The Journal of Medical Microbiology, Vol 30, Issue 1 51-58, Copyright © 1989 by Society for General Microbiology
Strategies for molecular characterisation of methicillin- and gentamicin-resistant Staphylococcus aureus in a Canadian nosocomial outbreak
N. Bigelow, L. K. Ng, H. G. Robson and J. R. Dillon
Antimicrobials and Molecular Biology Division, Laboratory Centre for Disease Control, Ottawa, Ontario, Canada.
Sixteen methicillin-resistant Staphylococcus aureus (MRSA) isolates, from a
single nosocomial outbreak, were tested for molecular and phenotypic
relationships. Two of the 16 outbreak strains were gentamicin resistant
(Gmr) and the plasmids that they carried were characterised by reverse
field electrophoresis, restriction endonuclease analysis and gene
hybridisation. The gentamicin-resistant (Gmr) strains harboured two
plasmids, a Gmr 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 HindIII fragment of the
32.8-kb plasmid and is similar to the 2.5-kb HindIII fragment also
described for S. aureus Gmr plasmids from Australia and the USA. The Gmr
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 Gmr and had identical plasmid and restriction endonuclease profiles to
the two Gmr strains studied initially. Two other S. aureus 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.
