A novel δ-hemolysis screening method for detecting heteroresistant vancomycin-intermediate Staphylococcus aureus and vancomycin-intermediate S. aureus

Abstract

We assessed a new screening method, based on δ-hemolysin production in the presence of 6 mg/liter vancomycin, to distinguish heteroresistant vancomycin-intermediate Staphylococcus aureus (hVISA) and vancomycin-intermediate S. aureus (VISA) from vancomycin-susceptible S. aureus (VSSA). On 37 clinical methicillin-resistant S. aureus (MRSA) isolates, hVISA and VISA displayed no δ-hemolysis whereas VSSA displayed strong δ-hemolysis, showing 91.6% sensitivity. These data, supported by real-time reverse transcription PCR (real-time RT-PCR) highlighting an hld downregulation, i.e., VSSA>hVISA>VISA, define this new assay as a valid screening method.

Fig 1

δ-Hemolysis assays on 5% Columbia sheep blood agar with 6 mg/liter VAN. (A) NRS149 (VSSA prototype); (B) VSSA clinical isolate; (C) Mu3 (hVISA prototype); (D) Mu50 (VISA prototype); (E) NRS155 (agr knockout strain).

Fig 2

Relative quantitative expression of hld in hVISA and VISA. Darker bars indicate hVISA; lighter bars indicate VISA. *, statistically significant differences between isolates and VSSA (NRS149) (P < 0.05).