Evaluation of the Etest GRD for the detection of Staphylococcus aureus with reduced susceptibility to glycopeptides.
OBJECTIVE
Continued glycopeptide-selective pressure has led to non-susceptible strains of Staphylococcus aureus including heterogeneously vancomycin-intermediate S. aureus (hVISA). The gold standard for identification of hVISA is the population analysis profile area under the curve ratio (PAP-AUC), though this method is time-consuming and labour-intensive. The objective of this study was to compare a new method for detection of hVISA, the Etest GRD, to PAP-AUC and to macro Etest.


METHODS
One hundred clinical hVISA and 50 clinical fully vancomycin-susceptible S. aureus (VSSA), confirmed by PAP-AUC, were evaluated. Microtitre and Etest MICs were determined by standard testing procedures on all isolates. Macro Etest was performed according to referenced procedures. The Etest GRD was tested using a 0.5 McFarland standard on Mueller-Hinton agar + 5% blood and read at 24 and 48 h. If either the vancomycin or the teicoplanin end of the GRD strip was >or=8 and the vancomycin Etest was <or=4, the isolate was considered hVISA.


RESULTS
Vancomycin MIC(50)/MIC(90) for hVISA and VSSA was 1.5/2 mg/L and 1/1.5 mg/L, respectively, by Etest and vancomycin MIC(50)/MIC(90) for hVISA and VSSA was 1/2 mg/L for both by microtitre; MIC values for hVISA being significantly higher (P <or= 0.023). At 24 h, the Etest GRD was 77% sensitive and 98% specific, and at 48 h, it was 93% sensitive and 82% specific compared with PAP-AUC. Macro Etest was 83% sensitive and 94% specific at 48 h.


CONCLUSIONS
Etest GRD was simple to perform and may be feasible for clinical microbiology laboratories. This test may be useful for clinical detection of hVISA.
Leonard
Steven N
SN
Anti-Infective Research Laboratory, Eugene Applebaum College of Pharmacy and Health Sciences, Wayne State University, Detroit, MI 48201, USA.
Rossi
Kerri L
KL
Newton
Karly L
KL
Rybak
Michael J
MJ
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D003470:Culture Media / Q000737:chemistry; D006020:Glycopeptides / Q000494:pharmacology*; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000379:methods; D013203:Staphylococcal Infections / Q000382:microbiology; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000302:isolation & purification; D013997:Time Factors; D020713:Vancomycin Resistance
D000900:Anti-Bacterial Agents; D003470:Culture Media; D006020:Glycopeptides