Evolution of Australian isolates of methicillin-resistant Staphylococcus aureus: a problem of plasmid incompatibility?

Methicillin-resistant Staphylococcus aureus (MRSA), currently causing problems in Australian hospitals, have chromosomal penicillinase and carry a new family of incompatibility group I (IncI) plasmids that encode resistance to nucleic acid-binding compounds (NAB). These plasmids may carry additional determinants for penicillinase production and resistance to gentamicin and trimethoprim. By comparison, earlier MRSA isolates from Australia were NAB-sensitive and the penicillinase determinants were carried on IncI plasmids. The possibility that these newer MRSA isolates have the same 'clonal' origin as other MRSA isolates has been investigated. Forcible maintenance of IncI penicillinase plasmids and NAC-resistance plasmids in the same cells resulted in various recombination events. Similar recombination events to those generated in the laboratory have been found in MRSA isolates.