Phenotypic and genetic characterisation of bacteriocin-producing strains of Staphylococcus aureus involved in bovine mastitis

Fifty strains of Staphylococcus spp. isolated from bovine mastitis cases in several herds from different Argentinian provinces were screened for antimicrobial substances. Twelve strains exhibited a high antagonistic activity against the indicator strain (Corynebacterium fimi) and were chosen for further characterisation. The antimicrobial substances were sensitive to proteolytic enzymes suggesting that they might be bacteriocins (Bac). These strains were identified as S. aureus by the amplification of the femA gene. Plasmid profile analysis of these strains revealed the presence of at least one plasmid. Eleven strains carried a plasmid with a size similar to that of pRJ6 (8.0kb), which encodes aureocin A70, a bacteriocin produced by the Brazilian S. aureus strain A70 isolated from commercial milk. The other strain harboured a much larger plasmid. PCR experiments, using specific primers for amplification of the bacteriocin operon found in pRJ6, showed that all strains had the expected 525bp amplicon, suggesting that the bacteriocin produced may be related to aureocin A70. The genomic DNA of all Bac(+) strains was then analysed by pulsed-field gel electrophoresis (PFGE) in order to investigate clonal relationships amongst strains. Based on the results of PFGE experiments, 10 out of the 12 Bac(+) strains belonged to the same clone. The remaining two strains are possibly related to the prevalent clone. The aureocin A70 producer-strain belonged to a distinct clone.