Genetic characterization of mcr-1-bearing plasmids to depict molecular mechanisms underlying dissemination of the colistin resistance determinant
Objectives To analyse and compare mcr-1 -bearing plasmids from animal Escherichia coli isolates, and to investigate potential mechanisms underlying dissemination of mcr-1 . Methods Ninety-seven ESBL-producing E. coli strains isolated from pig farms in China were screened for the mcr-1 gene. Fifteen mcr- 1-positive strains were subjected to molecular characterization and bioinformatic analysis of the mcr-1 -bearing plasmids that they harboured. Results Three major types of mcr-1 -bearing plasmids were recovered: IncX4 (∼33 kb), IncI2 (∼60 kb) and IncHI2 (∼216–280 kb), among which the IncX4 and IncI2 plasmids were found to harbour the mcr-1 gene only, whereas multiple resistance elements including bla CTX-M , bla CMY , bla TEM , fosA , qnrS , floR and oqxAB were detected, in various combinations, alongside mcr-1 in the IncHI2 plasmids. The profiles of mcr-1 -bearing plasmids in the test strains were highly variable, with coexistence of two mcr-1 -bearing plasmids being common. However, the MIC of colistin was not affected by the number of mcr-1 -carrying plasmids harboured. Comparative analysis of the plasmids showed that they contained an mcr-1 gene cassette with varied structures ( mcr-1 - orf , IS Apl1-mcr-1 - orf and Tn 6330 ), with the IncHI2 type being the most active in acquiring foreign resistance genes. A novel transposon, Tn 6330 , with the structure IS Apl1-mcr-1 - orf -IS Apl1 was found to be the key element mediating translocation of mcr-1 into various plasmid backbones through formation of a circular intermediate. Conclusions The mcr-1 gene can be disseminated via multiple mobile elements including Tn 6330 , its circular intermediate and plasmids harbouring such elements. It is often co-transmitted with other resistance determinants through IncHI2 plasmids. The functional mechanism of Tn 6330 , a typical composite transposon harbouring mcr-1 , should be further investigated.
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