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

Abstract

Staphylococcus aureus clinical isolates with vancomycin MICs of 2 g/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 timeconsuming and labor-intensive. A collection of 140 MRSA blood isolates with vancomycin MICs of 2 g/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 g/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 g/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.

Treatment options for methicillin-resistant Staphylococcus aureus (MRSA) infection: Where are we now?

“…However, laboratory determination of heteroresistance is difficult, with the gold-standard population analysis profiling being impracticable in the routine diagnostic laboratory. In the context of hVISA, teicoplanin antimicrobial sensitivity testing has been postulated as better predictor of heteroresistance and a better predictor of recurrence [31][32][33], but the literature is certainly far from clear on the issue of teicoplanin [34,35].…”

PREVALENCE OF HETEROGENEOUS GLYCOPEPTIDE INTERMEDIATE RESISTANCE IN METHICILLIN-RESISTANT <i>STAPHYLOCOCCUS AUREUS</i>

“…This study was done according to the method described by Satola et al (2011). In brief, 100 µL of cell suspension at 0.5 Macfarland suspension and serial 10 fold dilution of the suspension in BHI broth were spread on to BHI agar plates containing vancomycin at concentration ranging from1 to 10 µg mL −1 .…”

“…In the past few years, GRSA, GISA and hGISA have increasingly been reported (Kirby et al, 2010). hGISA strains are defined as for which vancomycin MICs are 1 to 4 µg mL −1 but which contain subpopulations that can grow on agar plates supplemented with 4 mg of vancomycin per liter (Satola et al, 2011).…”

Heteroresistance

“…In 2011, Satola et al evaluated the E-test Macrodilution method, E-test GRD and BHI screen agar plates containing 4 mg/L vancomycin and 16 g/L casein using 0.5 and 2.0 McFarland inocula. E-test Macrodilution was 57% sensitive and 96% specific, E-test 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 www.intechopen.com Infection Control -Updates 166 inoculums (Satola et al, 2011). In the same year, van Hal et al screened 458 MRSA bloodstream isolates to determine the most accurate testing strategy to detect the presence of heteroresistance.…”