The aim of our research group is the design of probiotic products for the reestablish of indigenous microbiota of different human and animal tracts. These products are applied for the prevention of urogenital infections in women, bovine metritis and mastitis, neonatal diarrhea in calves, and Red Leg syndrome in frogs. Also, the study of the mechanisms responsible of the protective effect of beneficial microorganisms is performed, through "in vitro" tests and "in vivo" assays in animal models. The selection of microorganisms with potentially probiotic properties characteristics is carried out by the isolation and identification of microorganisms from the ecological niche of the host where they are going to be applied. "In vitro" tests are applied to study probiotic properties, such as adhesion to surfaces and mucous, formation of biofilm, production of antagonistic substances against pathogen microorganisms, and resistance to antimicrobial agents. In the selected microorganisms, their technological characteristics are evaluated (optimal conditions of growth and expression of beneficial characteristics, viability during the time under different conditions of storage) and functional properties (resistance to the environmental conditions of the specific tract, and to the most frequents antimicrobial therapies). The studies of colonization ability, the no-production of side effects, and the capacity to prevent and/or to control infections by selected lactic bacteria (inoculated by different routes) are performed through "in vivo" tests.