Evaluation of the BBL Crystal MRSA ID System for detection of oxacillin resistance in Staphylococcus aureus

Aims - To evaluate the BBL(R) Crystal(TM) MRSA ID System for detection of oxacillin resistance in Staphylococcus aureus. Methods - 52 methicillin resistant S aureus (MRSA) from five different countries and 85 methicillin susceptible S aureus (MSSA) were included. The species was confirmed by tube coagulation and detection of the clumping factor using the Staphaurex Plus. Clonal non-identity of the MRSA isolates was shown by pulsed field gel electrophoresis. MIC values (oxacillin) were determined using the Etest. Polymerase chain reaction was carried out to detect the mecA gene. The BBL Crystal MRSA ID System was carried out according to the manufacturer's instructions. Results - The BBL Crystal MRSA ID System showed fluorescence in 15 of 52 MRSA (sensitivity 86.5%; negative predicitive value 92.2%), and the specificity was 97.6% (positive predicitive value 95.7%). Two of seven MRSA that failed to show fluorescence had MIC values (oxacillin) of 4 mg/litre. Conclusions - The BBL Crystal MRSA ID System is a valuable test for detecting oxacillin resistance in S aureus Its major advantage is the short time (4-5 hours) required to perform the test when organisms are grown on tryptic soy agar or sheep blood agar. Difficulties may arise in borderline resistant isolates.