Lactobacillus salivarius SNK-6 improves egg quality, yolk nutrient composition, and yolk flavor profile in laying hens via modulation of tissue metabolites and Cecal microbiomes

This study aimed to investigate the effects of Lactobacillus salivarius SNK-6 (L.a-SNK-6) supplementation on the laying performance, egg quality and yolk nutrition composition, including amino acid (AAs), fatty acids, and flavor profile, of Wenshang Barred hens. A total of 432 healthy 40-week-old hens were randomly assigned to three groups with 6 replicates each: control group (CON), a group supplemented with 2.0 × 10⁸ CFU/kg L.a-SNK-6 (T1), and a group supplemented with 2.0 × 10⁹ CFU/kg L.a-SNK-6 (T2). Compared with the CON group, both the T1 and T2 groups showed a reduction in the broken egg rate and increases in serum alkaline phosphatase, calcium (Ca) and calcitonin levels (P < 0.05). These groups also exhibited higher Haugh unit, eggshell strength and eggshell Ca content (P < 0.05). Furthermore, egg yolks from the T1 and T2 groups contained higher levels of α-linolenic acid, and oleic acid than those of the CON group. Total AAs content in the egg yolks was markedly elevated in both T1 and T2 groups, particularly the glutamic acid and aspartic acid (P < 0.05). Twenty volatile compounds that differed significantly among the three groups were identified, including d-limonene and β-pinene. Metabolomic analyses revealed 326, 237, and 108 differential metabolites in plasma, liver, and cecal content, respectively, between the T1 and CON group. These metabolites were significantly enriched in the tryptophan metabolism (map00380) pathway in both plasma and cecal content. 16S rRNA sequencing indicated that cecal microbiome diversity and abundance were higher in T1 group compared with the CON group, although the differences were not statistically significant. Combined analysis showed seventy-five host metabolite-microbiota pairs were significantly correlated (P-adjust < 0.05). Collectively, L.a-SNK-6 supplementation modulated metabolites across multiple host tissues and the cecal microbiome, leading to improved egg quality, enhanced yolk nutrient composition, and alterations in yolk volatile compounds.