Plasmid-Mediated Colistin Resistance Genes mcr-1 and mcr-4 in Multidrug-Resistant Escherichia coli Strains Isolated from a Healthy Pig in Portugal.

Antimicrobial resistance encoded by mobile colistin resistance () genes is a global and emergent threat. In this study, we report the occurrence of two different populations of colistin-resistant harboring and variants in the intestinal microbiome of a healthy pig. Following antimicrobial susceptibility determination, the presence of genes in two strains, isolated according to different selective microbiological procedures, was screened by PCR. Whole-genome sequencing confirmed that both strains were multidrug-resistant; INIAV_002EC was an AmpC producer carrying , , , genes, and INIAV_001EC carrying , , and genes, along with mutations in quinolone resistance-determining regions. In addition, both strains harbored , , and determinants. Further genome analysis revealed different plasmid replicons associated with the genes, IncX4 associated with , and ColE10 with . In addition, other replicons, including IncFIA, IncI1-Iγ, IncX1, IncY, in INIAV_002EC, and IncX1, IncI1, and p0111, in INIAV_001EC, were identified. Furthermore, both strains belonged to ST215 serotype O68:H12 and ST156 serotype O25:H28, respectively. This finding highlights the pig gut flora as a potential reservoir of mobile colistin resistance genes and reports the presence of the gene found for the first time in Portugal.