Incidence of plasmid DNA in Salmonella strains isolated from clinical sources in Ontario, Canada, during 1979 and 1980

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Abstract
Gel electrophoresis of DNA from 70 clinical strains of Salmonella revealed a heterogeneous plasmid population. Plasmid DNA, ranging in molecular weight from 1.4 × 106 to 145 × 106, was demonstrated in 26 of 32 antibiotic-resistant strains. Several resistant strains carried up to six plasmids; however, of these, five strains which were multiply resistant contained a single plasmid of molecular weight 54 × 106 to 145 × 106. Only one incompatibility group H2 (IncH2) plasmid (pDT28) was detected in a strain of S. heidelberg; thus, this represents a reduction in the prevalence of these plasmids in Ontario Salmonella strains since 1974. The pDT28 plasmid resembled other IncH2 plasmids by its high molecular weight (145 × 106) and by virtue of its temperature-sensitive mode of transfer, resistance to tellurium, and inhibition of coliphage development. Of the 38 antibiotic-susceptible Salmonella strains, approximately half contained plasmids, ranging in molecular weight from 1.4 × 106 to 60 × 106. The plasmid-containing antibiotic-susceptible strains carried either a group of two to four small plasmids, with molecular weights less than 4.5 × 106, or a single large plasmid of molecular weight 23 × 106 or 60 × 106.