| Summary: | The emergence and rapid spread of the plasmid-mediated colistin-resistant <i>mcr-1</i> gene introduced a serious threat to public health. In 2021, a multi-drug resistant, <i>mcr-1</i> positive <i>Escherichia coli</i> EC1945 strain, was isolated from pig caecal content in Croatia. Antimicrobial susceptibility testing and whole genome sequencing were performed. Bioinformatics tools were used to determine the presence of resistance genes, plasmid Inc groups, serotype, sequence type, virulence factors, and plasmid reconstruction. The isolated strain showed phenotypic and genotypic resistance to nine antimicrobial classes. It was resistant to colistin, gentamicin, ampicillin, cefepime, cefotaxime, ceftazidime, sulfamethoxazole, chloramphenicol, nalidixic acid, and ciprofloxacin. Antimicrobial resistance genes included <i>mcr-1, bla<sub>TEM-1B</sub>, bla<sub>CTX-M-1</sub></i>, <i>aac(3)-IId</i>, <i>aph(3’)-Ia</i>, <i>aadA5</i>, <i>sul2</i>, <i>catA1</i>, <i>gyrA</i> (S83L, D87N), and <i>parC</i> (A56T, S80I). The <i>mcr-1</i> gene was located within the conjugative IncX4 plasmid. IncI1, IncFIB, and IncFII plasmids were also detected. The isolate also harbored 14 virulence genes and was classified as ST744 and O101:H10. ST744 is a member of the ST10 group which includes commensal, extraintestinal pathogenic <i>E. coli</i> isolates that play a crucial role as a reservoir of genes. Further efforts are needed to identify <i>mcr-1</i>-carrying <i>E. coli</i> isolates in Croatia, especially in food-producing animals to identify such gene reservoirs.
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