Abstract
Objective To investigate the molecular mechanism of colistin resistance mediated by mcr-1and pmrAB genes in clinical Escherichia coli (E.coli) isolates. Methods A total of 1 988 clinical E. coli isolates were collected from the First People′s Hospital of Shangqiu from 2010 to 2017 and screened for colistin-resistant isolates using agar dilution method. The minimum inhibitory concentrations (MICs) of nine common clinical antibiotics were determined using broth microdilution method. PCR and sequencing analysis were performed to detect the colistin resistance genes of mcr-1 and pmrAB. Conjugation experiments were used to test the transferability of the plasmid carrying mcr-1 gene. S1-PFGE and Southern blot were used to locate the plasmid carrying mcr-1. All colistin-resistant E. coli isolates were typed by multilocus sequence typing (MLST). Results Six colistin-resistant E. coli isolates were obtained by agar dilution method. The results of susceptibility testing showed that all of the six isolates were multidrug resistant. PCR and sequencing analysis revealed that four out of the six strains carried mcr-1 gene, and the other two isolates both had an amino acid substitution (L167P) caused by pmrB gene mutation. The results of conjugation experiments, S1-PFGE and Southern hybridization analysis showed that the plasmids of four mcr-1 gene-positive E. coli strains were located on a conjugative plasmid about 60 kb in length. MLST analysis classified the six isolates into six distinct sequence types (STs). Conclusion This study suggested that mcr-1 gene and mutations in pmrAB gene were the main mechanisms mediating the resistance of E. coli to colistin. In clinical practice, the occurrence and spread of colistin-resistant E. coli should be further monitored, and the rational use of antibiotics should be promoted to prevent the spread of colistin-resistant strains. Key words: Escherichia coli; Colistin; mcr-1; pmrAB; Resistance mechanism
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