Anran Niu
Commensal bacteria have co-evolved with the host species to dominant the microbial community in the gastrointestinal tract. Intestinal symbiotic bacteria affect various aspects of host healthy status. Germ-free animals are usually associated with weaker immunity and higher susceptibility to disease. This review briefly summarizes the roles played by commensal bacteria in protecting the host and the mechanisms involved from the host point of view. Overall, these mechanisms can be divided into three actions including maintaining physical intestinal barrier function, stimulating production of host defence components, and modulating cellular immunity.