Occurrence of the blaZ gene in penicillin resistant Staphylococcus aureus isolated from bovine mastitis in Denmark.

Fifty-eight penicillin resistant Staphylococcus aureus isolates of known phage types from bovine mastitis in Denmark from the 1950'ties (10 isolates) and the 1990'ties (48 isolates) were tested for beta-lactamase production. Furthermore, the presence of blaZ and blaR1 and the location of blaZ was determined by PCR and hybridisation. All isolates produced beta-lactamase and contained blaZ and blaR1. The blaZ gene was located on the chromosome in 54 isolates and on plasmids of different sizes in 4 isolates. Sequence analysis of an internal region of blaZ in 2 isolates of bovine origin showed a high degree of homology to already published sequences from human isolates. BlaZ could be transferred from the 4 isolates with plasmid location whereas it was not possible to transfer blaZ from 3 isolates with chromosomal location of the gene. The blaZ gene and the blaR1 gene were located closely to each other as previously published. In contrast to observations among isolates of human origin, no correlation between penicillin resistance and phage pattern was indicated for the bovine isolates. Furthermore, in contrast to the observed shift towards increased occurrence of plasmid location of blaZ among isolates of human origin in Denmark, blaZ appears to remain predominately chromosomal located among isolates of bovine origin.