Occurrence and Molecular Characteristics of Mcr-1-Positive Escherichia coli from Healthy Meat Ducks in Shandong Province of China

Abstract

Colistin has been used as a growth promotant in livestock feed for many years. In China, <i>mcr-1</i>-positive <i>Escherichia coli</i> strains have been isolated from humans, chickens, and pigs. To date, there are few reports about the prevalence and molecular characteristics of fecal <i>E. coli</i> bearing <i>mcr-1</i> in the meat ducks. In this study, the prevalence of <i>mcr-1</i> gene was investigated among 120 fecal <i>E. coli</i> strains isolated from healthy meat ducks in Shandong province of China between October 2017 and February 2018. A total of nine <i>mcr-1</i>-containing <i>E. coli</i> strains were identified and two were identified as extra-intestinal pathogenic <i>E. coli</i> (ExPEC) among them. The clonal relationship of the nine <i>E. coli</i> strains was determined by multilocus sequencing typing (MLST) and pulsed field gel electrophoresis (PFGE), and the results indicated that all <i>mcr-1</i>-carrying isolates were clonally unrelated. Two different genetic contexts of <i>mcr-1</i> were 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. The <i>mcr-1</i> was 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 of <i>E. coli</i>, plasmid pHNSHP45 of <i>E. coli</i>, and plasmid pWF-5-19C of <i>Cronobacter sakazakii</i>, implying that pTA9-like plasmids may be epidemic plasmids that mediate the spread of <i>mcr-1</i> among <i>Enterobacteriaceae</i>. These results highlight that healthy meat duck is a potential reservoir for multidrug resistant <i>mcr-1</i>-containing <i>E. coli</i> strains.