Abstract
Enterobacteriaceae, including Escherichia coli, has been shown to acquire the colistin resistance gene mcr-1. A strain of E. coli, EC11, which is resistant to colistin, polymyxin B and trimethoprim-sulfamethoxazole, was isolated in 2016 from the feces of a dairy cow in Shanghai, China. Strain EC11 identifies with sequence type ST278 and is susceptible to 19 frequently used antibiotics. Whole genome sequencing of strain EC11 showed that this strain contains a 31-kb resistance plasmid, pEC11b, which belongs to the IncX4 group. The mcr-1 gene was shown to be inserted into a 2.6-kb mcr-1-pap2 cassette of pEC11b. Plasmid pEC11b also contained putative conjugal transfer components, including an oriT-like region, relaxase, type IV coupling protein, and type IV secretion system. We were successful in transferring pEC11b to E. coli C600 with an average transconjugation efficiency of 4.6 × 10-5. Additionally, a MLST-based analysis comparing EC11 and other reported mcr-positive E. coli populations showed high genotypic diversity. The discovery of the E. coli strain EC11 with resistance to colistin in Shanghai emphasizes the importance of vigilance in detecting new threats like mcr genes to public health. Detection of mcr genes helps in tracking, slowing, and responding to the emergence of antibiotic resistance in Chinese livestock farming.
Highlights
Antimicrobial resistance is becoming a great challenge to public health worldwide (Laxminarayan et al, 2014)
Out of 120 E. coli isolates collected from dairy cow fecal samples in May 2016 in Shanghai, only the E. coli isolate EC11 (Supplementary Figures S1, S2) carried the mcr-1gene, and none of these isolates carried mcr-2/3 determinants or the allelic variants
According to European Committee on Antimicrobial Susceptibility Testing (EUCAST) standards, the resistance cutoff of E. coli to colistin is 2 mg/L and the E. coli EC11 strain exhibited the lower level of colistin resistance (8 μg/mL) (Table 1)
Summary
Antimicrobial resistance is becoming a great challenge to public health worldwide (Laxminarayan et al, 2014). The rapid evolution of MDR Gram-negative bacteria is pushing humankind to the cusp of a post-antibiotic era. Colistin (polymyxins E) is a family of cationic polypeptide antibiotics which acts as the last line of defense in the treatment of severe bacterial infections by MDR or XDR bacteria. The MCR-1 encodes a PEA transferase that adds PEA to the lipid A of the lipopolysaccharide, leading to Gram-negative bacteria resistant to colistin (Anandan et al, 2017). This HGT mechanism of colistin resistance has alarmed the medical, media, academic and public health communities
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