Role of germ-free animal models in understanding interactions of gut microbiota to host and environmental health: A special reference to zebrafish

Highlights: • Functions of gut microbiota and probiotic bacteria in zebrafish are discussed. • Generation of germ-free zebrafish and isolation of gut bacteria are well described. • Application of germ-free zebrafish model is updated for recent research. • Prospects of germ-free zebrafish model are highlighted for evaluating the pollutants' toxicity. Numerous pieces of evidence documented the importance of gut microbiota in regulating human health and evaluating the toxicity of environmental pollutants, which are closely related to the host health in various aspects, including nutrition, energy translation, metabolism, pathogen resistance, and immune function. A variety of environmental factors can disrupt gut microbiota and their functions, and inevitably cause immune diseases, obesity and diabetes. However, deciphering the inner mechanisms involved in the functional interaction of gut microbes with host health is still needed extensive investigations. This review focused on the essential roles of intestinal microbes in host-related diseases and highlighted the development and applications of germ-free (GF) animal models, mainly zebrafish. Moreover, the generation, immunity characters, advantages and challenges of GF zebrafish models were also summarized. Importantly, the composition and isolation of zebrafish gut bacteria for further application and toxicity evaluation of aquatic environmental pollutants were also discussed. In conclusion, GF zebrafish play irreplaceable roles in understanding the potential functions and responses of customized microbiota towards human and environmental health implications.