In Ovo administration of Probiotics and Postbiotics: Novel Approaches for Early Immune Programming, Microbiota Modulation, and Performance Optimization in Poultry

The rapid rise of antimicrobial resistance has created an urgent need for sustainable alternatives to antibiotic use in poultry production. This doctoral thesis explores the potential of in ovo administration of probiotics and an innovative patented postbiotic formulation as strategies for early microbial programming, immune system development, and performance enhancement in chickens. A series of experimental studies were conducted to evaluate the effects of these biotic compounds on embryonic development, immune competence, microbiota establishment, growth performance, and disease resistance. Following careful dose optimization, in ovo delivery proved safe, with no adverse impact on hatchability or Marek’s Disease vaccination efficacy. Histological and immunohistochemical analyses revealed that treatment induced structural and immunological changes in the yolk sac, including increased macrophage recruitment, polarization, and angiogenesis, pointing to accelerated maturation of innate immunity. In the intestine, probiotic and postbiotic administration enhanced enterocyte activation, gut-associated lymphoid tissue maturation and villus morphometry, supporting more efficient nutrient absorption and barrier function. Microbiota profiling showed that treated birds exhibited higher alpha diversity and a more balanced gut ecosystem, with postbiotic administration driving the most pronounced effects. These microbial shifts translated into measurable functional improvements, including increased body weight, reduced feed conversion ratio, and greater overall zootechnical performance. Notably, reduced oocyst shedding and lesion severity following Eimeria spp. infection was observed, indicating strengthened host resilience. Together, these findings provide novel insights into the role of the yolk sac as a mediator of host–microbe interactions during embryogenesis and highlight the capacity of in ovo biotics to prime immunity and gut functionality before environmental exposure. This work establishes in ovo administration of probiotics and postbiotics as promising, safe, and sustainable alternatives to antibiotic growth promoters. By bridging immunology, microbiology, and poultry production, it contributes to One Health strategies aimed at improving animal welfare, reducing reliance on antimicrobials, and safeguarding public health.