| Summary: | Purpose: To investigate the increasing prevalence of colistin resistance in animal Escherichia coli isolates and their mcr-1-carrying plasmids, especially those shared by isolates from human and retail meats. Methods: E. coli from diseased swine and poultry recovered between 2012 and 2016 were studied. Susceptibility was determined using broth microdilution method or Vitek II system. Fifty-eight mcr-1-positive isolates were randomly selected for further testing, including pulsed-field gel electrophoresis (PFGE) for clonality determination, S1- or I-CeuI-PFGE and Southern blotting for localization of mcr-1, and conjugation for transmissibility. Whole genome sequencing (WGS) was performed for the genetic structure of plasmids. Results: Among the 1234 E. coli isolates from diseased swine, colistin resistance increased from 14.6% (14/96) in 2012 to 43.8% (63/144) in 2016 with a paralleled increase in mcr-1-positivity from 12.5% (12/96) to 33.3% (48/144) in 2016 (P < 0.001), but no such significant increase was observed in the 489 diseased poultry isolates. The 58 clonally diverse isolates were resistant to multiple antibiotics commonly used in humans. PFGE and Southern blotting revealed one chromosome-located mcr-1 and various mcr-1-borne plasmids, all of which were transferable to E. coli J53. WGS revealed that the prevalent 60-kb and 30-kb plasmid was the same as pHNSHP45 in China and pESTMCR in Estonia, respectively, which were both present in human isolates in Taiwan. Conclusion: Increased colistin resistance and mcr-1-positivity in diseased swine isolates and detection of mcr-1 carried on similar plasmids in isolates from animals and humans stress the need to monitor resistance in both sectors.
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