Figure 2.

Effects of the intestinal microbiota of layers on egg quality and safety. Suggested mechanisms of microbial action are as follows: ① Intestinal bacteria can spread to the oviduct through the cloaca leading to egg contaminations, and accelerating the formation of egg translucency through bacterial penetration. In addition, oviducal bacteria biosynthesize pigments affecting the eggshell color. Moreover, oviducal pathogenic bacteria activate the TLR4/NF-κB signaling pathway inducing the synthesis of IL-1β and IL-6, which affects the eggshell ultrastructure by inhibiting the protein expression of a calcium-binding protein and Ca²⁺ transport. ② Short-chain fatty acids (SCFAs) produced by intestinal microbiota reduce the luminal pH and enhances calcium solubility, utilization, and deposition to improve eggshell thickness. ③ SCFAs bind to G protein-coupled receptor 43 (GPR43) to activate Wnt signaling in the bone marrow stromal cells to improve the Ca²⁺ homeostasis of bone metabolism and eggshell quality. ④ Trimethylamine (TMA) produced by intestinal microbiota is deposited in egg yolks, resulting in an unpleasant fishy odor. ⑤ Intestinal probiotics can activate the Nrf2 signaling pathway in the liver to regulate the antioxidant status and metabolic functions, which may improve albumen quality. ⑥ Bile acids (BA) produced by intestinal microbiota modulate lipid metabolism and intestinal absorption to improve the egg yolk color. ⑦ SCFAs, BA, and indole derivatives can directly interact with intrinsic enteric neurons and gut-innervating vagal and spinal afferents to regulate the secretion of estradiol, which regulates the secretion of proteins in the oviducal magnum and finally completes the regulation of albumen quality.