Summary


      The concept of functional food is used to describe to foods supplemented with different types of ingredients and origins that can exercise beneficial effects on who ingests them. This concept was born in Japan and it has gone being popularized and expanding fundamentally toward other continents as Europe due to the concern about the nutrition, diet and health of the nowadays society. Probiotics represents a great area inside the functional foods and the investigation has been intensified to develop probiotic products and to penetrate into the knowledge on the effects on the human health.
      
      Probiotics are defined as “live microbial feed supplements that beneficially affect the host animal by improving its microbial intestinal balance.” They are microorganisms that stimulate protection functions of the digestive tract, they are also known as biotherapeutics, bioprotector or bioprophylactics, and they are used to prevent enteric and gastrointestinal infections. An effective probiotic microorganism should possess characteristics as: to be non-pathogen and non-toxic, to exercise beneficial effects on the human health, with human origin, to be technologically usable, to present a high percentage of viable cells, to be able to survive the intestinal microflora, to remain viable during its storage in refrigeration, capacity to adhere to the mucus surface, etc. The establishment of selection approaches and quality controls for these products are considered a priority due to the quick incorporation of these products in the market and their distribution in the international environment without the commonly accepted normative. 
      
      Recently, the interest for the products that contain probiotic microorganisms belonging to the genus Lactobacillus and Bifidobacterium has increased, this fact has given place to the commercial exploitation of some of these bacteria and to the appearance in the market of a great variety of probiotic products. Nowadays, many relative aspects are still ignored to their action mechanism; however their functionality is recognized in a lot of aspects, such as the prevention and treatment of gastrointestinal diseases; reduction of the lactose intolerance; modulation of the immune system; reduction of cholesterol levels, prevention certain cancers, inhibition of pathogens, alterants microorganisms and competitive flora. Their antimicrobial activity, is in great measure attributed to the production of organic acids (lactic and acetic acid), but it is much more complex and it is also, their biggest capacity to compete for nutritional compounds and to adapt to the ecological microflora and to synthesize other antimicrobial compounds like bacteriocins, heterocyclic compounds, dyacetil and others. 
      
      For all these, the aim of this work is the study of the fermented milks with bifidobacteria commercialized in the Spanish market, analyzing the content of microorganisms in these products, to identify them at genus and specie level and to molecular characterization, also, we want to know its viability to the storage conditions under refrigeration and simulated conditions throughout of gastrointestinal transit. The other hand, the selection and study of Bifidobacterium strains of human origin following the criteria of FAO-WHO (Food and Agriculture Organization of the United Nations- World Health Organization) such as the resistance to gastrointestinal conditions, antimicrobial activity against pathogens, alterants and other microorganisms, and capacity of adhesion to mucus) with the objective to obtain strains of human origin with possible use like probiotic strains for the elaboration dairy products.