-Mediated Colistin Resistance and Genomic Characterization of Antimicrobial Resistance in ESBL-Producing Infantis Strains from a Broiler Meat Production Chain in Italy.

This work aimed to evaluate phenotypically and genotypically the colistin susceptibility of 85 Infantis strains isolated in Italy from the broiler production chain, and to apply a whole-genome approach for the determination of genes conferring antimicrobial resistance (AMR). All isolates were tested by the broth microdilution method to evaluate the colistin minimum inhibitory concentrations (MICs). A multiplex PCR was performed in all isolates for the screening of , , , genes and whole-genome sequencing (WGS) of six . Infantis was applied. Three out of 85 (3.5%) . Infantis strains were colistin resistant (MIC values ranged from 4 to 8 mg/L) and positive. The and variants located on the IncX4 plasmid were detected in three different colistin-resistant isolates. The two allelic variants showed identical sequences. All six isolates harbored , and genes, mediating, respectively, beta-lactam, aminoglycoside and quinolone resistance. The pESI-megaplasmid carrying (tetracycline resistance), , (trimethoprim resistance) , (sulfonamide resistance) and (quaternary ammonium resistance) genes was found in all isolates. To our knowledge, this is the first report of the variant described in . Infantis isolated from broilers chickens. Our results also showed a low prevalence of colistin- resistance, probably due to a reduction in colistin use in poultry. This might suggest an optimization of biosecurity control both on farms and in slaughterhouses.