In order to exert their positive effect, the beneficial bacteria must reach the colon in relatively high viable cell concentrations. They must survive the transit through the stomach and the small intestines, where they are exposed to harsh conditions, such as low pH values and pepsin presence and high bile salt concentrations. Thus, the most important probiotic characteristic is the ability of surviving the harsh environment in the upper gastro-intestinal tract. Another key feature is the production of sufficient amount of cells during the cultivation process in the production facilities. Therefore, strains possessing high growth rates and capable of gaining high amount of biomass in a short period are more suitable for industrial production of probiotics and probiotic foods.