The number, frequency distribution and restriction enzyme fragmentation patterns of plasmids harboured by 163 methicillin-sensitive isolates of (MSSA) and 53 methicillin-resistant isolates (MRSA) were compared. Plasmids were demonstrated in less than half of the MSSA isolates; their frequency distribution did not differ from that predicted by a simple model of plasmid distributions. In contrast, all the MRSA isolates harboured plasmids, their distribution suggesting dissemination of a limited number of clones within the hospital. Among 72 MSSA isolates harbouring plasmids, 38 different restriction patterns were identified. There were fewer patterns among MRSA isolates; 11 were observed, and two predominant patterns accounted for 68% of those identified. These restriction patterns correlated with the presence or absence of aminoglycoside resistance. A multicopy plasmid of 2.6 kb was present in both MSSA and MRSA isolates that harboured more than one plasmid; it had the same restriction pattern irrespective of its source. The importance of these results in choosing a method of studying the spread of staphylococci is discussed.


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