In the complex ecosystem of gut microbiota, lactic acid bacteria (LAB)+B represent prevalent, abundant, and highly important subpopulation. LAB+B are well-known for their health-promoting effects: they are found to positively influence gut microbiota diversity, modulate immune response and intestinal permeability, and produce bioactive or regulatory metabolites. Bifidobacteriaceae represent critical early colonizers of the human gut, and the depletion of this microbial group in infants is correlated with allergy development. LAB+B are considered as one of the first human gut colonizers and recent studies have shown that humans might be exposed to them even in the womb. The primary role of the human gut microbiota, as in other animals, is its contribution to digestion. Several types of health-promoting dietary polyphenols are able to boost the abundance of LAB+B in human trials, but also in in vitro tests and animal models. A molecular methods-based insight into gut microbiota diversity showed presence of its disturbed composition—dysbiosis—in various diseases.