Prevalence, genetic diversity, and antimicrobial susceptibility profiles of Staphylococcus aureus isolated from bulk tank milk from Greek traditional ovine farms

Three hundred forty two milk bulk samples were tested for the presence of Staphylococcus aureus (S. aureus) originating from 114 ovine farms in Greece that followed 'transhumant breeding system'. The contamination of farms was low and was recorded at the level of 25.4%, 23.7% and 20.2% during the three sampling periods showing no significant difference. Eighty six out of 106 isolates (81%) were pansusceptible to the 16 antimicrobials tested, strongly suggesting that antibiotics are not used in a routine basis in this breeding system. Resistance was mainly to penicillin and found only at the rate of 6.6%. PFGE revealed 89 distinct pulsotypes, among 106 different isolates, clustering the majority of them into 6 main groups (80% similarity). PFGE showed that predominant clusters were circulating in this region during the study period and also that antimicrobial resistance didn't affect the predominance of these clusters. Basic biochemical tests can be used only as screening methods to identify S. aureus in raw ewe's milk, additional tests are needed such as STAPH-ID or PCR for confirmation. The findings of this work underscore the value of traditional ovine farming systems in food safety and antimicrobial resistance evolution. These traditional farming systems do not have high rates of antimicrobial agent consumption and subsequently low rates of antimicrobial resistance.