Bifi dobacteria: the Model Human Gut Commensal

This chapter discusses biology of bifidobacteria for probiotic and replacement therapy. Bifidobacteria were originally isolated from a breast-fed infant. Since then several species have been identified in the gastrointestinal tract (GIT) of mammals and insects. The availability of whole-genome sequences from bifidobacteria is extremely informative for the understanding of the processes underlying speciation and evolution in this genus as well as the adaptation to its specific habitat (e.g., the human intestine). Since bifidobacteria encounter a vast array of substrates in the GIT, information regarding their metabolic properties is crucial to formulate an optimal growth medium. Prebiotics are food substances which resist digestion in the proximal GIT and reach intact the distal region of the gut, where, besides their direct physiological effect, they also affect the GIT ecosystem by specifically stimulating the growth and activity of the probiotic components of the GIT microbiota (e.g., bifidobacteria), thereby eliciting a beneficial effect on the host’s health. Prebiotics are typically oligosaccharides that consist of a mixture of hexose oligomers with a variable extent of polymerization. The transmission of novel genes through horizontal gene transfer (HGT) is generally supported by vectors such as bacteriophages or plasmids which may become integrated into the bacterial chromosome. Generally, plasmids are rarely found in bifidobacteria. So far the majority of bifidobacterial plasmids have been identified within different B. longum biotype longum strains. The chapter ends with a discussion on taxonomy of bifidobacteria.