The gut microbiota and host innate immunity: Regulators of host metabolism and metabolic diseases in poultry?

The endogenous intestinal microbiota represents the multitudes of microbes residing in the intestine and is integral in multiple physiological processes of the host, including being a key factor involved in host metabolism, BW, and energy homeostasis. The gut microflora, together with other environmental factors such as diet and stress, can play a central role in both immune and nutritional physiological balance. The immune response and nutrient metabolism are 2 fundamental biological systems indispensable to maintaining and preserving life. Each of these systems is capable of modulating the activity of the other to ensure that the host animal is capable of coordinating the appropriate responses under any conditions. Thus, metabolic systems are integrated with pathogen-sensing and immune responses, and these pathways are evolutionarily conserved. Several important networks sense and manage nutrients and integrate with immune and inflammatory pathways to influence the physiological and pathological metabolic states. For example, the Toll-like receptors family of the innate immune system, found on immune cells, intestinal cells, and adipocytes, recognize specific microbial components (e.g., lipopolysaccharides, lipoproteins, nucleic acids, and so on) and can sense nutritional signals, such as elevated glucose levels and saturated fatty acids. Likewise, metabolism-signaling pathways, such as leptin and other hormones, can also regulate immune functions. Thus, any immune alteration, specifically inflammation, can cause disturbances in host metabolism. Gut microbiota have evolved with the host as a mutualistic partner, but dysbiosis in the form of altered gut microbiome and gut microbial activities, as well as environmental factors including stress, may promote the development of metabolic disorders of poultry. Using mammalian studies as the experimental models, this review will provide evidence to hypothesize that intestinal dysbiosis or recognition of nutrient-derived factors (fatty acids and glucose) by the avian intestinal innate immune system could activate signaling pathways that affect the avian gut microbiota and induce the dysfunction of the integrated immune and nutritional metabolic systems that could be responsible for initiating many metabolic disorders of poultry.