Conservation and plasticity of IncHI2 plasmid-borne metal resistance loci in Salmonella enterica Serovar Heidelberg

The incompatibility group (Inc) HI2 plasmid is frequently detected in Salmonella enterica serovar Heidelberg and is commonly associated with heavy metal and multidrug resistance genes. In this study, we assessed the characteristics of IncHI2A plasmids of eight copper-resistant Sal. Heidelberg isolates from animal and human sources. A comparative analysis of the genetic architecture of these plasmids with that of the parent plasmid R478 from Serratia marcescens and the reference plasmid pSH111_227 from Sal. Heidelberg revealed high conservation of the plasmid backbone, as well as copper/silver and tellurium resistance cassettes. Notably, the gene encoding the periplasmic silver-binding protein SilF was present in all Sal. Heidelberg isolates but was absent in R478, whereas the tellurite resistance gene terF was absent in all isolates. Additionally, except for two isolates, the IncHI2A plasmids lacked mercury resistance genes found in the parental plasmid R478. Tetracycline and aminoglycoside resistance genes are conserved among the IncHI2A plasmids of all Sal. Heidelberg isolates and the reference plasmids. In addition, all Sal. Heidelberg isolates included neomycin-family aminoglycoside resistance genes, which conferred resistance to neomycin and kanamycin. Analysis of the non-IncHI2A plasmids in these isolates showed that the majority of the isolates carried multidrug resistance (MDR) genes, whereas three isolates did not contain any detectable MDR genes. In conclusion, these variations within the heavy metal resistance (HMR) loci underscore the dynamic nature of IncHI2A plasmids and their potential role in the acquisition and retention of these genes under selective pressure.