Characterization of Borderline Oxacillin-Resistant Staphylococcus aureus Isolated from Food of Animal Origin

In this study, the molecular characteristics of food-derived oxacillin-resistant Staphylococcus aureus were determined. Eight borderline oxacillin-resistant strains with MICs of 2 to 4 mu g/ml were identified from 132 S. aureus isolates of food origin. One of the two isolates with a MIC of 4 mu g/ml was methicillin-resistant determinant (mecA) gene positive, and the other six with MICs of 2 mu g/ml were mecA negative. The mecA-positive isolate was classified as sequence type (ST)228, staphylococcal protein A (spa) type t041. and carried the staphylococcal cassette chromosome mec type I element. Two borderline oxacillin-resistant strains were classified as spa t008 and ST8, and the remaining five as spa 1164 and ST20. The mecA-positive strain and four borderline oxacillin-resistant strains were found enterotoxigenic. The enterotoxin genes detected in these strains included selp, egc1, and sed-sej-selr. The borderline-resistant S. aureus isolates from a manually handled product, i.e., minced pork, were shown genetically related to strains associated with human infections. This suggests that humans can be considered as a source of contamination of this food with oxacillin-resistant S. aureus strains. The genotypes of the investigated milk borderline-resistant isolates were shown to occur not only in cows, but also in humans. Since manual handling is reduced in raw milk production, a human origin of S. aureus seems unlikely. Because knowledge of the genotypes of animal staphylococci is limited, more research is needed to address the question of the origin of antibiotic-resistant S. aureus strains in food.