GENERAL SUMMARY The bacterium Erwinia amylovora is the causal agent of fire blight in the Rosaceae family, responsible of serious damages in several pome fruits and ornamental plants. Due to the easy dissemination and difficult control of fire blight, it is considered as a quarantine disease by the European Union. In spite of the numerous studies reported on E. amylovora, the knowledge of its life cycle is still incomplete, and the available information about its survival under stress conditions and/or out of its susceptible hosts is still scarce. Therefore, the objective for this thesis was to study the survival of E. amylovora in a mineral medium with copper, widely used for the control of bacterial plant diseases, as well as under starvation, common limitation in natural environments such as water. The results have proved that E. amylovora is able to survive under both stressful conditions adopting the viable but non-culturable (VBNC) state. When the environmental conditions turned favourable, the pathogen recovered its culturability and pathogenicity, so that the VBNC state can be considered as a survival strategy for this bacterium. Since it was observed that E. amylovora lost its culturability under adverse conditions, but maintaining its viability and pathogenicity, we proposed to increase its recovery efficiency on plates. Due to that copper is an essential micronutrient that can favour the bacterial growth in rich-nutrient media, we optimized one of the most employed culture media for the isolation of E. amylovora by adding copper sulphate. Thus, the growth of this pathogen under stress conditions was enhanced, and its loss of culturability was delayed. It was also observed that the presence of copper in the culture medium produced a notable increase in the mucus of the E. amylovora colonies. This fact, together with the observation by scanning electron microscopy of an increase in the thickness of the external layer only in the cells exposed to copper, leaded to study the role of exopolysaccharides (EPSs) in the survival of E. amylovora. We demonstrated that amylovoran and levan played a relevant role in the survival of this pathogen. Thus, both EPSs were able to bind free copper ions, decreasing consequently their toxicity, and in addition, it seemed that they could be used as carbon and energy sources under nutrient deprivation conditions. Overall, the studies collected in this thesis give new information on the survival strategies of E. amylovora under certain adverse conditions in the nature. A deeper knowledge on the life cycle and epidemiology of this plant pathogen will contribute to a better detection and an optimized control of the disease in the nature. 2