Bovine Staphylococcus aureus mastitis: strain recognition and dynamics of infection

Restriction enzyme fragmentation pattern (REFP) analysis was used to recogn ise Staphylococcus aureus strain variation in naturally occurring bovine su bclinical mastitis. Multiple colony REFP analysis identified eight distinct strains of S. aureus in addition to the original strains A and B that were infused via the intramammary route, indicating that individual quarters of the udder may be colonised simultaneously by more than one strain of S. au reus. Examination of multiple colonies per milk sample may benefit bacteria l strain recognition as an epidemiological tool in mastitis investigations. The dynamics of intramammary infection were determined using a novel doubl e crossover experimental challenge. Quarters remained persistently infected for several weeks following challenge in all four cows, irrespective of th e challenge strain. This indicated that no alteration of the original subcl inical infection, including the possible induction of clearance of the quar ters infected with S. aureus, or replacement of the original strain by the infused strain was induced by challenge. The persistent subclinical infecti on in all four animals supports previous reports on the chronicity of S. au reus intramammary infection in dairy cows.