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

Aims: To compare several methods for detection of methicillin resistance in Staphylococcus aureus isolates from food. Methods and Results: 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 center dot 7%) and mischaracterized three additional strains as MRSA (specificity of 98 center dot 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 center dot 3%). Fifteen MSSA strains displayed a beta-lactamase hyperproducer-like phenotype. The six MRSA (mecA-positive) strains resembled the characteristics of heteroresistant strains. Conclusions: As MRSA of animal origin may display atypical phenotypes, PCR appears to be more reliable for detection of methicillin resistance in animal strains. Significance and Impact of the Study: The study stresses the need for implementing the methods of screening S. aureus from food of animal origin for methicillin resistance.