Transferability of ESBL-encoding IncN and IncI1 plasmids among field strains of different Salmonella serovars and Escherichia coli

Objectives: This study aimed to sequence, assemble, and annotate three plasmids (two IncN and one IncI1) carrying the bla_CTX-M-1 gene and assess their transferability rates between homologous and heterologous serovars and/or species of bacteria.

Methods: First, the plasmids were sequenced, assembled, and annotated. They were then transferred from three donor strains (Escherichia coli/IncN, S. Heidelberg/IncN, and S. Heidelberg/IncI1) into nine recipient strains (S. Enteritidis, S. Heidelberg, S. Saintpaul, S. Cero, S. Infantis, S. Braenderup, E. coli 50, and E. coli 2010). The bla_CTX-M-1 gene polymerase chain reaction (PCR), plasmid isolation, and antimicrobial susceptibility testing were used on the transconjugants to confirm the successful transfer of extended-spectrum beta lactamase (EBSL) plasmids into the recipient strains.

Results: Both IncN plasmids were 42,407 bp in size and showed >99.4% similarity to the S. Bredeney pET1.2-IncN (GenBank accession CP043224.1), whereas the IncI1 plasmid was 107,635 bp in size and demonstrated >99.9% similarity to the E. coli pCOV33 plasmid (GenBank accession MG649046.1). Successful plasmid transfer was observed between donor ​E. coli (IncN) and all recipient strains except for E. coli 50 and between donor S. Heidelberg (IncN) and all recipient strains. Successful plasmid transfer was also observed between S. Heidelberg (IncI1) and E. coli 50.

Conclusion: Transfer of the bla_CTX-M-1 encoding IncN and IncI1 plasmids via conjugation is possible and yet occurs at different frequencies depending on the donor strain of bacteria, with S. Heidelberg (IncN) having the highest donor-dependent transfer frequency, followed by E. coli 9079 (IncN) and S. Heidelberg (IncI1).