Background: Rapid and reliable detection of methicillin-resistant Staphylococcus aureus(MRSA) carriers is crucial for the effective control of MRSA transmission in healthcare facilities. The aim of this study is to evaluate ChromID MRSA agar (MRSA ID) and mannitol salt agar containing 4 mg/L oxacillin (MSAox) for their ability to detect MRSA in nasal swab specimens from high risk intensive care unit (ICU) patients.
Materials and Methods: paired nasal swab specimens were taken from 160 ICU patients. Nasal swabs were suspended in 0.5 ml physiological (0.85%) saline for homogenization.One swab was directly plated on MRSA ID, MSA and MSAox , and the other swab was inoculated on enrichment broth composed of brain heart infusion supplemented with 7% NaCl for 24 h and subcultured on MRSA ID and MSAox. Selective agar plates were incubated at 37°Cand analyzed after 24 h. If no growth was observed, incubation was prolonged to 48 h. Presumptive Staphylococcus aureus(S. aureus)isolates (green-colonies on MRSA ID or yellow colonies on MSA and MSAox ) were identified by Gram staining, catalase production and tube coagulase testing. MRSA was confirmed using cefoxitin disk diffusion method according to CLSI recommendations.
Results: The sensitivity of MRSA ID was found to be higher than MSAox in detecting MRSA. The sensitivities were 81%versus 71% when directly plated (p=0.23). Broth enrichment increased the sensitivity of MRSA ID from 81% to 95% after 24h (p=0.07) and to 100% after48h (p=0.02), and also increased the sensitivity of MSAox from 71% to 76 %( p=0.37).
Conclusion: We conclude that MRSA ID combined with broth enrichment is a sensitive and specific medium for the isolation and identification of MRSA in surveillance swabs.