A new methicillin-and gentamicin-resistant Staphylococcus aureus in Dublin: Molecular genetic analysis

Abstract

In June 1985 two new strains of methicillin- and gentamicin-resistant Staphylococcus aureus (MGRSA) were isolated in a Dublin hospital. Of these, one strain spread rapidly, affecting a total of 65 patients during the following 18 months, and subsequently spread to a second Dublin hospital. Detailed laboratory studies, including plasmid screening, plasmid restriction enzyme digest pattern analysis, hybridisation analysis, location of resistance determinants, and bacteriophage typing with a set of experimental S. aureus typing phages, demonstrated that the "new" MGRSA organisms, termed Phenotype III Dublin isolates, were completely distinct from, and unrelated to, the MGRSA strains responsible for serious nosocomial infections in Dublin hospitals during the decade before June 1985. These Phenotype III isolates were very similar to MGRSA organisms isolated in a Baghdad hospital during 1984. Data from plasmid curing, plasmid transfer and hybridisation experiments indicated that 20% of the Phenotype-III isolates expressed chromosomally encoded, high level resistance to ethidium bromide (MIC 120 micrograms/ml), and that this was possibly due to chromosomal integration of a penicillinase-like plasmid.