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Background  Colonization of dairy cows by  Staphylococcus aureus (S. aureus) , especially those which are multi-drug resistant and toxin producing, is a concern for animal health and well-being as well as public health. The objective of this study was to investigate the prevalence, antibiotic resistance, gene content and virulence determinants of  S. aureus  in bulk tank milk samples ( BTM ) from U.S. dairy herds.    Results  BTM samples were collected, once in winter and once in summer, from 189 U.S. dairy herds. Of 365 BTM samples cultured, the sample and herd prevalence of  S. aureus  in BTM was 46.6% (170 of 365 samples) and 62.4% (118 of 189 herds), respectively. Among a subset of 138  S. aureus  isolates that were stored for further analysis, 124 were genome sequenced after being confirmed as  S. aureus  using phenotypic tests. The most commonly identified antimicrobial resistance-associated gene was  norA  (99.2%) and  mecA  gene responsible for methicillin resistance ( MRSA ) was identified in one isolate (0.8%). The most frequently detected putative virulence genes were  aur  (100%),  hlgB  (100%),  hlgA, hlgC, hlb  (99.2%),  lukE  (95.9%) and  lukD  (94.3%). In the 53 staphylococcal enterotoxin positive isolates,  sen  (37.9%),  sem  (35.5%),  sei  (35.5%) and  seg  (33.1%) were the most frequently detected enterotoxin genes. Among the 14 sequence types (ST) and 18  spa  types identified, the most common was ST2187 (20.9%) and t529 (28.2%), respectively. The most predominant clone was CC97 (47.6%) followed by CC unknown (36.3%). The single MRSA isolate belonged to ST72-CC8,  spa  type t126 and was negative for the  tst  gene but harbored all the other virulence genes investigated.    Conclusion  Our findings indicated a high prevalence of  S. aureus  in BTM of U.S. dairy herds, with isolates showing little evidence of resistance to antibiotics commonly used to treat mastitis. However, isolates often carried genes for the various enterotoxins. This study identified predominant genetic clones. Despite lower prevalence, the presence of MRSA and multi-drug resistant strains in BTM poses a significant risk to animal and public health if their number were to increase in dairy environment. Therefore, it is necessary to continuously monitor the use of antibiotics in dairy cows.

Prevalence, antibiotic resistance, virulence and genetic diversity of Staphylococcus aureus isolated from bulk tank milk samples of U.S. dairy herds