Characterization of bla-carrying IncC and rmtB-carrying IncI1/ST136 plasmids in an avian Escherichia coli ST224 strain.

To analyze characteristics and underlying evolutionary processes of IncC and IncI1 plasmids in a multidrug-resistant avian E. coli strain, antibiotic susceptibility testing, PCR, conjugation assays, and next-generation sequencing were performed. The type 1 IncC plasmid pEC009.1 harbored three antimicrobial resistance regions including ISEcp1-bla-blc-sugE, ARI-B resistance island, and ARI-A island that was a mosaic multidrug resistance region (MRR) comprised of a class 1 integron with cassette array |aac(6')-II(aacA7)|qacE∆1|sul1|, IS26-mphR(A)-mrx-mph(A)-IS26, IS26-fosA3-IS26, and mercury resistance cluster merRTPABDE. It is the first report of three different size circular forms derived from IS26-mphR(A)-mrx-mph(A)-IS26-fosA3-IS26 in ARI-A of type 1 IncC plasmid. In IncI1/ST136 pEC009.2, the truncated transposon Tn1722 carrying bla, rmtB, aac(3)-IId(aacC2d), and a class 1 integron with cassette array |dfrA12|orfF|aadA2|, inserted into the plasmid backbone generating 5-bp direct repeats (DRs, TATAA) at the boundaries of the region, which was highly similar to that of other IncI1 plasmids, and differed by the arrangements of resistance determinants. Comparison among two epidemic plasmid lineages showed complex MRRs respectively located in the specific position in type 1 IncC and IncI1/ST136 plasmids with conserved backbones, and these have evolved via multiple events involved in mobile elements-mediated loss and gain of resistance genes and accessory genes. Strains harboring these plasmids may serve as a reservoir for antibiotic resistance genes, thereby contributing to the rapid spread of resistance genes and posing a public health threat.