Abstracting review presents current information from the field of colonization of gastrointestinal tract of a newborn, defining the role of bifidobacterias. It is pointnig out/mention suggest scientifically confirmed effects of some probiotics, regarding particular phases of colonization of gastrointestinal tract of a child. The review is suggesting confirmed and predicted impacts and interactions of microflora of host macroorganism. Latest non-cultivating molecular – genetic methods enabled identification of new species of microorganisms, that are found on „naturally sterile places“ of human organism (lower respiratory tract, amniotic fluid, meconium). Ecological-physiological relations between macroorganism and microorganisms are complicated. Recently we are able to define only several stages of development, pattern and mechanisms of the development of human microbion (complex of genomes of all microorganisms in certain enviroment). Microflora of gastrointestinal tract narrowly cooperates with immune system of the host, providing him accessory ability of digestion, many beneficial metabolitesand vitamins, as well as defence against pathogens. We are going to able to influence more successfully the development and health state of an individual by understanding and respecting these interactions.