Plasmid-mediated antibiotic resistance in methicillin-resistant Staphylococcus aureus.

Strains of methicillin-resistant Staphylococcus aureus (MRSA), isolated from clinical sources in Victoria, were screened for the presence of plasmid DNA. Resistance to gentamicin, kanamycin, and tobramycin was found to reside on an 18-megadalton plasmid, and resistance to chloramphenicol was mediated by a 3-megadalton plasmid. No extra-chromosomal locus was found for resistance to several other antibiotics, including methicillin. Local strains of MRSA differ from their international counterparts by the possession of extensive chromosomal resistance determinants. Together with those encoded on plasmids, these determinants are capable of neutralising almost all available antistaphylococcal antibiotics.