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Objective The occurrence of colistin-resistant Escherichia coli in food-producing animals presents a global public health hazard. We aimed to determine the prevalence and molecular characteristics of mobilized colistin resistance ( mcr) -carrying E. coli isolated from colistin-use and non-use pig farms. Methods Antimicrobial susceptibility of isolated 552 E. coli from pig feces (n = 334) and their farm environments (n = 218) was assessed using broth microdilution. Molecular characterization of colistin-resistant E. coli was determined by polymerase chain reaction, multi-locus sequence typing, and next-generation sequencing (NGS). Results Colistin-resistant E. coli was detected in 4.7% (26/552) of isolates, from pigs (6.3%) and farm environments (2.3%), with significantly higher levels in colistin-use farms ( p < 0.05). Among them, all carried mcr-1 , except one isolate carrying mcr-3 . We found 61.5% of isolates transferred the mcr-1 to E. coli J53 by conjugation. Replicon type IncI2 (57.7%) was predominantly detected in mcr-1 -carrying plasmids. Common clones were ST101, ST10, and ST88, with identical STs found in different farms, suggesting clonal transmission. NGS analysis revealed the co-presence of mcr-1, bla CTX-M-55 , and qnrS1 genes on megaplasmids. The plasmids also contain aminoglycosides, beta-lactams, sulfonamides, phenicols, and tetracycline resistance genes. Transposable elements IS 26 , IS Ecp1 , and orf477 were identified in the genetic environment of the bla CTX-M-55 . Conclusion The significant incidence of mcr-1 in pig farms may be due to the use of colistin, clonal spread, and horizontal spread of resistance genes. Thus, prudent antimicrobial use and biosecurity measures are necessary to prevent antimicrobial resistance in pig farms.
Hwang et al. (Wed,) studied this question.