D2 Comparison of BBL^TM CHROMagar ^TM MRSA to Currently Used Media for the Detection of Methicillin Resistant Staphylococcus aureus.

S. KIRCHER*, N. DICK, V. RITTER, K. STURM, N. VAZIRI, M. STANOJEVIC.  Becton Dickinson Diagnostic Systems, Cockeysville, MD.

Objective: To compare the recovery and identification of MRSA strains on BBL CHROMagar MRSA(CMRSA), Oxacillin Resistance Screening Agar Base (ORSAB, Oxoid Limited, Hampshire) and BBL Oxacillin Screen Agar (Ox). CMRSA contains a unique beta lactam agent for the detection of methicillin resistant Staphylococcus aureus (MRSA) in surveillance cultures.

Methods:  Forty-one mecA positive strains of MRSA; 20 S. aureus non-MRSA (mecA negative); 20 coagulase- negative-staphylococci; and 10 other organisms; including streptococci, enterococci, Neisseria and Candida were evaluated at 24 and 48 hours. For plating on CMRSA and ORSAB, MRSA strains were inoculated at a concentration of 10³ CFUs and all other organisms at a concentration of 10⁵ CFUs.  Ox plates were spot-inoculated using 10 ul of each organism suspension adjusted to a 0.5 McFarland turbidity standard per the manufacturer's instructions.  All plates were read by two separate readers and the scores averaged.  

Results: At 24 hours, CMRSA detected 37 of 41 MRSA strains (sensitivity 90%), ORSAB detected 30 of 41 (sensitivity 73%) and Ox detected 41 0f 41 (sensitivity 100%).  At 48 hours the sensitivity increased for CMRSA and for ORSAB to 100% and 95%, respectively. CMRSA demonstrated a specificity of 96% (48 of 50) at 24h, ORSAB 92% (46 of 50) and Ox 76% (38 of 50).    

Conclusion: CMRSA provides  reliable identification of MRSA and may potentially offer the user a direct screen for clinical samples, pending prospective clinical studies.