Intestinal microbiota of layer hens and its association with egg quality and safety

Abstract

The intestinal microbiota has attracted tremendous attention in the field of the poultry industry due to its critical role in the modulation of nutrient utilization, immune system, and consequently the improvement of the host health and production performance. Accumulating evidence implies intestinal microbiota of laying hens is a potential mediator to improve the prevalent issues in terms of egg quality decline in the late phase of laying production. However, the regulatory effect of intestinal microbiota on egg quality in laying hens remains elusive, which requires consideration of microbial baseline composition and succession during their long lifespans. Notable, although Firmicutes, Bacteroidetes, and Proteobacteria form the vast majority of intestinal microbiota in layer hens, dynamic intestinal microbiota succession occurs throughout all laying periods. In addition to the direct effects on egg safety, intestinal microbiota and its metabolites such as short-chain fatty acids, bile acids, and tryptophan derivatives, are suggested to indirectly modulate egg quality through the microbiota-gut-liver/brain-reproductive tract axis. These findings can extend our understanding of the crosstalk between intestinal microbiota and the host to improve egg quality and safety. This paper reviews the compositions of intestinal microbiota in different physiological stages of laying hens and their effects on egg quality and proposes that intestinal microbiota may become a potential target for modulating egg quality and safety by nutritional strategies in the future.

Keywords: egg quality; intestinal microbiota; layer hen; microbial succession; short-chain fatty acids.

Figure 1

Intestinal microbial compositions in different stages of layer hens.

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.