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Journal of Clinical Microbiology, September 2008, p. 3042-3047, Vol. 46, No. 9
0095-1137/08/$08.00+0     doi:10.1128/JCM.00265-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Evaluation of a New Etest Vancomycin-Teicoplanin Strip for Detection of Glycopeptide-Intermediate Staphylococcus aureus (GISA), in Particular, Heterogeneous GISA{triangledown}

Anne Yusof,1* Anette Engelhardt,1 Åsa Karlsson,1 Lina Bylund,1 Pamela Vidh,1 Karen Mills,1 Mandy Wootton,2 and Timothy R. Walsh2

AB Biodisk, Dalvägen 10, 169 56, Solna, Sweden,1 Department of Medical Microbiology, Medical School, Cardiff University, Cardiff CF14 4XN, United Kingdom2

Received 8 February 2008/ Returned for modification 25 March 2008/ Accepted 20 June 2008

Glycopeptide-intermediate Staphylococcus aureus (GISA) and, in particular, heterogeneous GISA (hGISA) are difficult to detect by standard MIC methods, and thus, an accurate detection method for clinical practice and surveillances is needed. Two prototype Etest strips designed for hGISA/GISA resistance detection (GRD) were evaluated using a worldwide collection of hGISA/GISA strains covering the five major clonal lineages. A total of 150 strains comprising 15 GISA and 60 hGISA strains (defined by population analysis profiles-area under the curve [PAP-AUC]), 70 glycopeptide-susceptible S. aureus (GSSA) strains, and 5 S. aureus ATCC reference strains were tested. For standardized Etest vancomycin (VA) MIC testing, the modified Etest macromethod with VA and teicoplanin (TP) strips tested with a heavier inoculum using brain heart infusion agar (BHI) and two glycopeptide screening agar plates (6 µg/ml VA/BHI and 5 µg/ml Mueller-Hinton agar [MHA]) were tested in parallel with the two new Etest GRD strips: a VA 32 (0.5-µg/ml)-TP 32 (0.5-µg/ml) double-sided gradient (E-VA/TP) with one prototype overlaid with a nutrient (E-VA/TP+S) to enhance the growth of hGISA. The Etest GRD strips were tested with a standard 0.5-McFarland standard inoculum using MHA and MHA plus 5% blood (MHB) and were read at 18 to 24 and 48 h. The interpretive MIC cutoffs used for the new Etest GRD strips at 24 and 48 h were as follows: for GISA, TP or VA, ≥8, and a standard VA MIC of ≥6; for hGISA, TP or VA, ≥8, and a standard VA MIC of ≤4. The results on MHB at 48 h showed that E-VA/TP+S had high specificity (94%) and sensitivity (95%) in comparison to PAP-AUC and was able to detect all GISA (n = 15) and 98% of hGISA (n = 60) strains. In contrast, the glycopeptide screening plates performed poorly for hGISA. The new Etest GRD strip (E-VA/TP+S), utilizing standard media and inocula, is a simple and acceptable tool for detection of hGISA/GISA for clinical and epidemiologic purposes.

* Corresponding author. Mailing address: AB Biodisk, Dalvägen 10, 169 56, Solna, Sweden. Phone: 46 (0) 8 730 0760. Fax: 46 (0) 8 838 158. E-mail: anne.yusof{at}abbiodisk.se

{triangledown} Published ahead of print on 2 July 2008.

Journal of Clinical Microbiology, September 2008, p. 3042-3047, Vol. 46, No. 9
0095-1137/08/$08.00+0     doi:10.1128/JCM.00265-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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