B. M. WILLEY, L. PEARCE, D. CHEN, T. C. MOORE, B. TENNANT, G. RUZO, A. MCGEER, D. E. LOW, M. SKULNICK. Mount Sinai and Princess Margaret Hospitals, Univ of Toronto, Toronto, Ontario, Canada.

Due to the increased rates of MRSA worldwide, it is crucial for laboratories to have rapid, accurate and simple methods for the detection and confirmation of MRSA. Although the NCCLS oxacillin plate is an excellent screening method, breakthrough of some susceptible S. aureus is not uncommon due to subtle media failures and the occurrence of borderline oxacillin-resistant strains of S. aureus. These factors make the confirmation of all new MRSA essential. This study challenged a MRSA slide agglutination kit (Denka Sieken, Med-Ox, Ottawa, ON) which contains monoclonal antibodies to the mecA PBP 2’ protein, to detect oxacillin resistance in 371 clonally diverse strains of Canadian and Argentinian S. aureus. Isolates were characterized using the oxacillin screen plate followed by the BBL Crystal MRSA ID kit as a confirmatory test. SmaI PFGE typing was performed on all MRSA. PBP 2’ agglutination was performed blind once the isolates had been identified by the above methods. PCR amplification of the mecA and nucA genes was performed on isolates with discrepant results. 230/371 (62%) of S. aureus were Crystal positive, whereas 225 (61%) were positive for PBP 2’ protein with the Denka kit. PCR found 3/5 discrepant isolates to be mecA negative even though one of these belonged to a known MRSA clone as determined by PFGE and spa gene sequencing. On repeat testing, the 2 discrepant mecA positive isolates agglutinated in the Denka at 4 min rather than 3 min as recommended by the manufacturers, and the Crystal was negative on repeat of the 3 mecA-negative Crystal-positive isolates. The Crystal and the Denka had resulting sensitivities of 100% and 99% and specificities of 98.6% and 100%, respectively. In conclusion, the Denka Kit was found to be a simple and accurate test for the rapid (15 min) identification of MRSA.

Presented at:

AMERICAN SOCIETY OF MICROBIOLOGY (ASM) 99^th General Meeting, Chicago, IL, May 30 –June 3, 1999