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The plasmid-mediated tigecycline resistance gene cluster tmexCD1-toprJ1 and other variants have been detected in a variety of strains from multiple sources, including human-derived strains. In addition to tigecycline, these tmexCD-toprJ -like gene clusters reduce susceptibility of the host strain to many other antimicrobials.

The rise of the plasmid-encoded colistin resistance gene mcr-1 is a major concern globally. Here, during a routine surveillance, an unexpectedly high prevalence of Escherichia coli with reduced susceptibility to colistin (69.9%) was observed in a Chinese broiler farm. Fifty-three (63.9%) E. coli isolates were positive for mcr-1 . All identified mcr-1 -positive E. coli (MCREC) were multidrug resistant and carried other clinically significant resistance genes. Furthermore, the mcr-1 genes were mainly located on the IncI2 and IncHI2 plasmids. Conjugation experiments unraveled the co-transfer of mcr-1 with other antibiotic resistance genes ( bla CTX-M-55 , bla CTX-M-14 , floR , and fosA3 ) via the IncI2 ( n =3) and IncHI2 ( n =4) plasmids. The stable genetic context mcr-1 - pap2 was common in the IncI2 plasmids, whereas IS Apl1 - mcr-1 - pap2 -IS Apl1 was mainly found in the IncHI2 plasmids. The dominance of mcr-1 -bearing IncI2 and IncHI2 plasmids and co-selection of mcr-1 with other antimicrobial resistance genes might contribute to the exceptionally high prevalence of mcr-1 in this broiler farm. Our results emphasized the importance of appropriate antibiotic use in animal production.

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We aimed to characterize NDM-5-producing Enterobacteriaceae from aquatic products in Guangzhou, China. A total of 196 intestinal samples of grass carp collected in 2019 were screened for carbapenemase genes. Characterization of bla NDM-5 positive isolates and plasmids was determined by antimicrobial susceptibility testing, conjugation experiments, Illumina HiSeq, and Nanopore sequencing. One Citrobacter freundii and six Escherichia coli strains recovered from seven intestinal samples were verified as bla NDM-5 carriers (3.57%, 7/196). The bla NDM-5 genes were located on the IncX3 ( n =5), IncHI2 ( n =1), or IncHI2-IncF ( n =1) plasmids. All bla NDM-5 -bearing plasmids were transferred by conjugation at frequencies of ~10 −4 –10 −6 . Based on sequence analysis, the IncHI2 plasmid pHNBYF33-1 was similar to other bla NDM-5 -carrying IncHI2 plasmids deposited in GenBank from Guangdong ducks. In all IncHI2 plasmids, bla NDM-5 was embedded in a novel transposon, Tn 7051 (IS 3000 -ΔIS Aba125 -IS 5 -ΔIS Aba125 - bla NDM-5 - ble MBL - trpF - tat -∆ dct -IS 26 -∆ umuD -∆IS Kox3 -IS 3000 ), which was identical to the genetic structure surrounding bla NDM-5 found in some IncX3 plasmids. The IncHI2-IncF hybrid plasmid pHNTH9F11-1 was formed by homologous recombination of the bla NDM-5 -carrying IncHI2 plasmid and a heavy-metal-resistant IncF plasmid through ∆Tn 1721 . To the best of our knowledge, this is the first report on the characterization of bla NDM-5 -bearing plasmids in fish in China. The IncHI2 plasmid pHNBYF33-1 may be transmitted from ducks, considering the common duck-fish freshwater aquaculture system in Guangdong. Tn 7051 is likely responsible for the transfer of bla NDM-5 from IncX3 to IncHI2 plasmids in Enterobacteriaceae , resulting in the expansion of transmission vectors of bla NDM-5 .