Members of the genus Bifidobacterium are high G + C Gram positive bacteria belonging to the phylum Actinobacteria, and represent common inhabitants of the gastro-intestinal tract (GIT) of mammals, birds and certain cold-blooded animals. The overall microbial population that resides in the GIT, referred to as the "gut microbiota", is an extremely complex community of microorganisms whose functions are believed to have a significant impact on human physiology. Different ecological relationships between bifidobacteria and their host can be developed, ranging from opportunistic pathogenic interactions (e.g. in the case of Bifidobacterium dentium) to a commensal or even health-promoting relationship (e.g. in the case of Bifidobacterium bifidum and Bifidobacterium breve species). Among the known health-promoting or probiotic microorganisms, bifidobacteria represent one of the most dominant group and some bifidobacterial species are frequently used as the probiotic ingredient in many functional foods. However, despite the generally accepted importance of bifidobacteria as constituents of the human microbiota, there is only limited information available on their phylogeny, physiology and genetics. Moreover, host-microbiota interactions and cross-talk between different members of the gut microbiota are far from completely understood although they represent a crucial factor in the development and maintenance of human physiology and immune system. The aim of this review is to highlight the genetic and functional features of bifidobacteria residing in the human GIT using genomic and ecology-based information. (C) 2010 Elsevier B.V. All rights reserved.