Antimicrobial resistance-encoding plasmid clusters with heterogeneous MDR regions driven by IS26 in a single Escherichia coli isolate.

Background: IS26-flanked transposons played an increasingly important part in the mobilization and development of resistance determinants. Heterogeneous resistance-encoding plasmid clusters with polymorphic MDR regions (MRRs) conferred by IS26 in an individual Escherichia coli isolate have not yet been detected. Objectives: To characterize the complete sequence of a novel bla(CTX-M-65)- and fosA3-carrying IncZ-7 plasmid with dynamic MRRs from an E. coli isolate, and to depict the mechanism underlying the spread of resistance determinants and genetic polymorphisms. Methods: The molecular characterization of a strain carrying bla(CTX-M-65) and fosA3 was analysed by antimicrobial susceptibility testing and MLST. The transferability of a plasmid bearing bla(CTX-M-65) and fosA3 was determined by conjugation assays, and the complete structure of the plasmid was obtained by Illumina, PacBio and conventional PCR mapping, respectively. The circular forms derived from IS26-flanked transposons were detected by reverse PCR and sequencing. Results: A novel IncZ-7 plasmid pEC013 (similar to 118kb) harbouring the bla(CTX-M-65) and fosA3 genes was recovered from E. coli isolate EC013 belonging to D-ST117. The plasmid was found to have heterogeneous and dynamic MRRs in an individual strain and the IS26-flanked composite transposon-derived circular intermediates were identified and characterized in pEC013. Conclusions: The heterogeneous MRRs suggested that a single plasmid may actually be a cluster of plasmids with the same backbone but varied MRRs, reflecting the plasmid's heterogeneity and the survival benefits of having a response to antimicrobial-related threatening conditions in an individual strain.