Co-occurrence of mcr-1 in the chromosome and on an IncHI2 plasmid: persistence of colistin resistance in Escherichia coli

• Published in: International journal of antimicrobial agents 2018-06, Vol.51 (6), p.842-847
• Main Authors: Sun, Jian, Li, Xing-Ping, Fang, Liang-Xing, Sun, Ruan-Yang, He, Yu-Zhang, Lin, Jingxia, Liao, Xiao-Ping, Feng, Youjun, Liu, Ya-Hong
• Format: Article
• Language: English
• Subjects: Chromosome; Colistin resistance; Escherichia coli; mcr-1
• ISSN: 0924-8579; 1872-7913
• DOI: 10.1016/j.ijantimicag.2018.01.007

•Co-occurrence of mcr-1 in the chromosome and on a plasmid in Escherichia coli.•mcr-1 was in triplicate in the chromosome, with another copy encoded on a pHNSHP45-2-like IncHI2 plasmid.•A single copy of mcr-1 could result in LPS modification and cause colistin resistance in E. coli.•Acquisition of multiple mcr-1 copies, especially chromosomal, aids persistence of colistin resistance in the host strain. Two colistin-resistant Escherichia coli strains (FS13Z2S and FS3Z6C) possessing chromosomally encoded mcr-1 isolated from swine were characterised. Whole-genome sequencing revealed that in strain FS13Z2S mcr-1 occurred in triplicate in the chromosome with another copy encoded on a pHNSHP45-2-like IncHI2 plasmid, whereas in strain FS3Z6C only one copy mcr-1 was inserted in the chromosome. It seems likely that the triplication of chromosomal copies of mcr-1 in FS13Z2S is due to intramolecular transposition events via a composite transposon containing an mcr-1 cassette bracketed by two copies of insertion sequence ISApl1, and the pap2 gene at the insertion site was truncated by an IS1294-like element. In plasmid pFS13Z2S and the chromosome of strain FS3Z6C, only a single copy of ISApl1 was present upstream of the mcr-1 cassette. The two strains exhibited similar colistin minimum inhibitory concentrations (MICs) and featured phosphoethanolamine addition to lipid A, without regard to the copy number of mcr-1. The mcr-1-harbouring plasmid was unstable in wild-type strain FS13Z2S and was quickly lost after 7 days of passage on colistin-free Luria–Bertani broth containing 0.5% SDS, but the mcr-1 copies on the chromosome persisted. These results reveal that the single copy of mcr-1 could result in modification of lipopolysaccharide (LPS) and cause colistin resistance in E. coli. Acquisition of multiple copies of mcr-1, especially on the chromosome, would facilitate stable persistence of colistin resistance in the host strain.