Molecular characterization of two novel NDM-1-producing atypical enteroaggregative Escherichia coli isolates from patients

To investigate NDM-1-producing atypical Enteroaggregative Escherichia coli (aEAEC) of sequence type 349 from hospitalized patients, the isolates 13ZX28 and 13ZX36 were subjected to antimicrobial susceptibility testing, conjugation and whole genome sequencing. Only one single nucleotide mutation was detected in chromosomes despite different plasmid profiles. Both isolates were positive for bla_NDM-1 mediating resistance to carbapenem. A novel plasmid p13ZX28–272 (~272-kb) from 13ZX28 encodes bla_NDM-1. Interestingly, its sequence was identical to the two plasmids p13ZX36–200 (~200-kb) and p13ZX36–70 (~70-kb) from 13ZX36. Formation of the former episome possibly involved homologous recombination through a 4948-bp large fragment located on each of the two latter plasmids. Furthermore, plasmid p13ZX28–272 could be resolved into a ~ 98-kb daughter plasmid by IS26 rearrangement following conjugation. The plasticity of the plasmids is recognized, which warrants further investigation to evaluate the underlying public health risk and understand how antibiotic selection pressure drives this process.