Phenotypic characterization of the shiga toxin mediated pathogenic potential of enterohemorrhagic Escherichia coli (EHEC) of the O157:H7 serotype

Escherichia coli O157:H7 poses a major threat to public health. This zoonotic food and water-borne human pathogen, notorious for its low infectious dose and its capacity to cause severe disease and potentially lethal complications such as hemorrhagic colitis (HC) and hemolytic uremic syndrome (HUS). A major virulence hallmark is the production of a potent Shiga toxin (stx) which are encoded on lysogenic lambdoid phages that is a direct mediator of human disease. This study presents the characterization of different E. coli O157:H7 strains based on phenotypic traits and identification of its specific virulence traits. More than 25 epidemiological diverse E. coli O157:H7 strains that represent all nine established phylogenetic clades were selected from our collections to determine their geno- and phenotypic virulence states. Chromosomal phage locations were determined using Shiga-toxin bacteriophage insertion sites (SBI) assay. We treated cultures with the antibiotic ciprofloxacin, a known phage-inducing agent to determine efficiency of Stx phage mobilization using Digoxygenin labels, and consequently stx transcription by qPCR, toxin production and cytotoxicity assay. In summary, we characterized variation in Stx production and mobilization across and within different phylogenetic clades, and categorized isolates according to their Stx mediated pathogenic potential; and identified previously unknown types of Stx-producing phages. This approach allowed us to identify high Stx producers and correlate recorded virulence phenotypes to underlying bacterial loci and pathomechanisms responsible for Stx production levels, and consequently human disease severity.