High carriage rate of mcr-1 and antimicrobial resistance profiles of mcr-1-positive Escherichia coli isolates in swine faecal samples collected from eighteen provinces in China

The mcr-1 gene is a plasmid-borne and globally distributed colistin-resistance gene, which threatens the last-line role of colistin in treatment of infections caused by multi-drug resistant(MDR) Gram-negative bacteria in humans. The aim of this study was to obtain a comprehensive understanding of mcr-1 prevalence in the pig industry in China. A total of 600 faecal samples were collected from 60 swine farms in 18 provinces of China. Faecal DNA was extracted and subjected to PCR screening to detect the presence of mcr-1. Positive samples were randomly selected for isolation of colistin-resistant bacteria. A total of 152 mcr-1-positive isolates collected from different provinces were characterized by antimicrobial susceptibility testing and pulsed-field gel electrophoresis (PFGE) typing. Extremely high rate of faecal carriage of mcr-1(457/600, 76.2%) was observed, with positive rates ranging from 45.0%-100% among the provinces. All colistin-resistant colonies were identified as Escherichia coli and mcr-1-positive. The 152 representative mcr-1-positive E. coli isolates showed high level resistance to ampicillin (79.60%), tetracycline(94.74%), doxycycline(93.42%), florfenicol(78.29%), enrofloxacin(44.08%), and ciprofloxacin(38.82%) and low level resistance to ceftiofur(26.32%), cefoxitin(0.66%), and amikacin(0.66%). No resistant isolates were detected for gentamicin, meropenem, and tigecycline. The mcr-1-positive E. coli strains exhibited highly diverse PFGE patterns, which suggested that horizontal transfer through plasmids or other mobile elements was the main mechanism for the wide dissemination of mcr-1 in pig farms. Similar studies are warranted to continuously monitor colistin resistance and mcr-1 prevalence in food-animal production.