Characteristics of colistin-resistant Escherichia coli from pig farms in Central China

Zhong Peng,Huanchun Chen,Menghong Dai,Zizhe Hu,Chen Tan,Chaoying Jia,Xiangru Wang,Xiaosong Li,Xiaoxue Zhang,Zugang Li

doi:10.1186/s44149-021-00009-5

Abstract

The emergence and dissemination of colistin resistance in Enterobacteriaceae mediated by plasmid-borne mcr genes in recent years now pose a threat to public health. In this study, we isolated and characterized colistin-resistant and/or mcr-positive E. coli from pig farms in Central China. Between 2018 and 2019, 594 samples were collected and recovered 445 E. coli isolates. Among them, 33 with colistin resistance phenotypes and 37 that were positive for mcr genes were identified, including 34 positive for mcr-1, one positive for mcr-3, and two positive for both mcr-1 and mcr-3. An insertion of nine bases (“CTGGATACG”) into mcr-1 in four mcr-positive isolates led to gene dysfunction, and therefore did not confer the colistin resistance phenotype. Antimicrobial susceptibility testing revealed that 37 mcr-positive isolates showed severe drug resistance profiles, as 50% of them were resistant to 20 types of antibiotics. Multilocus sequence typing revealed a heterogeneous group of sequence types in mcr-positive isolates, among which ST10 (5/37), ST156 (5/37), and ST617 (4/37) were the predominant types. Plasmid conjugation assays showed that mcr-carrying plasmids of 25 mcr-positive isolates were conjugated with E. coli recipient, with conjugation frequencies ranging from 1.7 × 10-6 to 4.1 × 10-3 per recipient. Conjugation of these mcr genes conferred a colistin resistance phenotype upon the recipient bacterium. PCR typing of plasmids harbored in the 25 transconjugants determined six types of plasmid replicons, including IncX4 (14/25), FrepB (4/25), IncI2 (3/25), IncHI2 (2/25), FIB (1/25), and IncI1 (1/25). This study contributes to the current understanding of antibiotic resistance and molecular characteristics of colistin-resistant E. coli in pig farms.

Highlights

Having first been discovered in the 1940s, polymyxins are an old family of chemically distinct lipopeptide antibiotics produced by the gram-positive bacterium Paenibacillus polymyxa (Li et al 2019a)
Isolation of colistin-resistant E. coli and mcr-positive E. coli from pig farms in Central China By performing bacterial isolation and identification (Fig. 1A), 445 E. coli strains were recovered from 594 farm-origin samples in Central China between 2018 and 2019 (Fig. 1B)
Results revealed that several mcr-positive E. coli did not display a colistin resistance phenotype, which might be because base mutations were present, thereby leading to gene dysfunction

Summary

Introduction

Having first been discovered in the 1940s, polymyxins are an old family of chemically distinct lipopeptide antibiotics produced by the gram-positive bacterium Paenibacillus polymyxa (Li et al 2019a). In addition to mcr-1, nine plasmid-borne mcr genes (mcr-2~mcr-10) have been identified to date (Wang et al 2020a). These 10 mcr genes have been detected in bacterial isolates from humans, animals, foods of animal origin, and environment, and they confer resistance to polymyxins (Andrade et al 2020). The emergence of these genes may accelerate global movement towards a postantibiotic era (Du et al 2016; Paterson and Harris 2016). It is of great importance to monitor the prevalence of mcr-bearing bacteria in clinical activities

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Discussion

Conclusion