Enhanced detection of methicillin-resistant Staphylococcus aureus using chromogenic media compared to traditional culture methods

Abstract

ISSUE: Methicillin-resistant S. aureus (MRSA) has become a widespread nosocomial pathogen. The use of active surveillance cultures to detect nasal carriage of MRSA is recommended to identify carriers and initiate contact precautions. OBJECTIVE: To evaluate time to detection (TTD) of MRSA using chromogenic media compared to traditional culture methods from nasal surveillance cultures. METHODS: A total of 1133 nasal samples were plated to sheep blood agar (SBA) and BBL CHROMagar™ MRSA (C-MRSA, BBL, Sparks, Maryland). MRSA appear as mauve colonies on C-MRSA. All other organisms including methicillin-susceptible staphylococci are inhibited or produce a distinctly different colony color. If mauve colonies are detected at 24 hours, MRSA may be reported without further testing. At 48 hour detection, a slide coagulase needs to be completed. MRSA was identified from SBA by slide coagulase, exogenous DNase, and mannitol fermentation. Susceptibility testing was performed using agar dilution. RESULTS: Of the 1133 samples tested, 78 (7.4%) were positive for MRSA. Forty-one of 78 (53%) had a TTD of 24 hours from C-MRSA and 48 hours from SBA. Eleven of 78 (14%) had a TTD of 24 hours from C-MRSA and greater than 48 hours from SBA. At 24 hours, an additional 11 (14%) isolates were recovered on C-MRSA that were not detected on SBA. After 48 hours incubation, seven of 78 (9%) were detected on both media, while an additional five of 78 (6%) had TTD of 48 hours on C-MRSA and equal or greater than 72 hours on SBA. An additional two isolates were recovered on C-MRSA at 48 hours that were not detected on SBA and one isolate had a TTD of 72 hours from SBA and was not recovered on C-MRSA. CONCLUSION: Sixty-three of 78 (81%) of MRSA were reported at 24 hours using C-MRSA versus 48 hours or greater using traditional culture methods. Eleven of 63 (17%) were reported 3 or more days later than the C-MRSA result. Eleven of 63 (17%) were missed by traditional culture methods. A more rapid positive report from nares surveillance cultures may assist the infection control practitioner in identifying colonized patients, initiate contact precautions, and potentially decrease the spread of MRSA.