Colistin resistance mediated by mcr-1 in ESBL-producing, multidrug-resistant Salmonella Infantis in broiler chicken industry, Italy (2016-2017)

We report the first evidence of mcr-1-mediated colistin resistance in four multidrug resistant (MDR) out of 324 Salmonella Infantis from the Italian antimicrobial resistance (AMR) monitoring (2001-2017) in broilers and broiler meat. Two were also ESBL-producing isolates. In-depth characterization by whole genome sequencing (WGS), located mcr-1.1 on an incX4 plasmid. Phylogenetic relationships with selected Italian S. Infantis demonstrated that the ESBL-producing, mcr-1-positive isolates belonged to the emerging, pESI-positive, ESBL-producing clone, described in Italy in 2015.