Abstract:Germ-free animals refer to the animals in which bacteria, fungi, actinomycetes, mycoplasma, chlamydia, spirochetes, rickettsia, viruses, protozoa, and parasites cannot be detected in any part of the body and in vitro by modern technologies. Since germ-free animals do not carry any microorganism, they can be modeled into the animals carrying specific microorganisms. Because of the dormant immune system, germ-free animals are extremely sensitive to microbial infections. A variety of gnotobiotic animal models can be established for specific microbial infection experiments and pathogenic mechanism research. As a key tool, germ-free animals are pivotal in studying the relationship between microbiome and diseases and play an irreplaceable role in the research on the relationship between microbiome and host health and the mechnisms of infections. We briefly introduce germ-free animals and review the applications of germ-free animals in the research on the mechanisms of host-microorganism interactions.