Plasmid-Mediated Colistin Resistance ( mcr-1 ) in Escherichia coli from Non-Imported Fresh Vegetables for Human Consumption in Portugal

In this study, we report the presence of the plasmid-mediated colistin resistance (PMCR)-encoding gene in an isolate, INSali25, recovered from lettuce produced and marketed in Portugal. Colistin MIC from the vegetable isolate-determined by microdilution broth method according to EUCAST guidelines-revealed a non-wild-type phenotype of colistin (MIC 16 mg/L). To understand the genetic background of INSali25, we performed whole genome sequencing. Plasmid sequencing was also performed after plasmid DNA extraction from the transconjugant TcINSali25 ( ). Directed bioinformatics analysis identified the gene in a 39,998 bp length contig, with an upstream region including the antibiotic resistance gene in a partial transposon Tn truncated by the insertion sequence IS and showing >99% identity with previously described -harboring IncHI2 plasmids. Further analysis showed the presence of additional genes conferring resistance to β-lactams ( ), aminoglycosides ( , , , ), macrolides ( type), phenicol ( type), tetracycline ( ), and sulphonamides ( ). INSali25 isolate belonged to the ST1716 lineage and showed the and alleles. Lettuce is a vegetable that is commonly consumed fresh and not subjected to any cooking process, which may amplify human food safety risks. Moreover, the occurrence of plasmid-mediated colistin resistance in a sample that was not imported and was acquired in a large retail store reinforces the widespread distribution of .