Evaluation of oxoid mannitol salt agar with cefoxitin for the detection of methicillin-resistant Staphylococcus aureus (MRSA) from surveillance specimens

Abstract

OBJECTIVES: Challenged by an increasing demand for MRSA surveillance and declining resources, clinical laboratories require sensitive and specific screening media. METHODS: This study compares Mannitol Salt Agar base™ (Difco Laboratories, Detroit, Michigan) containing either 4 ug/mL oxacillin (MSO) or 10 ug/mL cefoxitin (MSF) to Aniline Blue Salt agar with 10 ug/mL cefoxitin (ABF) (all manufactured by Oxoid Inc., Nepean, Ontario). A total of 2561 MRSA screens from 1313 patients (775 nasals, 733 rectal, 266 wound, 682 nasal/axilla/groin/perineum, 105 others) were planted onto MSO, MSF, and ABF. MSO was always planted first, while the order of inoculation for MSF and ABF was varied every 200 specimens. Media were incubated at 35°C and read blinded at 18-24 hours and 48 hours. Yellow colonies from MSO and MSF, and blue colonies from ABF were tested for capsular antigens 5 and 8 (Pastorex Staph Plus, Sanofi Diagnostics Pasteur, Redmond, Washington), tube coagulase (Remel Inc., Lenexa, Kansas), and PBP2a (Denka Seiken Co., Ltd., Tokyo) to identify MRSA. PBP2a reactions were confirmed using the NCCLS 6 ug/mL oxacillin agar. Pulsed-field gel electrophoresis (PFGE) was performed on all discrepant MRSA isolates. RESULTS: Overall, 158 MRSA grew in the 48 hour study period: 136 from MSF, 104 from MSO, 102 from ABF, while 75 grew on all three media. The sensitivity for each media was: MSF (87.8%), MSO (74.5%), and ABF (55.1%). From MSF, 41/136 (30.1%) MRSA grew CONCLUSION: MSF plates were less costly and significantly more sensitive than both MSO and ABF (p