Comparison of Detection Methods for Heteroresistant Vancomycin-Intermediate Staphylococcus aureus, with the Population Analysis Profile Method as the Reference Method

• Type of Material: Article
• Authors: Sarah Satola, Emory University; Monica Farley, Emory University; Karen F. Anderson, Emory University; Jean B. Patel, Emory University
• Language: English
• Date: 2011-01
• Publisher: American Society for Microbiology (ASM)
• Publication Version: Final Published Version
• Copyright Statement: © 2011, American Society for Microbiology
• Final Published Version (URL): http://jcm.asm.org/content/49/1/177
• Title of Journal or Parent Work: Journal of Clinical Microbiology
• Volume: 49
• Issue: 1
• Start Page: 177
• End Page: 183
• Grant/Funding Information: Financial support for this study was from CDC's Emerging Infections Program.
• Research Categories: Biology, Microbiology

Abstract

Staphylococcus aureus clinical isolates with vancomycin MICs of 2 μg/ml have been associated with vancomycin therapeutic failure and the heteroresistant vancomycin-intermediate S. aureus (hVISA) phenotype. A population analysis profile (PAP) with an area under the curve (AUC) ratio of ≥0.9 for the AUC of the clinical isolate versus the AUC for hVISA strain Mu3 is most often used for determining hVISA, but it is time-consuming and labor-intensive. A collection of 140 MRSA blood isolates with vancomycin MICs of 2 μg/ml by reference broth microdilution and screened for hVISA using PAP-AUC (21/140 [15%] hVISA) were tested by additional methods to detect hVISA. The methods included (i) Etest macromethod using vancomycin and teicoplanin test strips, brain heart infusion (BHI) agar, and a 2.0 McFarland inoculum; (ii) Etest glycopeptide resistance detection (GRD) using vancomycin-teicoplanin double-sided gradient test strips on Mueller-Hinton agar (MHA) with 5% sheep blood and a 0.5 McFarland inoculum; and (iii) BHI screen agar plates containing 4 μg/ml vancomycin and 16 g/liter casein using 0.5 and 2.0 McFarland inocula. Each method was evaluated using PAP-AUC as the reference method. The sensitivity of each method for detecting hVISA was higher when the results were read at 48 h. The Etest macromethod was 57% sensitive and 96% specific, Etest GRD was 57% sensitive and 97% specific, and BHI screen agar was 90% sensitive and 95% specific with a 0.5 McFarland inoculum and 100% sensitive and 68% specific with a 2.0 McFarland inoculum. BHI screen agar with 4 μg/ml vancomycin and casein and a 0.5 McFarland inoculum had the best sensitivity and specificity combination, was easy to perform, and may be useful for clinical detection of hVISA.