BHIT52.0, and BHIT5S]). The sensitivity of all methods except MHT50.5 and MHT52.0 was 100%. The specificity ranged from 4 to 82%. BHIT50.5 yielded the best performance, with a specificity of 84% for detecting isolates with teicoplanin Etest macromethod values of ≥8 mg/liter. Screening on BHIT50.5 is useful where screen-positive isolates are investigated with the Etest macromethod and confirmed by vPAP-AUC and tPAP. The prevalence of hGISA among patients with blood culture isolates recovered in Irish hospitals between 1999 and 2003 was 2.6%, whereas the prevalence among patients with isolates from all specimen sites collected during a 2-week survey in 1999 was 12%. The prevalence in one hospital decreased from 5.3% in 2003 to 1.5% in 2004.

* Corresponding author. Mailing address: National MRSA Reference Laboratory, St. James's Hospital, James's St., Dublin 8, Ireland. Phone: 353 1 410 3662. Fax: 353 1 410 3666. E-mail: arossney{at}stjames.ie

{triangledown} Published ahead of print on 8 August 2007.

Journal of Clinical Microbiology, October 2007, p. 3263-3269, Vol. 45, No. 10
0095-1137/07/$08.00+0     doi:10.1128/JCM.00836-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.



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