Occurrence and Molecular Characteristics ofMcr-1-PositiveEscherichia colifrom Healthy Meat Ducks in Shandong Province of China

Simple Summary Colistin has been used as a growth promotant in livestock feed for many years. To date, there are few reports about the prevalence and molecular characteristics of fecalEscherichia colibearingmcr-1in the meat ducks. In this study, among 120 fecalEscherichia colistrains isolated from healthy meat ducks, a total of ninemcr-1-containingE. colistrains were identified and two were identified as extra-intestinal pathogenicE. coli. The 9mcr-1-bearingE. coliisolates were clonally unrelated, carried two different genetic contexts ofmcr-1, and the colistin-resistant phenotype of them was successfully transferred to the recipient strains. These results highlight that healthy meat duck is a potential reservoir for multidrug resistantmcr-1-containingE. colistrains. Colistin has been used as a growth promotant in livestock feed for many years. In China,mcr-1-positiveEscherichia colistrains have been isolated from humans, chickens, and pigs. To date, there are few reports about the prevalence and molecular characteristics of fecalE. colibearingmcr-1in the meat ducks. In this study, the prevalence ofmcr-1gene was investigated among 120 fecalE. colistrains isolated from healthy meat ducks in Shandong province of China between October 2017 and February 2018. A total of ninemcr-1-containingE. colistrains were identified and two were identified as extra-intestinal pathogenicE. coli(ExPEC) among them. The clonal relationship of the nineE. colistrains was determined by multilocus sequencing typing (MLST) and pulsed field gel electrophoresis (PFGE), and the results indicated that allmcr-1-carrying isolates were clonally unrelated. Two different genetic contexts ofmcr-1were identified among these isolates. Colistin-resistant phenotype of all the isolates was successfully transferred to the recipient strains by conjugation experiments and seven transconjugants carried a single plasmid. Themcr-1was located on three replicon plasmids: IncI2 (n = 4), IncFII (n = 2) and IncN (n = 1). Complete sequence analysis of a representative plasmid pTA9 revealed that it was strikingly similar with plasmid pMCR1-IncI2 ofE. coli, plasmid pHNSHP45 ofE. coli, and plasmid pWF-5-19C ofCronobacter sakazakii, implying that pTA9-like plasmids may be epidemic plasmids that mediate the spread ofmcr-1amongEnterobacteriaceae. These results highlight that healthy meat duck is a potential reservoir for multidrug resistantmcr-1-containingE. colistrains.