Genetic characterization of multidrug-resistant Escherichia coli harboring colistin-resistant gene isolated from food animals in food supply chain

Colistin is widely used for the prophylaxis and treatment of infectious disease in humans and livestock. However, the global food chain may actively promote the dissemination of colistin-resistant bacteria in the world. Mobile colistin-resistant ( ) genes have spread globally, in both communities and hospitals. This study sought to genomically characterize -mediated colistin resistance in 16 strains isolated from retail meat samples using whole genome sequencing with short-read and long-read platforms. To assess colistin resistance and the transferability of genes, antimicrobial susceptibility testing and conjugation experiments were conducted. Among the 16 isolates, 11 contained , whereas three carried and two contained and . All isolates had minimum inhibitory concentration (MIC) for colistin in the range 1-64 μg/mL. Notably, 15 out of the 16 isolates demonstrated successful transfer of genes via conjugation, indicative of their presence on plasmids. In contrast, the KK3 strain did not exhibit such transferability. Replicon types of -containing plasmids included IncI2 and IncX4, while IncFIB, IncFII, and IncP1 contained . Another single strain carried - on IncX4 and - on IncP1. Notably, one isolate contained located on a chromosome and carrying - on the IncFIB plasmid. The chromosomal location of the gene may ensure a steady spread of resistance in the absence of selective pressure. Retail meat products may act as critical reservoirs of plasmid-mediated colistin resistance that has been transmitted to humans.