Verocytotoxin-producing Escherichia coli (VTEC) of serogroup O157 are among the most important causes of severe cases of foodborne disease and outbreaks worldwide. As little is known about the characteristic of these strains in Romania, we aimed to provide reference information on the virulence gene content, phylogenetic background, and genetic diversity of 7 autochthonous O157 strains collected during 2016 and 2017 from epidemiologically non-related cases. These strains were typed by a combination of phenotypic and molecular methods routinely used by the national reference laboratory. Additionally, 4 of them were subjected to whole-genome sequencing (WGS), and public web-based tools were used to extract information on virulence gene profiles, multilocus sequence types (MLST), and single nucleotide polymorphism (SNP)-based phylogenetic relatedness. Molecular typing provided evidence of the circulation of a polyclonal population while distinguishing a cluster of non-sorbitol-fermenting, glucuronidase-negative, phylogenetic group E, MLST 1804 strains, representing lineage II and clade 7, which harbored vtx2c, eae-gamma, and ehxA genes. A good correlation between the routine typing methods and WGS data was observed. However, SNP-based genotyping provided a higher resolution in depicting the relationships between the O157:H7 strains than that provided by Pulse-field gel electrophoresis. This study should be a catalyst for improved laboratory-based surveillance of autochthonous VTEC.
Keywords: Escherichia coli O157; molecular characterisation; whole genome sequencing.