Dietary Spirulina platensis enhances immune responses and modulates gut microbiota and microbial function in laying hens.

Growing restrictions on antibiotic use in poultry nutrition have heightened the need for natural feed additives that promote health and performance. Spirulina platensis (SP) has gained attention for its immunomodulatory and gut health benefits. This study evaluated the effects of SP supplementation on immune responses, gut microbiota composition, and microbial functional potential in White Leghorn laying hens. A total of 192 White Leghorn laying hens (39 weeks old) were randomly assigned to four dietary groups: Control (0g SP/kg feed), T1 (2.5g SP/kg feed), T2 (5g SP/kg feed), and T3 (10g SP/kg feed). Each group received its respective diet for a six-weeks duration. Serum biomarkers, including cytokines, chemokines, and growth factors, were analyzed, and microbial profiling was performed using 16S rRNA sequencing followed by PICRUSt2-based functional predictions. Immune data were analyzed using one-way ANOVA, followed by orthogonal polynomial contrasts and Duncan's multiple test (p < 0.05). Microbial data were analyzed using Kruskal-Wallis and UniFrac distances with Tukey's HSD. LEfSe (LDA>4.0, p < 0.05) was used for identifying differentially abundant taxa. SP supplementation, particularly at 10g SP/kg, significantly enhanced (p < 0.05) pro-inflammatory and anti-inflammatory cytokines, chemokines, and growth factors. Microbial shifts included increased Firmicutes and Lactobacillus, and reduced Bacteroidota and Fusobacterium, with distinct clustering of microbial communities in SP-treated groups. Functional predictions showed significant upregulation (p < 0.05) of glycolysis, amino acid, and nucleotide metabolism, and downregulation (p < 0.05) of teichoic acid and lipid metabolism. These findings underscore the potential of SP, particularly at 10g SP/kg, as a sustainable feed additive to enhance immunity and gut health in antibiotic-free poultry production.