Plasmid analysis of 219 methicillin-resistant Staphylococcus aureus strains with uncommon profiles isolated in Lisbon.

During the years 1986 and 1987, 219 methicillin-resistant Staphylococcus aureus (MRSA) strains were isolated in a Lisbon hospital. Antimicrobial susceptibilities and genetic analysis showed that resistance to penicillin, methicillin, erythromycin (inducible phenotype), tetracycline, gentamicin, tobramycin, kanamycin, streptomycin, neomycin and trimethoprim were chromosomally encoded. Plasmid DNA was absent from 38.8% of the isolates. Constitutive erythromycin resistance was coded by three plasmids of c. 2.3 Md, c. 2.0 Md and c. 1.6 Md. Chloramphenicol resistance was mediated by two plasmids of c. 2.9 Md and c. 1.8 Md. Small cryptic plasmids of c. 1.65 Md, c. 1.2 Md and c. 1.0 Md were also detected. The majority of the strains revealed antigen 17, were lysed by phages 75, 89 and/or 85, and were either devoid of plasmid DNA, or possessed a c. 1.6 Md plasmid coding for constitutive erythromycin resistance or a c. 1.0 Md cryptic plasmid. These observations cannot rule out that the MRSA Lisbon isolates are a homogeneous group of strains that might have a common origin, and seem to be different from MRSA recently isolated in other countries.