Strains "fil", stress-resistant under fermentation and/or growth conditions

Process for obtaining new eukaryotic strains, preferably new yeast strains, and even more preferentially strains of Saccharomyces cerevisiae conserving stress resistance in the presence of fermentable sugars such as glucose, comprising the following steps:

• a classic mutagenic treatment is carried out on the cells of a starting strain,
• the cells having undergone the said mutagenic treatment are cultured so as to obtain a stationary phase,
• the said cells in stationary phase are incubated in the presence of at least one fermentable sugar selected from the group comprising glucose, maltose, and sucrose, this sugar being present in a quantity such that the cells enter an active metabolic state (fermentation and/or growth) of this sugar,
• said cells in active metabolic state are subjected to one or several stresses leading to a mortality rate of at least 99% with respect to the starting population,
• the surviving cells are isolated and
• those of the surviving cells which respond to the following criteria which characterize the fil phenotype are selected, i.e.:
• a growth, evaluated by production or production yield of biomass over sugar in a given time or by a growth rate, under identical culture conditions, at least equal to 80% of the starting strain, and preferably at least equal to 90% of the starting strain,
• a CO2 release, or a metabolite production, in identical conditions, at least equal to 80%, and preferably at least equal to 90% of the starting strain,
• a stress resistance, corresponding to a survival rate at least 2 times higher, preferably at least 3 times higher, more preferentially at least 5 times higher, and even more preferentially at least 10 times higher than the survival rate of the starting strain, under identical phase conditions corresponding to a growth or active metabolism followed by a heat shock of at least 20 minutes at 52° C., or at least 1.5 times higher, preferably at least 2 times higher, more preferably at least 3 times higher, and even more preferentially at least 5 times higher than the survival rate of the starting strain, under identical conditions of growth phase followed by freezing for a period of at least 24 hours at −20° C. or at a lower temperature,
• maintenance of these properties after repeated cultures on non selective medium, such as YPD medium, so as to verify that the fil phenotype obtained by the mutation is perfectly stable and permanent.