Comparative analysis of an mcr-4 Salmonella enterica subsp. enterica monophasic variant of human and animal origin

Objectives: In this study we compared the recently described mcr-4-positive Salmonella enterica monophasic variant, isolated in 2016 in two Italian patients affected by gastroenteritis, with the first mcr-4-positive Salmonella isolate identified in 2013 in a pig at slaughter in Italy. Methods: WGS of the two Salmonella isolates of human origin was performed using a MiSeq instrument (Illumina). The phylogenetic analysis was performed by SNP analysis, comparing genomes of the mcr-4-positive isolates of swine and human origin with 82 Salmonella genomes downloaded from the EnteroBase Salmonella database. Complete sequences of plasmids carrying mcr-4.2 were obtained and compared. Transformation experiments were performed to transfer the mcr-4 plasmids into a colistin-susceptible Escherichia coli recipient strain. Results: Comparative genomics demonstrated that the Salmonella of swine origin did not cluster with the isolates of human origin, The mcr-4.2 gene variant identified in the Salmonella of human origin was located on a plasmid. This plasmid showed different replication and mobilization genes with respect to those previously described in the ColE plasmid carrying the mcr-4.1 variant, identified in Salmonella of swine origin. Conclusions: The divergence in genomes, plasmids and gene variants demonstrated that there was not a unique mcr-4-positive, monophasic Salmonella lineage circulating in animals and causing gastroenteritis in humans in Italy. There was no horizontal transfer of the same plasmid among Salmonella strains of animal and human origin, but the mcr-4 gene and a fragment of the plasmid identified in the animal strain were mobilized by an IS1294 into a different ColE plasmid.