Purpose: The aim of this study was to determine the prevalence and transmission mechanism of mcr-3  in Aeromonas  spp. isolated from chicken cloaca.
Materials and Methods: A. veronii  w55 was isolated from chicken in 2008. PCR assay was used to detect mcr  genes and putative circular intermediate. Susceptibility testing was identified by the microdilution method. WGS was performed to obtain the whole sequence. S1-PFGE and DNA southern hybridization were used to study the location of mcr-3.6 .
Results: PCR-based analysis indicated that 1 out of 55 Aeromonas  spp. isolates was mcr-3 -positive. Whole-genome sequencing revealed that the strain A. veronii  w55 belonged to novel sequence type ST514 and had two adjacent chromosomally located mcr  variants, mcr-3.6  and mcr-3 -like. The mcr-3.6  and mcr-3 -like genes showed 93.67% and 82.84% nucleotide sequence identity, respectively, to original mcr-3  from E. coli. A. veronii  w55 also exhibited resistance to extended-spectrum β-lactams and was positive for bla _PER-3, and this is the first time to report bla _PER-3 in A. veronii . Genetic environment analysis revealed that the segment of mcr-3.6-mcr-3 -like-dgkA  was flanked by five insertion sequence elements originated from Aeromonas  species, and the structure of ISAs2 -ISAhy2 -ISAs20-mcr-3.6-mcr-3 -like-dgkA -ISAs2  was designated as a novel transposon Tn 6518 , in which an 8405-bp circular intermediate carrying two mcr-3  variants can be looped out.
Conclusion: This result suggested the mcr-3 variant genes could be disseminated between various Aeromonas  species via transposon-mediated transmission.
Keywords: mcr-3.6 , Tn 6158 , Aeromonas veronii