Abstract
IncX4 plasmids are associated with the dissemination of the mcr-1 genes in Enterobacteriaceae. We screened IncX4 plasmids among 2,470 isolates of Enterobacteriaceae and determined the mcr-1 positive isolates. Forty-three isolates were observed to carry IncX4 type plasmid, among which 13 were identified to carry mcr-1 gene. Three representative mcr-1-positive IncX4 plasmids were selected for high-throughput sequencing. Comparative genomics showed that the mcr-1-carrying IncX4 plasmids exhibit remarkable similarity in the backbone, and the major distinction lies in the region containing mcr-1. The major variable regions of all the IncX4 plasmids were fully characterized by PCR-RFLP. The results revealed that the mcr-1 was located on the Variable Region I of IncX4 plasmids in 11 E. coli isolates. Among them, nine E. coli strains possess an epidemic pCSZ4-like IncX4 plasmid containing mcr-1. ISApl1 was presumably involved in the transposition of the mcr-1 cassette and then was lost. Similar genetic contexts were found in different plasmids, even the E. coli chromosome, implying the acquisition of mcr-1 by a unique common mechanism.
Highlights
Among them, IncX4 is one of the most prevalent plasmid type in E. coli (7.6% to 34.0%)[3, 5]
Following 16S-based identification of these IncX4-positive isolates, 41 species were assigned to E. coli, one isolate was classified into K. pneumoniae, and the remaining one was Citrobacter freundii
Antimicrobial susceptibility tests revealed that all the mcr-1-positive E. coli isolates have higher minimum inhibitory concentration (MIC) (4–8 μg/mL) in relative to E. coli 25922 (0.25 μg/mL)
Summary
IncX4 is one of the most prevalent plasmid type in E. coli (7.6% to 34.0%)[3, 5]. The dissemination of the gene blaCTX-M-14b by the IncX4 type plasmid pSAM78 was observed amongst the following three species (E. coli, Enterobacter cloacae, and Salmonella enterica). It suggested that IncX4 plasmids can be transferred between different species of Enterobacteriaceae. IncX4 plasmid is widespread in various species (E. coli, Salmonella enteric, and Klebsiella pneumonia) of diversified origins ranging from human, animals, to animal products in many countries, like China, Denmark, United Kingdom, etc.[15,16,17, 22,23,24,25,26,27,28,29]. We discussed the possible mode/pattern for the integration of mcr-1 into IncX4 plasmids
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