Genetics of streptomycin resistance in methicillin-sensitive multiply-resistant Staphylococcus aureus

Comparison of genes involved in penicillin resistance in staphylococci of bovine origin

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.

Conjugative transfer of high-level mupirocin resistance and the mobilization of non-conjugative plasmids in Staphylococcus aureus

A 31-kb conjugative plasmid, pXU12, encoding high-level mupirocin resistance via the mupA gene, was isolated from a multiply resistant Staphylococcus aureus isolate, MB494. pXU12 was derived by a deletion of an 8.6-kb EcoRI fragment from a approximately 40-kb plasmid in the parental isolate during curing and conjugation experiments. It transferred rapidly in conjugation experiments, with transconjugants being obtained after 15 min of mating, and mobilized a 3.0-kb erythromycin resistance plasmid, pXU13, from the parental isolate at high frequencies. The cotransfer of pXU13 by pXU12 was unaffected by varying the donor-recipient ratios in the mating mixtures or the length of incubation. pXU12 also mobilized 11 other nonconjugative plasmids belonging to different incompatibility groups and cotransferred at high frequencies. The ability of pXU12 to mobilize different nonconjugative plasmids suggested that it can be used to transfer and isolate non-conjugative plasmids from resistant S. aureus strains in the laboratory, especially from strains where phage-dependent methods of transfer are not applicable.