Ten
penicillin-resistant and -susceptible staphylococci, isolated from
bovine mastitis milk, were studied for the presence of genes that are, or may be, involved in resistance against
penicillin. The repressor (blaI), antirepressor (blaR1), and structural (blaZ) genes of the
beta-lactamase-operon were found to be closely linked in all
penicillin-resistant strains. The
beta-lactamase gene cluster was more commonly located on chromosomal rather than plasmid
DNA in the strains studied. The
transposase (p480) gene, which has been identified in the Staphylococcus aureus
beta-lactamase transposon Tn552, was found in only one single
penicillin-resistant S. aureus strain. The other
penicillin-resistant S. aureus isolates contained IS1181 in close location with the
beta-lactamase gene cluster. In only one S. haemolyticus isolate was the
beta-lactamase gene cluster found in close association with IS257.
Penicillin-resistant S. aureus strains, which were additionally resistant to
tetracycline, contained IS257 in close association with the tetracycline resistance gene (tetK). Sequence analysis of blaI, blaR1, and blaZ in two
penicillin-resistant S. aureus strains revealed 94-96% sequence homology with bla in staphylococci of human origin. The results indicate a predominance of class I bla transposons rather than Tn3 family class II transposons in the isolates used in this study.