The Evolution of Shiga Toxin-Producing Escherichia coli Characterization and Risk Categorization Strategies

Shiga toxin-producing Escherichia coli (STEC) are a group of E. coli that cause food and waterborne enteric illness. STEC are genotypically and phenotypically diverse as they do not have a single evolutionary origin, instead multiple STEC lineages have appeared independently following horizontal gene transfer events. The diversity of STEC is also reflected in an apparent variability in the potential of individual STEC strains to cause severe illness (bloody diarrhea or hemolytic uremic syndrome) or be involved in large foodborne outbreaks. In the context of food safety decision making, the variable characteristics of STEC strains and their potential health outcomes have resulted in an ongoing debate on how STEC should be categorized into risk groups. The aim of this review is to clarify that debate by examining how STEC characterization and risk categorization strategies have evolved over time in response to changes in analytical techniques, understanding of STEC epidemiology, and pathogenicity mechanisms. The origin of established perspectives and practices is explored, from the recognition of STEC O157:H7 as an emerging pathogen in the 1980s, through serotype-based identification and characterization of STEC, to contemporary approaches based on molecular markers for virulence. The evidence for the role of specific virulence factors in STEC pathogenicity is reviewed and their utility in STEC risk categorisation assessed. Finally, a binary categorization scheme is proposed, based on evidence for the correlation of hemolytic uremic syndrome, with the presence of Shiga toxin subtypes Stx2a, Stx2c, Stx2d, and colonisation factors including the Locus of Enterocyte Effacement (intimin/eae) and Enteroaggregative attachment (aggR/aaiC).