Identification and antimicrobial resistance analysis of mcr-1 positive Salmonella Weltevreden

  Objective  To identify mcr-1 positive Salmonella and analyze its antibiotic resistance characteristics and mechanism.

  Methods  Pathogen biochemical identification and serotyping with serological agglutination test were conducted for the Salmonella strains isolated in 2015 through intestinal tract pathogen surveillance platform, the mcr-1 gene was detected with PCR and microbroth dilution method was used for drug susceptibility testing of the strains. S1 nuclease pulsed-field gel electrophoresis (S1-PFGE) and southern-blot analysis was performed to locate the resistance gene mcr-1. To verify the horizontal transfer ability of mcr-1 plasmids, plasmid conjugative transfer assay was used. The strains were sequenced by the miseq platform sequencer of Illumina, and the assembled plasmid sequences were annotated and analyzed.

  Results  A mcr-1-positive Salmonella Weltevreden was identified, which was resistant to polymyxin. S1-PFGE and Southern-blot revealed that mcr-1 gene was located on a plasmid which was about 30 kb size. The result of plasmid conjugative transfer assay showed that the plasmid could be transferred horizontally. The plasmid sequencing analysis indicated that the plasmid, named as pS68, had a size of 32914 bp, belonged to IncX4, carried five virulence genes.

  Conclusion  We should strengthen the surveillance of foodborne pathogens and the molecular epidemiology research of mcr-1 resistance gene transfer, so as to lay a sound foundation for better prevention of spread of drug resistant strains.