Establishment of In Ovo Salmonella Enteritidis Infection and Synbiotic Delivery Models in Chick Embryos and Their Effects on Early Gut Health

Early microbial exposure during embryogenesis may shape post-hatch gut development in poultry, yet the effects of sublethal pathogenic exposure and in ovo synbiotics remain unclear. This model-establishment study preliminarily established in ovo Salmonella Enteritidis (SE) infection and synbiotic (SYN) delivery models and evaluated their effects on hatchability, cecal microbiota, intestinal morphology, epithelial turnover, and barrier function in newly hatched chicks. In one group, the air cells of specific pathogen-free White Leghorn eggs were injected with SE on embryonic day 12; in another group, a synbiotic consisting of Lactobacillus plantarum, Pediococcus acidilactici, and inulin was injected into the amniotic cavity on embryonic day 17.5. Sterile saline was injected as the vehicle-only procedural control at the corresponding time points and injection sites. Based on their impacts on hatchability, SE1-L and SYN-H were selected. SE1-L reduced cecal microbial diversity, expanded Proteobacteria and Escherichia–Shigella, increased ileal apoptosis and crypt depth, decreased the villus height-to-crypt depth ratio, downregulated jejunal tight-junction genes, upregulated ileal MYD88 and TNF-α, and increased plasma lipopolysaccharide and D-lactate. In contrast, SYN-H maintained hatchability, promoted early Pediococcus colonization, suppressed potential pathogens, increased ileal villus height and villus height-to-crypt depth ratio, enhanced proliferation, reduced apoptosis, and improved mucosal barrier-related indices. These findings provide preliminary evidence that embryonic SE infection and synbiotic delivery differentially influence early intestinal microbiota succession and gut development in newly hatched chicks.