Purpose: The purpose of this study was to locate the antibiotic-resistant genes in antibiotic-resistant plasmids in Salmonella isolates, and to determine the prevalence of Salmonella with different antibiotic-resistant plasmid types and antibiotic-resistant genes.
Methods: The incompatibility groups (Inc) of 78 Salmonella isolates were typed by multiplex PCR. The plasmid elimination assay was conducted with 32 antibiotic-resistant Salmonella isolates in 5% SDS at 44.5°C.
Results: The types of the harboring plasmids in 78 Salmonella isolates were comprising of nine plasmid incompatibility groups (Inc P, Inc HI2, Inc N, Inc A/C, Inc FIIS, Inc FIA, Inc FIB, Inc FIC and Inc I1). Among them, the Inc P group and Inc HI2 group were the largest groups. Based on the carrying of antibiotic-resistant genes and the typing of plasmids, it was found that all the isolates which carrying blaOXA-1 gene and blaCMY-2 gene contained Inc HI2 plasmids; the isolates which carrying qnrA gene basically contained Inc A/C plasmids (4/5, 80.0%). After plasmid elimination assay, 15 isolates were changed from antibiotic-resistant to antibiotic-susceptible isolates, and the elimination efficiency was 46.9% (15/32). Later, the plasmids of 15 antibiotic-resistant mutants were extracted, and the plasmid profiles were significantly changed after the plasmid elimination assay. The lost plasmids were mainly Inc HI2 group, and the β-lactamases antibiotic-resistant genes (blaOXA-1, blaTEM-1 and blaCMY-2) were lost accordingly with the loss of Inc HI2 plasmids, as well as the antibiotic-resistance phenotypes.
Significance: These data suggest that the horizontal transfer of antibiotic-resistant genes in Salmonella was mainly related to Inc HI2 plasmids, but not Inc P plasmids.