Detection and epidemiology of mcr-1-positive Escherichia coli strains in Dongyang area of Zhejiang Province
Objective 
To investigate the prevalence of mcr-1 gene, a plasmid-mediated polymyxin resistance gene, in Escherichia coli (E.coli) strains isolated in Dongyang of Zhejiang Province and to understand the epidemiological characteristics of E. coli strains carrying mcr-1 gene in order to provide local clinicians with a theoretical basis for prevention and control of the spread of mcr-1-bearing E. coli strains. 
 
 
Methods 
A total of 315 E. coli strains were collected in the People′s Hospital of Dongyang, Zhejiang Province from January to December 2016. All strains were isolated from specimens of blood, urine, respiratory tract, etc. PCR was performed to detect the genes confering resistance to polymyxin (mcr-1 gene), β-lactamase and carbapenem. Minimal inhibitory concentrations (MIC) of antibiotics against mcr-1-positive strains were determined by micro-broth dilution method. Conjugation test was performed to confirm whether the mcr-1 gene was located on the transferable plasmid. Multilocus sequence typing (MLST) was used for molecular typing of mcr-1-positive strains. 
 
 
Results 
Five mcr-1-positive strains were identified from 315 E. coli strains with a positive rate of 1.6%. Two out of the five mcr-1-positive E. coli strains contained β-lactamase resistance genes, blaTEM-1 and blaCTX-M-14. Both of them were resistant to the first, second and third generation of cephalosporins and one was also resistant to cefepime. All of the five mcr-1-positive E. coli strains were sensitive to ciprofloxacin and levofloxacin, but resistant to ticarcillin/clavulanic acid. No carbapenem resistance genes were detected. One transconjugant was successfully obtained by transconjugation assay. MLST analysis showed that a total of four sequence types were identified, including ST131 (two strains), ST43 (one strain), ST69 (one strain) and ST349 (one strain). 
 
 
Conclusion 
Only 1.6% of all E. coli strains isolated in Dongyang area of Zhejiang Province carry mcr-1 gene, indicating that there is no epidemic of mcr-1 gene-positive E. coli infection. The coexistence of mcr-1 gene and β-lactamase resistance genes in E. coli strains isolated in Dongyang suggests that local clinicians should avoid antibiotic abuse to prevent the spread of drug-resistant E. coli. 
 
 
Key words: 
Escherichia coli; Polymyxin; mcr-1; Multidrug resistance