Assessment of the microstructural changes that occur in wheat doughs during fermentation due to the Lactobacillus type (Lactobacillus plantarum, Lactobacillus brevis and {\it Lactobacillus sanfranciscensis), and the changes associated to mechanical properties. To dough ​​with a dough yield (DY) of 150 were added 50 mL of inoculum (109 CFUmL^-1) and fermented at 35°C for 24 h (76% RH). The uniaxial extensibility of the dough was determined at 0, 6, 12, 18 and 24 h of fermentation using a texture analyzer equipped with a Kieffer rig. The microstructure was observed by means of a Scanning Electron Microscope (SEM), and the images were analyzed to determine the fractal dimension (FD_SDBC) and Entropy (Ent). In all cases, the maximum extensibility (Em) and the maximum resistance (R_m) of the doughs decreased with the progress of fermentation. Lb. plantarum and Lb. sanfranciscensis reduced E_m by 40 and 42%, respectively, while DY decreased by more than 85%. Fermentation increased the structural complexity of the dough by raising its FD_SDBC and Ent values, with differences depending on the type of bacteria. These results provide adequate criteria for the selection of bacteria for the development of sourdoughs and for understanding the structural and mechanical changes that occur during fermentation.