Characterization of a blaIMP-4-carrying plasmid from Enterobacter cloacae of swine origin

To characterize an MDR blaIMP-4-harbouring plasmid from Enterobacter cloacae EC62 of swine origin in China.

Plasmid pIMP-4-EC62 from E. cloacae EC62 was transferred by conjugation via filter mating into Escherichia coli J53. Plasmid DNA was extracted from an E. coli J53 transconjugant and sequenced using single-molecule real-time (SMRT) technology. MIC values for both the isolate EC62 and the transconjugant were determined using the broth microdilution and agar dilution methods. Plasmid stability in both the isolate EC62 and the transconjugant was assessed through a series of passages on antibiotic-free media.

Plasmid pIMP-4-EC62 is 314351 bp in length, encodes 369 predicted proteins and harbours a novel class 1 integron carrying blaIMP-4 and a group II intron. The blaIMP-4-bearing plasmid belongs to the IncHI2/ST1 incompatibility group. Sequence analysis showed that pIMP-4-EC62 carries four MDR regions and several gene clusters encoding heavy metal resistance. Plasmid pIMP-4-EC62 was stably maintained in both the E. cloacae EC62 isolate and the transconjugant E. coli J53-pIMP-4-EC62 in the absence of selective pressure. Analysis of the evolutionary relatedness of selected IncHI2 plasmids indicates that ST1-type plasmids are key carriers of carbapenemase genes among IncHI2 plasmids.

pIMP-4-EC62 represents the first fully sequenced IncHI2-type blaIMP-4-harbouring plasmid from E. cloacae in China. Co-location of blaIMP-4 with other resistance genes on an MDR plasmid is likely to further accelerate the dissemination of blaIMP-4 by co-selection among bacteria from humans, animals and the environment under the selective pressure of other antimicrobial agents, heavy metals and disinfectants.