Carbapenem resistance in members of the order Enterobacterales is a growing public health problem that is associated with high mortality in developing and industrialized countries. Moreover, in the field of veterinary medicine, the occurrence of New Delhi metallo-β-lactamase-producing Escherichia coli isolates in animals, especially food-producing animals, has become a growing concern in recent years.

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Objectives To clarify the transmission mechanism of the blaCTX-M-64 gene between Escherichia coli and Salmonella isolates from food animals. Methods A total of 329 E. coli and 60 Salmonella isolates collected from food animals in 2016 were screened for the presence of blaCTX-M-64 genes. The blaCTX-M-64-positive isolates were typed and plasmid and chromosome DNA was sequenced to determine the genetic context of blaCTX-M-64 and the plasmid types present. Results The blaCTX-M-64 gene was identified in only three E. coli isolates but was the predominant gene in the Salmonella isolates (n = 9). These 12 CTX-M-64-positive isolates were all resistant to ampicillin, cefotaxime, ceftiofur, ceftriaxone, ceftazidime and florfenicol and 9 were resistant to ciprofloxacin. The blaCTX-M-64 gene was located on transferable IncI2 plasmids and an IncHI2 plasmid in three E. coli and one Salmonella isolate, respectively. The remaining eight Salmonella isolates contained blaCTX-M-64 integrated into the chromosome. Different genetic contexts of blaCTX-M-64 genes were found among the 12 isolates: ISEcp1-blaCTX-M-64-orf477-A/C on IncI2 plasmids of 3 E. coli isolates; ΔISEcp1-blaCTX-M-64-orf477-A/C in the chromosome of 1 Salmonella isolate; and ISEcp1-blaCTX-M-64-orf477 on the IncHI2 plasmid and chromosome of 8 Salmonella isolates. Conclusions To the best of our knowledge, this is the first report of chromosomally encoded CTX-M-64 in Salmonella isolates. ISEcp1-mediated transposition is likely to be responsible for the spread of blaCTX-M-64 between different plasmids and chromosomes in Enterobacteriaceae especially E. coli and Salmonella.

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We identified 33 bla NDM -harboring Escherichia coli in 470 poultry samples from 9 Shandong and Guangdong provinces during 2016 to 2017 and included the subtypes bla NDM-1 , 10 bla NDM-5 and bla NDM-9. The bla NDM gene possessed by all strains was plasmid-borne and ranged in 11 size from 46 to 265 kb. The plasmids included the replicon types IncX3, IncY, IncB/O/K/Z and 12 IncHI2 and most were transferable and stable in recipient host bacteria. Sequences flanking 13 bla NDM in IncX3 and IncB/O/K/Z plasmids were diverse and the mobile element IS26 played an 14 important role in the evolution of the IncB/O/K/Z bla NDM plasmids. 15