Which test is commonly used to screen MRSA phenotypically in AST workflows?

Which test is commonly used to screen MRSA phenotypically in AST workflows?

• A. Oxacillin or cefoxitin resistance
• B. Vancomycin susceptibility
• C. AmpC beta-lactamase activity
• D. Penicillin G susceptibility

Answer: (A)

Screening for MRSA phenotypically relies on detecting resistance to beta-lactams produced by mecA or mecC, which alter the target PBPs and confer a methicillin-resistant phenotype. In routine antimicrobial susceptibility testing workflows, the most reliable way to flag MRSA is to test for resistance to oxacillin or, more robustly, to cefoxitin. Cefoxitin acts as a strong inducer of mecA-mediated resistance, and disk diffusion with cefoxitin (or related phenotypic screens) typically provides a clear, reproducible separation between susceptible and resistant Staphylococcus aureus. When an isolate resists cefoxitin, it strongly indicates an MRSA phenotype, prompting confirmatory molecular tests or alternative care pathways. Other tests address different concerns. Vancomycin susceptibility checks resistance to vancomycin rather than methicillin-class beta-lactams and is used to identify VISA/VRSA concerns. AmpC beta-lactamase activity is more relevant for certain Gram-negative bacteria and does not inform MRSA status. Penicillin G susceptibility is unreliable for S. aureus because many strains produce beta-lactamase, making penicillin G ineffective regardless of MRSA status.

Screening for MRSA phenotypically relies on detecting resistance to beta-lactams produced by mecA or mecC, which alter the target PBPs and confer a methicillin-resistant phenotype. In routine antimicrobial susceptibility testing workflows, the most reliable way to flag MRSA is to test for resistance to oxacillin or, more robustly, to cefoxitin. Cefoxitin acts as a strong inducer of mecA-mediated resistance, and disk diffusion with cefoxitin (or related phenotypic screens) typically provides a clear, reproducible separation between susceptible and resistant Staphylococcus aureus. When an isolate resists cefoxitin, it strongly indicates an MRSA phenotype, prompting confirmatory molecular tests or alternative care pathways.

Other tests address different concerns. Vancomycin susceptibility checks resistance to vancomycin rather than methicillin-class beta-lactams and is used to identify VISA/VRSA concerns. AmpC beta-lactamase activity is more relevant for certain Gram-negative bacteria and does not inform MRSA status. Penicillin G susceptibility is unreliable for S. aureus because many strains produce beta-lactamase, making penicillin G ineffective regardless of MRSA status.