Prevalence and Genomic Characteristics of <i>mcr</i> -Positive Escherichia coli Strains Isolated from Humans, Pigs, and Foods in China

Abstract

Colistin is one of the last-resort antibiotics for treating infections caused by multidrug-resistant (MDR) Gram-negative bacteria. However, <i>mcr</i> genes conferring resistance to colistin have been widely identified, which is considered a global threat to public health. Here, we investigated the prevalence and characteristics of <i>mcr</i>-harboring Escherichia coli strains isolated from humans, animals, and foods in China by PCR, antimicrobial susceptibility testing, conjugation experiments, molecular typing, genome sequencing, and bioinformatics analysis. In total, 135 <i>mcr-1</i>-harboring E. coli isolates were acquired from 847 samples, and 6 isolates carried <i>mcr-3</i>. Among them, 131 isolates were MDR bacteria. Sixty-five resistance genes conferring resistance to multiple antimicrobials were identified in 135 isolates. The diverse pulsed-field gel electrophoresis (PFGE) patterns and sequence types (STs) of <i>mcr-1</i>-carrying isolates demonstrated that clonal dissemination was not the dominant mode of <i>mcr-1</i> transmission. Seven types of plasmids were able to carry <i>mcr-1</i> in this study, including IncI2, IncX4, IncHI2, p0111, IncY, and two hybrid plasmids. The genetic structures carrying <i>mcr-1</i> of 60 isolates were successfully transferred into the recipient, including 25 IncI2 plasmids, 23 IncX4 plasmids, and an IncHI2 plasmid. <i>mcr-1-pap2</i> was the dominant <i>mcr-1</i>-bearing structure, followed by IS<i>Apl1-mcr-1-pap2-</i>IS<i>Apl1</i> (Tn<i>6330</i>) and IS<i>Apl1-mcr-1-pap2</i>, among 7 <i>mcr-1</i>-bearing structures of 135 isolates. In conclusion, IncI2, IncX4, and IncHI2 plasmids were the major vectors spreading <i>mcr-1</i> from different geographical locations and sources. The prevalence of Tn<i>6330</i> may accelerate the transmission of <i>mcr-1</i>. Continuous surveillance of <i>mcr-1</i> and variants in bacteria is vital for evaluating the public health risk posed by <i>mcr</i> genes. <b>IMPORTANCE</b> The spread of polymyxin-resistant Enterobacteriaceae poses a significant threat to public health and challenges the therapeutic options for treating infections on a global level. In this study, <i>mcr-1</i>-bearing ST10 E. coli was isolated from pigs, pork, and humans simultaneously, which demonstrated that ST10 E. coli was an important vehicle for the spread of <i>mcr-1</i> among animals, foods, and humans. The high prevalence of <i>mcr-1</i>-positive E. coli strains in pigs and pork and the horizontal transmission of <i>mcr-1</i>-bearing plasmids in diverse E. coli strains suggest that pigs and pork are important sources of <i>mcr-1</i>-positive strains in humans and pose a potential threat to public health. Additional research on the prevalence and characteristics of <i>mcr-1</i>-positive E. coli is still required to facilitate early warning to improve polymyxin management in hospitals.