Distribution of plasmid-borne resistance to nucleic acid binding compounds in methicillin-resistant Staphylococcus aureus.

Methicillin-resistant Staphylococcus aureus (MRSA) which are epidemic in Australian hospitals have been found to be genetically different from strains previously isolated in Australia and in Europe. A characteristic feature of the recent Australian isolates is a plasmid which encodes resistance to nucleic acid binding compounds (NAB) such as propamidine isethionate. Of 753 MRSA strains isolated from around the world in 1980-1983, c. 60% were NAB resistant. The determinant for this resistance could be transferred from 90% of strains to a recipient strain in mixed culture. A plasmid analysis revealed that, in general, the largest plasmid of MRSA isolates coded for NAB resistance and may carry other determinants for penicillinase production and resistance to gentamicin, trimethoprim, neomycin, tetracycline, cadmium and mercury. Some plasmids exhibited unusual behaviour with the appearance of deletion mutants after transfer and, in one case, a high-frequency alteration in the expression of gentamicin resistance from high-level to low-level resistance, correlated to a deletion of c. 0.5 megadalton of plasmid DNA. These results demonstrate that these NAB-resistance plasmids are not unique to Australian MRSA strains but are widely distributed throughout the world.