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National audience; Epidemiologic studies reveal that only few virulence factors are frequently associated with Shiga toxin-producing Escherichia coli (STEC) strains that cause severe disease in humans. Among them, Shiga toxins (Stx) is required for the development of the most severe symptoms in STEC infected patients but is not strictly sufficient to make an E. coli strain pathogenic to human. Indeed, many STEC strains isolated from rearing animals have never been implicated in human disease. It therefore appears crucial to increase our knowledge on STEC pathogenesis at the molecular level. We developed a RIVET (Recombination-based in vivo expression technology) strategy in order to identify STEC genes specifically induced in vivo during mouse infection. Construction and screening of a RIVET promoter library from the typical STEC O157:H7 strain EDL933 resulted in the identification of 31 in vivo induced (ivi) genes. Most but not all, have an attributed function and are involved in either metabolism or stress response pathways, indicating that STEC has to adapt to the intestinal ecosystem. Additionally, some identified ivi genes belong to the dispensable genome of E. coli and analysis of their distribution among E. coli strains revealed a strong prevalence for some of them specifically in STEC strains. We also observed correlations between ivi genes and virulence genes, serotypes and/or seropathotypes. Finally, we assessed their contribution to STEC pathogenesis by infecting mice with mutants inactivated for selected ivi genes. This work may thus help to improve STEC molecular risk assessment schemes by giving new insights into molecular aspects of STEC pathogenesis.

National audience