Which testing approach best detects mecA-mediated oxacillin resistance in Staphylococcus aureus during routine broth microdilution susceptibility testing?

Which testing approach best detects mecA-mediated oxacillin resistance in Staphylococcus aureus during routine broth microdilution susceptibility testing?

• A. Addition of 4% NaCl
• B. Incubation at 30C
• C. Incubation for 48 hours
• D. Testing with cefoxitin

Answer: (D)

The key idea is how mecA makes Staphylococcus aureus resistant and how to reliably reveal that resistance in testing. The mecA gene encodes PBP2a, a penicillin-binding protein with very low affinity for beta-lactams, which is what causes methicillin resistance. To detect mecA-mediated resistance, you need an agent that strongly induces mecA expression so the resistance becomes evident in the test result. Cefoxitin is a potent inducer of mecA expression, more reliably triggering the resistant phenotype than oxacillin in routine broth microdilution. When you test with cefoxitin, MRSA isolates carrying mecA typically appear resistant at established breakpoints, providing a clearer, more accurate detection of mecA-mediated resistance. Other approaches like adding salt, lowering the temperature, or extending incubation don’t induce mecA as effectively or consistently, so they’re less reliable for this purpose.

The key idea is how mecA makes Staphylococcus aureus resistant and how to reliably reveal that resistance in testing. The mecA gene encodes PBP2a, a penicillin-binding protein with very low affinity for beta-lactams, which is what causes methicillin resistance. To detect mecA-mediated resistance, you need an agent that strongly induces mecA expression so the resistance becomes evident in the test result. Cefoxitin is a potent inducer of mecA expression, more reliably triggering the resistant phenotype than oxacillin in routine broth microdilution. When you test with cefoxitin, MRSA isolates carrying mecA typically appear resistant at established breakpoints, providing a clearer, more accurate detection of mecA-mediated resistance. Other approaches like adding salt, lowering the temperature, or extending incubation don’t induce mecA as effectively or consistently, so they’re less reliable for this purpose.