A novel screening agar for the detection of vancomycin non-susceptible Staphylococcus aureus

Abstract number: P592

Objectives: The ability to accurately identify isolates of Staphylococcus aureus with reduced susceptibility to vancomycin is of critical clinical importance. Guidance for susceptibility testing for S. aureus has recently been in flux. In January 2006, CLSI updated breakpoints for vancomycin susceptibility testing for S. aureus such that a minimum inhibitory concentration (MIC) greater than 2 mg/L was considered to be non-susceptible to vancomycin. In 2009, the CLSI deemed that disk diffusion was no longer an acceptable means for susceptibility testing for vancomycin in staphylococci. The objective of this study was to design a cost-effective medium to detect vancomycin non-susceptible S. aureus.

Methods: A medium consisting of brain heart infusion agar with 3 mg/L vancomycin (BHI-V3) was created to screen for isolates of S. aureus with reduced susceptibility to vancomycin. The agar was inoculated with 10 uL of a 0.5 McFarland standard suspension of organism and incubated at 35 C for 24 h. Any growth on BHI-V3 was interpreted as positive. This agar was validated using a collection of 100 S. aureus strains previously characterized by the CDC using broth microdilution. This collection included 55 vancomycin susceptible isolates and 45 vancomycin intermediate (VISA) isolates. Once validated, BHI-V3 was incorporated into routine use in the Barnes-Jewish Hospital Microbiology Laboratory. The vancomycin MIC of all isolates growing on BHI-V3 was also determined using multiple methods.

Results: All of the VISA isolates (45) and 19 of the vancomycin susceptible isolates in the challenge set grew on BHI-V3, for 100% sensitivity and 65% specificity. In the first 60 days the agar was implemented in clinical practice, we identified 17 potential VISA isolates out of 421 S. aureus strains tested. Thirteen of these isolates were confirmed as VISA. The MIC of the confirmed VISA isolates was determined using four different methods. The Microscan Pos MIC Panel 26 had the highest sensitivity of VISA detection (92%), followed by Etest (85%), and Sensititre GPALL (54%). Vitek2 GP67 was the least sensitive, detecting only 1 of the 13 VISA isolates.

Conclusions: BHI-V3 has excellent sensitivity for detection of VISA. We recommend that clinical laboratories use this media to screen all isolates of S. aureus for reduced susceptibility to vancomycin. Isolates that do not grow on the agar can be considered vancomycin susceptible.

Session Details

Date: 10/04/2010
Time: 00:00-00:00
Session name: Abstracts 20th European Congress of Clinical Microbiology and Infectious Diseases
Location: Vienna, Austria, 10 - 13 April 2010
Presentation type:
Back to top