The Cytolethal Distending Toxin (CDT) is a virulence factor produced by many pathogenic bacteria (Escherichia coli, Helicobacter hepaticus ...) colonizing the oral cavity, the gastrointestinal tract, the liver etc. The presence in the gut of genotoxin producing bacteria, such as E. coli, could be a predisposing factor for the development of cancer, including colon cancer responsible of 500 000 deaths per year in the world. CDT was discovered in clinical isolates from patients with diarrhea caused by an E. coli infection. CDT is essential for H. hepaticus and C. jejuni to sustainable colonize the gastrointestinal tract and induce severe mucosal and liver inflammation, in mouse models. In addition, the expression of CDT by H. hepaticus leads to dysplasic liver nodules. It is therefore important to characterize the CDT mechanism of action in eukaryotic cells. CDT induces DNA double-strand breaks (DSB), leading to cell cycle arrest, activation of DNA repair mechanisms and increased cell death by apoptosis. The induction of DSB by CDT activates a signal pathway similar to the one observed in response to irradiation. However, we have shown that at low doses of CDT (1000 times lower than those used in the literature), single-strand breaks are induced in S phase, leading to a replicative stress before DSB production. These damages cause the activation of a pathway involving RPA, ATR and CHK1 proteins, characteristic of a replicative stress, before activation of the ATM kinase. Our future goals are to characterize 1- the cell sensitivity, according to the cell cycle phases, 2- the type of damage, and 3- (the) DNA repair mechanism(s) involved in response of such damages.