Chousalkar, KapilMcWhorter, AndreaKhan, SamiullahZhang, Jiawei2025-07-232025-07-232025https://hdl.handle.net/2440/146278The sustained production of eggs is essential to meeting global protein demands, with per capita egg consumption steadily increasing. Extending the productive lifespan of layer flocks is of great interest to the Egg industry worldwide. However, extending late-lay periods (hens over 60 weeks old) often presents production challenges. Maintaining gut health during this period is important and gut microbiota plays a vital role in digestion, immunomodulation, and overall productivity. Understanding microbiota structure in late-lay flocks may offer strategies to sustain high egg production. The gut microbiota composition of 65-week-old free-range flocks was examined. Flocks with high egg production exhibited significantly different gut microbiota diversity compared to low-production flocks. Genera such as Anaerobiospirillum, Succinatimonas, and Fecalibacterium were more abundant in highproduction flocks, while Ruminococcus and Odoribacter dominated in low-production flocks. The current study found no significant impact of early disease histories, including Spotty Liver Disease, Fowl cholera, or Big Liver and Spleen disease, on gut microbiota composition in late-lay flocks. The gut microbiota of high-production caged flocks was also investigated. While the dominant phyla (Bacteroidota and Firmicutes) were consistent between states, Queensland flocks exhibited higher microbiota richness, potentially reflecting environmental or dietary differences. Significant differences in diversity were observed within the flocks under uniformed management. Lactobacillus species isolated from high-production flocks. Eleven species were identified, including Lactobacillus kitasatonis, Lactobacillus johnsonii, Lactobacillus agilis, Lactobacillus crispatus, Lactobacillus gallinarum, Lactobacillus reuteri, Lactobacillus salivarius, Lactobacillus saerimneri, Lactobacillus ingluviei, Lactobacillus oris and Lactobacillus mucosae. The in-vitro tests revealed strong probiotic traits, such as culturability in challenging conditions (low pH, bile salts, and simulated gastric juice), adhesion to epithelial cells, and pathogen inhibition. These results highlight the potential of specific Lactobacillus strains as probiotic candidates to enhance gut health and productivity. Further, a gel-based delivery system was evaluated for administering Lactobacillus strains to chickens. A formulation using 2% xanthan gum maintained high bacterial viability after 24 hours of aerobic incubation at 25°C. A single time point administration of L. mucosae, L. ingluviei, or L. oris via the gel demonstrated successful colonization of the caeca over four weeks. This innovative delivery system presents a reliable alternative to traditional feed and water methods, ensuring the viability and effective gut colonization of probiotics. Overall the studies conducted as part of this thesis demonstrated that high egg production in late-lay flocks is associated with richer gut microbiota diversity. Several Lactobacillus strains from high-production flocks exhibited probiotic potential, and a gel-based delivery method proved effective for delivering viable probiotics. Future studies could be conducted to better understand disease impacts on gut microbiota and practical strategies to extend production cycles in layer hens.enGut microbiotaLayer hensLate-layLactobacillusProbioticsGut healthEgg productionGel-based probiotic deliveryThesis