Sixteen methicillin-resistant (MRSA) isolates, from a single nosocomial outbreak, were tested for molecular and phenotypic relationships. Two of the 16 outbreak strains were gentamicin resistant (Gm) and the plasmids that they carried were characterised by reverse field electrophoresis, restriction endonuclease analysis and gene hybridisation. The gentamicin-resistant (Gm) strains harboured two plasmids, a Gm plasmid of 36.5 kb and a cryptic plasmid of 25.4 kb, whereas the other 14 isolates contained only the cryptic plasmid. Gentamicin resistance was encoded by a 2.5-kb III fragment of the 32.8-kb plasmid and is similar to the 2.5-kb III fragment also described for Gm plasmids from Australia and the USA. The Gm plasmid was non-conjugal and was cured by ethidium bromide at a frequency of 4%. Two MRSA strains isolated subsequently from the same hospital were also Gm and had identical plasmid and restriction endonuclease profiles to the two Gm strains studied initially. Two other isolates from the original carrier detected in this study and from his son were methicillin and gentamicin susceptible and had novel profiles. Since large plasmids show anomalous migration in agarose gels, more definitive analyses than simple plasmid identification should be considered when studying nosocomial outbreaks.


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