The global poultry industry is a cornerstone of modern food systems, supplying over 105 million metric tons of chickens annually as per USDA 2025 projections and fulfilling a substantial portion of worldwide protein demand (Zampiga et al., 2021). This vast scale of production, driven by intensive farming practices, has introduced significant health challenges for poultry, including compromised gut health, weakened immunity, and increased susceptibility to infectious diseases. Historically, these issues were managed through the widespread use of antibiotics as growth promoters (AGPs). However, the overuse of AGPs has contributed to the escalating global public health crisis of antimicrobial resistance (AMR), whereby pathogens evolve resistance to critical antibiotics used in both human and veterinary medicine (Etienne et al., 2025). Recent reports estimate that antimicrobial resistance affects up to 30 % of poultry pathogens globally, posing a severe threat to food safety, animal welfare, and production economics (Caneschi et al., 2023; Salam et al., 2023). These trends underline the urgent need for effective antibiotic alternatives in poultry farming worldwide.

In response, many countries, including members of the European Union, banned the use of AGPs as early as 2006, with others following through regulatory or voluntary measures (Castanon, 2007). This paradigm shift left a considerable gap in production management, creating an urgent need for effective, sustainable alternatives that can address the health and performance challenges previously mitigated by antibiotics. Among the most promising substitutes are probiotics. These are live microorganisms which, when administered in adequate amounts, confer beneficial effects on the host (Halder et al., 2024). Probiotics support robust and resilient intestinal environments in animals via multiple established mechanisms such as competitive exclusion of pathogens through niche occupation and resource competition; immune modulation that enhances innate defenses; and improved nutrient utilization through positive impacts on gut morphology and absorption (Naeem & Bourassa, 2025). Despite these documented benefits, many commercially available probiotics lack consistent efficacy due to limited strain resilience under gastrointestinal stresses or processing conditions, highlighting the need for robust strains with enhanced stability and multifaceted benefits.

While probiotics efficacy in enhancing growth performance, feed efficiency, and overall health is extensively documented, these benefits are highly strain-dependent and influenced by dosage and environmental context (Negi & Chen, 2025). This underscores the critical need to explore novel microbial sources for strains with more potent and consistent effects. One such candidate is Lactococcus lactis, a well-known probiotic genus widely used in the food industry and recognized for its benefits in poultry. The strain investigated here, L. lactis L25_4, is unique due to its origin isolated from deep ocean water off Taiwan's eastern coast at a depth of 312 m (Negi, Kuo, et al., 2025). This extreme environment, characterized by high pressure and low temperature, fosters microorganisms with unique adaptive traits that may translate into novel and enhanced biological functionalities, distinguishing them from more conventional terrestrial or dairy-derived strains. The unique adaptive traits of deep-sea microorganisms not only offer novel bioactive potentials but also align with One Health principles by supporting sustainable, antibiotic-free solutions that integrate animal health, food safety, and environmental stewardship.

Preliminary studies provide a solid foundation for this investigation. Beyond terrestrial applications, Lactococcus lactis L25_4 exhibits potent antimicrobial and immunomodulatory properties, effectively reducing methicillin-resistant Staphylococcus aureus (MRSA) biofilm formation and upregulating host antimicrobial peptides such as Cecropin (Negi, Kuo, et al., 2025). In aquaculture, dietary supplementation with L25_4 improved survival of whiteleg shrimp (Litopenaeus vannamei) against Vibrio vulnificus infection by modulating gut microbiota, enhancing gut barrier integrity, and upregulating key immune genes including HSP70 and Lysozyme C (Negi, Yadav, et al., 2025). This strain resilience to extreme environmental pressures and temperatures further suggests its metabolic products including its cell-free supernatants (CFS) possess superior stability and potency. Such features are critical for industrial feed formulations, especially in contexts lacking consistent cold storage, subsequently offering commercially viable, multifactorial probiotic alternatives. Thus, evaluating both live probiotic candidate and derivatives presents a comprehensive strategy for advancing sustainable poultry production.

This study aimed to evaluate an encapsulated deep-sea Lactococcus lactis L25_4(C/A)₂ as a multifunctional potential probiotic system for sustainable broiler production. Utilizing a feeding trial followed by an acute lipopolysaccharide (LPS) challenge model in Arbor Acres broilers, we investigated the strain's impact on metabolic efficiency, carcass characteristics, immune and oxidative responses, and intestinal microbiota composition. Furthermore, the post-mortem application of a CFS-based hydrogel was assessed for its efficacy in preserving breast meat quality and inhibiting microbial spoilage. The combined findings establish a proof-of-concept that this deep-sea–derived, encapsulated Lactococcus system provides a holistic, antibiotic-free solution that simultaneously supports animal performance and enhances product shelf life.