The human gut is home to trillions of microorganisms, including 1000 species of bacteria, and are collectively termed the intestinal microbiota. The microbiota is in constant interaction with the host and has various effects on the host, such as differentiation of immune cells i.e. Foxp3+ regulatory T cells (Treg) and T helper 17 cells (Th17). Treg cells play important roles in suppressing inflammatory responses, especially in unwelcome situations such as autoimmune disease. We are interested in elucidating how gut commensals like B. bifidum, which colonize during early stage of life, induces Foxp3+ Treg cells, and aim to find the bacteria derived effector molecules involved in this process. To fulfill our goal we use germ free and B. bifidum monoassociated mice. Our studies provide a mechanistic scenario of how gut commensal bacteria induce Treg cells, moreover, it also provides potential candidates for therapy of inflammatory immune disorders.