In MRSA detection, what does a positive cefoxitin screen suggest?

In MRSA detection, what does a positive cefoxitin screen suggest?

• A. Susceptible to methicillin
• B. Likely MRSA; cefoxitin resistance indicates mecA/mecC-mediated methicillin resistance.
• C. MRSA cannot be detected by cefoxitin screen
• D. Cefoxitin screen indicates vancomycin resistance

Answer: (B)

Cefoxitin serves as a reliable indicator for methicillin resistance in Staphylococcus aureus because it strongly induces the expression of mecA/mecC, the genes that encode PBP2a with low affinity for beta-lactams. A positive cefoxitin screen means the isolate likely carries mecA or mecC and is resistant to beta-lactam antibiotics, i.e., MRSA. This is why cefoxitin testing is preferred for MRSA detection—its resistance phenotype more accurately reflects mecA/mecC-mediated resistance than some older methicillin tests. It does not imply susceptibility to methicillin, and it does not address vancomycin resistance, which requires separate testing. Confirmatory methods include molecular detection of mecA/mecC or PBP2a protein testing.

Cefoxitin serves as a reliable indicator for methicillin resistance in Staphylococcus aureus because it strongly induces the expression of mecA/mecC, the genes that encode PBP2a with low affinity for beta-lactams. A positive cefoxitin screen means the isolate likely carries mecA or mecC and is resistant to beta-lactam antibiotics, i.e., MRSA. This is why cefoxitin testing is preferred for MRSA detection—its resistance phenotype more accurately reflects mecA/mecC-mediated resistance than some older methicillin tests. It does not imply susceptibility to methicillin, and it does not address vancomycin resistance, which requires separate testing. Confirmatory methods include molecular detection of mecA/mecC or PBP2a protein testing.