| Summary: | Aquaculture systems are widely recognised as hotspots for horizontal gene transfer, and the need for screening for bacteria carrying antimicrobial resistance genes in aquaculture systems is becoming more important. In this study, we characterised seventeen bacterial strains (<i>Escherichia coli</i>, <i>Klebsiella pneumoniae</i>, <i>Acinetobacter baumannii,</i> and <i>A. nosocomialis</i>) resistant to colistin originating from retailed aquaculture products imported from Vietnam to the Czech Republic. The <i>mcr-1.1</i> gene was found located on plasmid types IncHI2, IncI2, and IncX4, as well as on the rarely described plasmid types IncFIB-FIC and IncFIB(K), phage-like plasmid p0111, and on the chromosome of <i>E. coli</i>. One <i>E. coli</i> strain carried the <i>mcr-3.5</i> gene on IncFII(pCoo) plasmid in addition to the <i>mcr-1.1</i> gene located on IncHI2 plasmid. <i>K. pneumoniae</i> was found to carry the <i>mcr-1.1</i> and <i>mcr-8.2</i> genes on IncFIA(HI1) plasmid. The <i>mcr-4.3</i> gene was found on similar untypeable plasmids of <i>A. baumannii</i> and <i>A. nosocomialis</i> strains, pointing to the possible interspecies transfer of plasmids carrying the <i>mcr-4</i> gene. Our results highlight that some aquaculture products of Asian origin can represent an important source of variable plasmids carrying <i>mcr</i> genes. The results showed an involvement of phages in the incorporation of the <i>mcr-1</i> gene into plasmids or the chromosome in <i>E. coli</i> strains from aquaculture. The detection of <i>E. coli</i> with the <i>mcr-1</i> gene in the chromosome points to the risks associated with the stabilisation of the <i>mcr</i> genes in the bacterial chromosome.
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