Colistin Resistance Mediated by mcr-1 in ESBL-Producing, Multidrug Resistant Salmonella Infantis in Broiler Chicken Industry, Italy (2016–2017)

Colistin-resistance mediated by mobilisable and plasmid-borne mcr genes has emerged worldwide, threatening the efficacy of colistin, a last resort antibiotic increasingly used for treating human invasive infections by multidrug-resistant or extensively drug-resistant Enterobacteriaceae. In this study, 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 Extended Spectrum Beta-Lactamases (ESBL)-producing isolates. Characterization by whole genome sequencing (WGS), located mcr-1.1 on an incX4 plasmid. Phylogenetic analysis of these isolates with selected Italian S. Infantis previously isolated from animals, meat and human clinical cases with unknown epidemiological relationship, demonstrated that ESBL-producing, mcr-1-positive isolates belonged to the emerging pESI-like-positive-ESBL-producing clone described in Italy in 2015.