Detection of the colistin resistance gene mcr-1 in pathogenic Escherichia coli from pigs affected by post-weaning diarrhoea in Italy

Objectives: The aim of this study was to investigate the presence of plasmid-mediated colistin resistance genes in Escherichia coli from pigs affected by post-weaning diarrhoea (PWD). Methods: DNA samples collected from 51 E. coli isolates from Italian pigs affected by PWD in 20152016 were studied. Isolates were classified as presumptively resistant to colistin by routine susceptibility testing and were investigated for the presence of the mcr-1 gene of plasmid origin by PCR. E. coli isolates testing negative for mcr-1 were analysed for the presence of a novel plasmid-mediated gene, mcr-2. Isolates were characterised for fimbrial [F4 (k88), F5 (k99), F6 (987P), F18 and F41] and toxin (LT, STa, STb and Stx2e) determinants by PCR as well as for the occurrence of haemolysis by phenotypic observation. Susceptibility to apramycin, cefquinome, enrofloxacin, florfenicol, gentamicin, tetracycline and trimethoprim/sulfamethoxazole (SXT) was also determined by disk diffusion. Results: Most of the isolates showed the presence of at least one virulence factor, confirming their pathogenic potential. The presence of mcr-1 was shown in 37 (72.5%) of the 51 isolates. All of the mcr-1-negative isolates tested negative for the mcr-2 gene. Moreover, 80.4% of the isolates were resistant to apramycin, 9.8% to cefquinome, 54.9% to enrofloxacin, 52.9% to florfenicol, 76.5% to gentamicin, 96.1% to tetracycline and 78.4% to SXT. Conclusions: This is the first report documenting the presence of the mcr-1 gene in pathogenic E. coli isolated from pigs affected by PWD in Italy. (C) 2017 International Society for Chemotherapy of Infection and Cancer. Published by Elsevier Ltd. All rights reserved.