Detection of heterogeneous vancomycin-intermediate Staphylococcus aureus isolates using a combination of δ-hemolysis assay and Etest.

This study evaluated the δ-hemolysis assay for detection of heterogeneous vancomycin-intermediate Staphylococcus aureus (hVISA) isolates. The assay was performed using Columbia or trypticase soy blood agar plates without vancomycin at 37°C with 5% CO2, and the test isolate was streaked perpendicularly 1mm away from S. aureus RN4220. One hundred thirty-eight methicillin-resistant S. aureus (MRSA) bloodstream isolates with vancomycin MICs of 0.75-2 μg/mL were screened for hVISA by the δ-hemolysis assay and Etest glycopeptide resistance detection (GRD) method. The δ-hemolysis assay could be read at 24h and was more sensitive and specific than the Etest GRD at 24h and 48 h for detection of hVISA. Because most hVISA isolates have a vancomycin MIC of 2 μg/mL, we recommend screening MRSA isolates first using Etest for those with an MIC of 2 μg/mL and then performing the δ-hemolysis assay on these isolates for hVISA.