Comparison of methods for the detection of methicillin resistance in Staphylococcus aureus isolates from food products

To compare several methods for detection of methicillin resistance in Staphylococcus aureus isolates from food. Two hundred S. aureus isolates from food of animal origin were screened for methicillin resistance by a PCR assay specific for the mecA gene, an oxacillin agar screen test and a cefoxitin disk diffusion test. Six out of 200 strains (3%) were found to be methicillin-resistant Staphylococcus aureus (MRSA) by PCR. The oxacillin agar screen test detected only one of the MRSA isolates (sensitivity of 16·7%) and mischaracterized three additional strains as MRSA (specificity of 98·45%). None of the MRSA strains was detected by the cefoxitin test (sensitivity of 0%), while 15 methicillin-susceptible S. aureus (MSSA) strains were misclassified as resistant (specificity of 92·3%). Fifteen MSSA strains displayed a β-lactamase hyperproducer-like phenotype. The six MRSA (mecA-positive) strains resembled the characteristics of heteroresistant strains. As MRSA of animal origin may display atypical phenotypes, PCR appears to be more reliable for detection of methicillin resistance in animal strains. The study stresses the need for implementing the methods of screening S. aureus from food of animal origin for methicillin resistance.