Food can be considered as functional if it can be demonstrated that it is benefitial for one or more functions of the human body, over and above the expected nutritional effects, being essential for improving health and well-being and/or for reducing the risk of illness.
       Probiotics belong to the group of functional foods and FAO defines them as “‘Live microorganisms which when administered in adequate amounts confer a health benefit on the host”.
       Although it is difficult to specify which are the most desirable features that a microorganism must show for being a good probiotic, the minimum requirements are: genus, species and strain designation; strains should be deposited in an internationally recognized cultura collection; in vitro and animal assessment must be carried out in order to get a better understanding of the phisiological properties of the strain; safety of the strain must be completely considered before its use; controlled studies must be undertaken for reporting some health benefit; its capability for keeping the minimum required numbers of viable bacteria by the end of shelf-life must be assessed.
       The use of functional dairy products makes possible to combine widely used foods, successful and tolerance, with biologically active molecules. Some species of lactobacilli and bifidobacteria in combination (or not) with Streptococcus thermophilus are the main bacteria used as probiotics in yoghurt and other fermented milks.
       However, microorganisms must be in high numbers in the final product  and survive to the gastrointestinal passage for confering a health benefit on the host.
       In the mid-nineties, an important commercial brand, considered the idea of getting into the yoghurt market with the launching of a pasteurised after fermentation dairy product that could be preserved for three months without any refrigeration. In addition to this, the company defended that its product had the same effect as the traditional yoghurt since its live cultures could not reach the intestine. It started a fight called the yogurt war with the aim of getting the consent of the Spanish government for qualifying the product as yoghurt.
       In this context, there is a discrepancy among the different research groups when confirming the beneficial effects of traditional yoghurt. Some of them maintain that its bacteria are not capable of surviving the passage through the gastrointestinal tract while others confirme the opposite.
       Based on the previous arguments, this work considered the development of quick methods other than the traditional culture ones for studying the possible properties as probiotics of bacteria contained on different dairy products. Firstly, lactic bacteria present on 11 commercial yoghurts (Lactobacillus delbrueckii subsp bulgaricus and Streptococcus thermophilus) were isolated, identificated and molecularly characterized. Then, PCR and FISH techniques were developed for the detection of both microorganisms. DVC-FISH technique was developed for the detection of viable cells of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus that allowed to determine in vitro resistance to the gastrointestinal juice of the strains, demonstrating its variability on a strain level. Finally, an in vivo study of the survival to the gastrointestinal tract of BAL strains from yoghurt was undertaken with the application of the developed protocols on fecal samples, studying the evolution of intestinal microbiota all along the test. This test allowed to demonstrate the presence of viable Streptococcus thermophilus on feces after the intake of the product.
       DVC-FISH technique showed to be the most quick and effective method for determining LAB viability on complex matrices such as human feces. Also, it was observed with cultura media that yoghurt consumption for a determined period produces an increase on lactobacilli endogenous microbiota and a decrease on the enterobacteria population.