The cefoxitin surrogate agent test is preferred to other methods (CLSI, 2021). This surrogate test predicts results for mecA-mediated methicillin (oxacillin) resistance. Most methicillin (oxacillin) resistance is mediated by mecA gene encoding production of PBP2a (PBP2' or penicillin-binding protein). Testing for both mecA and PBP2a are the most definitive tests for the detection of methicillin (oxacillin) resistance for Staphylococcus.
Susceptibility or resistance to a variety of beta-lactam antimicrobial agents may be deduced from testing only cefoxitin. Refer to current CLSI guidelines for detail. A 30 µg cefoxitin disk is used in combination with Mueller-Hinton agar, incubated for 16-18 hours, and zone sizes of the S. aureus growth are read in mm. A zone of ≥22 mm is interpreted as negative for mecA-mediated resistance, while a zone of ≤21 mm is interpreted as positive for mecA-mediated resistance. Report these isolates as methicillin (oxacillin)-resistant, not cefoxitin-resistant. (Other beta-lactam drugs, except for ceftaroline, should be reported as resistant.)
PBP2a
PBP2a latex tests are commonly used, but false-positive and false-negative results have been observed.
Mechanisms of resistance other than mecA are rare and include mecC. PBP2a tests cannot predict mecC resistance.
Molecular assays
Current molecular assays target mecA (or other functional regions) and detect methicillin (oxacillin)-resistant S. aureus. Always confirm the result with the isolate identification (biochemical tests) and Gram reaction, making sure the isolate is not mixed. Resolve discrepancies by proper confirmatory or alternate testing.
