Staphylococcus aureus isolates collected from sites of intramammary infection during a 10-month period and from extramammary sites (dairy cow teat skin, teat canals, and skin lesions; milking liners; and hands and nostrils of milking personnel) at two separately managed Finnish dairy herd establishments were analyzed to study the sources and reservoirs of bovine S. aureus intramammary infection. Selected isolates were subjected to pulsed-field gel electrophoresis (PFGE) typing and PCR analysis for genes encoding hemolysins (hla to hlg), leukocidins (lukED and lukM), superantigens (sea, sec, sed, seg to seo, seu, and tst), adhesins (fnbA and fnbB), and penicillin and methicillin resistance (blaZ and mecA). S. aureus was found throughout the herds in 94% of the cows. Nine PFGE types were found, with the herds each having their own predominant type and sharing one type. The degree of diversity of PFGE types in herd II, which integrated foreign heifers, was higher than that in herd I. For both herds, the majority of the PFGE-typed isolates both from milk and from extramammary sites represented the predominant PFGE types. In isolates from herd I, the most prevalent genes were hla-hlg, lukED, and fnbA; in those from herd II, they were hla, hld, hlg, lukED, and fnbA. The other genes were pulsotype linked within the herds. The predominant PFGE types carried both fnbA and fnbB; only fnbA was detected in the other PFGE types. No connection between specific virulence genes and the origins of isolates was found. The results suggest that for the two herds, most S. aureus isolates from extramammary sites were indistinguishable from the isolates infecting the mammary gland and that those sites can thus act as origins and reservoirs of intramammary infections. However, contamination in the opposite direction cannot be excluded.

Aims:  To screen putative virulence genes in Staphylococcus aureus causing persistent and nonpersistent bovine intramammary infections (IMI) with different clinical characteristics. To examine, whether a possible relationship exists between genetic profile and infection persistence, clinical signs of infection, clonal type determined by pulsed‐field gel electrophoresis (PFGE), and antimicrobial resistance. Methods and Results:  One‐hundred and sixty‐one S. aureus isolates derived from bovine IMI, consisting of 17 different PFGE types, were screened by conventional and multiplex‐polymerase chain reaction (PCR) for 24 virulence genes for haemolysins (hla‐hlg), leukocidins (lukED, lukM), exfoliative toxins (eta, etb), enterotoxins (sea‐seo, seu), toxic‐shock syndrome toxin (tst), and genes encoding penicillin (blaZ) and methicillin resistance (mecA). The majority of S. aureus isolated at the onset of mastitis carried haemolysin genes (76·7–97·4%), lukED (96·6%), and at least one gene for pyrogenic toxin superantigen (PTSAg) (69·0%). Strains carrying PTSAg‐encoding genes were more common among predominant PFGE types and in persistent IMI. Strains concomitantly possessing sed, sej, and blaZ, putatively plasmid‐encoded, were typically found in connection with persistent IMI. Conclusions:  Our results suggest that certain genetic elements are over‐representative in S. aureus isolates especially from persistent bovine mastitis. This phenomenon seems to be in connection with clonal type and is often concomitant with penicillin resistance. Significance and Impact of the Study:  This is the first study to investigate associations between a large number of bacterial factors and outcome of S. aureus mastitis. The finding that widespread clonal types of S. aureus causing bovine mastitis of low treatment response may harbour characteristic genes could be improved for strain‐specific diagnostic purposes.