Summary. Strains of methicillin-resistant Staphylococcus aureus (MRSA) from Australia and the UK were compared by digesting their chromosomal DNA with the low-frequencycutting restriction enzyme SmaI and separating the restriction fragment length polymorphisms (RFLPs) by contour-clamped homogeneous electric field (CHEF) electrophoresis. The numbers of restriction fragments produced were in the range 14-17 and the sizes of the bands were 7-700 kb. Generally, the results confirmed previous conclusions based on antimicrobial resistance and plasmid profiles. The earlier MRSA isolates were different from more recent isolates, and the epidemic MRSA from eastern Australia (EA MRSA) was the same as the epidemic MRSA (EMRSA) found in London hospitals. However, contrary to previous results, the EA MRSA did not constitute a homogeneous group. The results showed that comparison of RFLPs by CHEF electrophoresis is a useful technique for studying the epidemiology of MRSA.

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Summary. Twenty-six clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) collected from six Australian hospitals by a National Staphylococcal Study Group were examined by analysis of restriction fragment length polymorphisms (RFLPs) of chromosomal DNA with pulsed field gel electrophoresis. Digestion with the restriction endonuclease SmaI produced 13-17 bands of 7-700 kb. The digestion patterns were easily distinguished and isolates could be classified into 17 groups based on their RFLPs. Isolates giving a pattern associated with one group were from four hospitals in four different states. In another group, the isolates responsible were from three hospitals in two states and in a further group, the isolates were derived from two hospitals in different states. The remaining groups comprised only one member each. The method has promise for typing and studying the epidemiology of MRSA.

A multiply resistant Staphylococcus aureus isolate, WBG7410, harbours plasmids of 38, 26, 2.8, 2.4 and 1.9 kb and transfers trimethoprim and kanamycin resistance at high frequencies by conjugation. The transconjugants contained the 38-kb plasmid, pWBG707, and the 2.8-kb plasmid. Plasmid pWBG707 was shown to encode trimethoprim resistance, was conjugative and mobilised at high frequencies the 2.8-kb plasmid which presumably encodes kanamycin resistance. Plasmid pWBG707 was isolated mostly in the open circular form and analysis with EcoRI restriction endonuclease suggests that pWBG707 is a new conjugative plasmid distinct from the other conjugative plasmids reported in S. aureus.