Concluding remarks

The work described in this PhD thesis developed throughout the entire axis of exposure to STEC infections, from the animal reservoir to the development of disease. Given the complex epidemiology of this infectious disease, the proposed work has been focused on the less known segments of this axis, as the investigation of STEC isolates whose impact on public health was not completely clear as well as on the identification of new reservoirs and vehicle of dissemination of

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the pathogens throughout the whole food chain, up to the less known action of the STEC on the host intestinal microbiota.

The next generation sequencing technologies, extensively used in the studies carried out, provided a wealth of information on all the topics touched and allowed to recognize new STEC types causing HUS, new possible pathways for the spreading of STEC and other pathogenic E. coli and the risk of their transfer to the food chain through the use of soil improvers, as well as the possibility that biosolids may represent a cradle for the development of STEC with shuffled virulence genes asset.

All these findings allowed a better comprehension of some of the less known aspects of the epidemiology of STEC infections, such as the involvement of the Stx subtypes in the development of the disease in humans. As a matter of fact, our findings showed that Stx subtypes previously regarded as being adapted to an animal host, as Stx2f, may be able to induce HUS in humans in presence of an efficacious repertoire of determinants that allow establishing a successful colonization of the host intestine.

Our work also provided evidence that the investigation of the threats associated with the re-use of wastes in the framework of the green economy is necessary and has to be faced through an extensive risk assessment exercise. Our work focused on STEC but it was evident from our findings that other threats should also be investigated.

Finally, the investigation of the changes induced in the intestinal microbiota upon infection with a STEC O26:H11 led to the identification of the unbalance in the taxonomic units of the microbiota, which appears to be similar to that observed during infections with other enteric pathogens and described in the literature. This may either be a response of the microbiota to the interaction with the STEC or a general adaptation of the intestinal microflora to the colonization by a pathogen. In any case the knowledge of these dynamics is essential to the comprehension of the pathogenetic mechanisms and paves the way to the development of non-pharmacological approaches to mitigate the burden of intestinal infections. This aspect is particularly interesting for STEC as the antibiotic treatment is not recommended for the treatment of the infections, which mainly relies on prevention and supportive therapy.

In conclusion, the work presented in this thesis aimed at providing knowledge to specifically fill the gaps in our understanding of the complex epidemiology of STEC infections, particularly considering the less knows STEC types and their natural reservoirs, the interactions with the intestinal microbiota in the human hosts and the environmental involvement in the spreading of the infections.

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