Abstract
Shiga-toxin-producing Escherichia coli (STEC) represents a significant cause of foodborne disease. In the last years, an increasing number of STEC infections associated with the consumption of raw and pasteurized milk cheese have been reported, contributing to raise the public awareness. The aim of this study is to evaluate the main genomic features of STEC strains isolated from a semi-hard raw milk cheese, focusing on their pathogenic potential. The analysis of 75 cheese samples collected during the period between April 2019 and January 2020 led to the isolation of seven strains from four stx-positive enrichment. The genome investigation evidenced the persistence of two serotypes, O174:H2 and O116:H48. All strains carried at least one stx gene and were negative for eae gene. The virulence gene pattern was homogeneous among the serogroup/ST and included adherence factors (lpfA, iha, ompT, papC, saa, sab, hra, and hes), enterohemolysin (ehxA), serum resistance (iss, tra), cytotoxin-encoding genes like epeA and espP, and the Locus of Adhesion and Autoaggregation Pathogenicity Islands (LAA PAIs) typically found in Locus of Enterocyte Effacement (LEE)-negative STEC. Genome plasticity indicators, namely, prophagic sequences carrying stx genes and plasmid replicons, were detected, leading to the possibility to share virulence determinants with other strains. Overall, our work adds new knowledge on STEC monitoring in raw milk dairy products, underlining the fundamental role of whole genome sequencing (WGS) for typing these unknown isolates. Since, up to now, some details about STEC pathogenesis mechanism is lacking, the continuous monitoring in order to protect human health and increase knowledge about STEC genetic features becomes essential.
Highlights
Shiga toxin Escherichia coli (STEC) infections in the EU have increased in the last years, being the third most commonly reported zoonosis (EFSA and ECDC, 2019) and causing different types of diseases, ranging from uncomplicated diarrhea to hemorrhagic colitis and hemolytic–uremic syndrome (HUS) (Karmali, 2017)
The STEC-associated risk lies in the presence of the main virulence factors Shiga toxins (Stx), classified in two forms, Stx1 and Stx2, each type being further categorized into four variants for Stx1 (a, c, d, and e) and 12 variants for Stx2 (a–i) (EFSA and ECDC, 2020)
In order to verify the vitality of E. coli cells, the enrichment broth was streaked into Tryptone Bile X-glucuronide (TBX) plates incubated at 44◦C ± 1◦C for 18/24 h, and 50 colonies were isolated in nutrient agar (NA) and incubated at 37◦C for 18/24 h
Summary
Shiga toxin Escherichia coli (STEC) infections in the EU have increased in the last years, being the third most commonly reported zoonosis (EFSA and ECDC, 2019) and causing different types of diseases, ranging from uncomplicated diarrhea to hemorrhagic colitis and hemolytic–uremic syndrome (HUS) (Karmali, 2017). The serogroup is one of the factors used to identify STEC strains that have the capacity to cause human illness (Preueß et al, 2013). The STEC-associated risk lies in the presence of the main virulence factors Shiga toxins (Stx), classified in two forms, Stx and Stx, each type being further categorized into four variants for Stx (a, c, d, and e) and 12 variants for Stx (a–i) (EFSA and ECDC, 2020). Among these subtypes, Stx2a is considered potentially related to a more severe illness. The stx genes are carried by lambdoid phages, which are highly mobile genetic elements, and, thanks to their mobility, the horizontal transfer and the dissemination, as much as the loss of the stx genes, are facilitated (Werber and Scheutz, 2019)
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