Evolution of virulence in pathogenic escherichia coli strains:impact on public health

Abstract

Diarrheagenic Escherichia coli (DEC), including Verocytotoxin-producing E. coli (VTEC), are a significant public health issue worldwide. The management of the infections caused by these bacterial pathogens is complicated by their extreme heterogeneity, including strains causing a plethora of symptoms spanning from uncomplicated diarrhoea to life-threatening systemic sequelae such as the haemolytic-uremic syndrome (HUS). Considerable efforts have been devoted by the scientific community to understand the evolutionary forces leading to the emergence of new E. coli pathogenic clones. However, E. coli genome is very dynamic and evolves continuously through horizontal gene transfer. The main objective of the present piece of research was to investigate the molecular bases of the evolution of pathogenic E. coli, mainly DEC. We focused on VTEC as a model of dynamic pathogenic group (pathotype) encompassing many established Escherichia coli pathotypes. VTEC pathogenicity mainly relies on two aspects, the ability to efficiently colonize the host gut and the production of the erocytotoxins (VTs). Genes encoding the VTs (vtx) are carried by lambdoid bacteriophages whose genome is normally integrated into the E. coli chromosome. We have investigated the distribution of several Mobile Genetic Elements (MGEs) encoding virulence features in different VTEC subpopulations and studied their evolution through the differentiation of the virulence genes into different allelic variants. Additionally, we studied the different bacteriophages transporting the vtx genes into different VTEC types and their role in manipulating the host biology. Finally, we assessed the possibility that the spreading of VT-phages may or may not be subjected to host-related barriers by probing the possible acquisition of such phages by E. coli strains with a genetic background different from VTEC. The work presented here has been largely based on the genomic comparison of VTEC strains isolated from human and animal sources and held in the collections of the EU RL VTEC and of the collaborating institutions. This approach led to the identification of a pathogenicity island encoding an allelic variant of the Subtilase cytotoxin (subAB2, Chapter 3) and to the description of the allelic variants of the colonizationassociated virulence factor toxB (toxB1 and toxB2, Chapter 4). Moreover, the distribution analysis of the two virulence factors on a large panel of E. coli strains allowed making inference on their role in the VTEC pathogenetic process. The genomic approach has also been used to identify and characterize different VT-phages present in E. coli strains associated to the most severe form of infection, the HUS. In detail, a VT-phage present in VTECO157 strains and able to influence the regulation of genes involved in the colonization mechanism has been described (Φ-8, Chapter 5). Additionally, the complete sequence of the VT-phage isolated from a recently described E. coli pathotype, the Enteroaggregative Haemorrhagic E. coli (EAHEC) has been obtained and compared to other VT-phages sequences with the aim to help unravelling their complicate biology (Phi-191, Chapter 5). With the same aim, the evaluation of the stable acquisition of VT-phages by non and pathogenic E. coli strains belonging to all the known pathotypes led to the conclusion that such phages show a host range broader than expected (Chapter 6). This observation, together with other evidences from the literature raises the hypothesis that probably any Enterobacteria equipped with an efficient colonization machinery could be potentially acquire a VT-phage generating clones with augmented virulence potential for humans, as it happened with the EAHEC O104:H4 that caused a large outbreak of HUS in Germany in 2011.