Fractionation for valorization of hemicelluloses from lignocellulosic biomass is a key step in the biorefinery of plants. The use of lignocellulolytic enzymes is one of the ways used for the hydrolysis of biomasses because it is more eco-responsible. It is then necessary to have efficient enzymes and to optimize their production. This thesis aims to improve the production of hemicellulases by a thermophilic bacterium, Thermobacillus xylanilyticus. This hemicellulolytic bacterium secretes an enzyme of the GH11 family with a strong xylanase activity but this activity tends to decrease during the cultivations. The thesis focused on the study of population factors impacting the fall of the activity during successive cultivations. A phenotypic diversification leading to the presence of a cheating subpopulation was identified as responsible for the loss of activity. Strategies to control phenotypic diversification were then established. They were based on the introduction of environmental changes (carbon sources) or by changing the physiological state of the cells (age of inocula) during successive cultivations to improve enzyme production. From these approaches, xylanase activity could be increased up to 15-fold by alternating the carbon source. The complementary activities β-xylosidase and acetyl esterase were increased by changing the age of the inocula (cells in stationary phase). In the last part of the thesis, the strategy of environmental changes was done with segregated cultures which allowed to control the phenotypic diversification of the strain in bioreactors. XylanFactory