Antibiotic resistance assessment in S. aureus strains isolated from raw sheep's milk cheese

In vitro activities of 16 antibiotics were tested against 36 Staphylococcus aureus (SA) strains isolated from raw sheep's milk cheese from six dairies. The minimum inhibitory concentration (MIC) was determined using a broth microdilution method (CLSI). All 36 isolates were analyzed for the presence of the accessory gene regulator gene, agr (I-IV), and genes encoding resistance to methicillin (mecA), erythromycin (ermA), penicillin (blaZ), and vancomycin (vanA-B). The isolates were also analyzed for similarities in pulsed-field gel electrophoresis (PFGE) patterns. SA strains showed resistance to ampicillin (36.1%), penicillin (33.3%), tetracycline (11.1%), and cloxacillin (2.8%) but were susceptible (>= 94.4%) to 12 out of 16 tested antimicrobials. The overall susceptibility of the strains to oxacillin, vancomycin, and erythromycin was confirmed by the absence of the mecA, vanA-B, and ermA genes. The PFGE results showed that 32 strains belonged to 10 different clusters (P1-P10) while four strains were untypeable.