Antibiotic resistances and plasmids in Staphylococcus aureus from Italian hospitals

A total of 473 Staphylococcus aureus isolates from six Italian hospitals was examined for susceptibility to several antimicrobial agents and for plasmid content. Methicillin-resistant S. aureus (MRSA) were characterised by a plasmid of mol. wt (106) 18–22 or 25 that carried the determinants for penicillinase production, resistance to cadmium ions and resistance to tetracycline. MRSA isolates usually harboured other smaller plasmids of mol. wt (106) 2·8, 2·6 and 1·65 that encoded resistance to tetracycline, chloramphenicol and erythromycin, respectively, and cryptic plasmids of mol. wt (106) c. 2 and 1 were found frequently. Methicillin-sensitive S. aureus (MSSA) that produced penicillinase often carried plasmids of mol. wt (106) 11 or 13. No particular difference was found in plasmid patterns of strains from the various sources. Analysis of plasmids by EcoRI digestion showed that plasmids of similar mol. wt and phenotypic characteristics may have different restriction patterns, but often share one or more fragments in common.