Dietary Encapsulation of a Novel Lysinibacillus sp. PWR01 Probiotic Modulates Growth, Antioxidant, Immune Gene Expression, and Gut Health in Nile tilapia (Oreochromis niloticus) Against Aeromonas hydrophila Infection

Encapsulated probiotics, which are promising approaches for improving aquaculture species' performance and health, have incompletely characterized dose-dependent physiological and immunological effects. This study represents the first report evaluating the probiotic efficacy of a novel encapsulated <i>Lysinibacillus</i> sp. PWR01, originally isolated from rubber latex nodules, in Nile tilapia (<i>Oreochromis niloticus</i>). A total of 300 Nile tilapia (10.80 ± 0.07 g) was allocated to four experimental groups receiving 0, 10⁴, 10⁵, and 10⁶ CFU/g of encapsulated <i>Lysinibacillus</i> sp. PWR01 in their diet. At 8 weeks of the feeding trial, growth performance, feed utilization, serum antioxidant status, intestinal bacterial counts and immune-related gene expression were analyzed. Results demonstrated that fish fed 10⁶ CFU/g achieved a final weight of 51.48 g, representing a 9.88% increase compared with the control (46.85 g), while weight gain (WG) improved by 12.82% and specific growth rate (SGR) by 6.34%. Feed conversion ratio (FCR) was reduced by up to 18.42% at 10⁵ CFU/g relative to the control. Encapsulated groups enhanced serum superoxide dismutase activity without altering malondialdehyde levels, increased total intestinal bacterial and lactic acid bacteria counts, and selectively upregulated <i>TLR2</i> and <i>MHC II</i> mRNA transcript levels. Histological analysis further revealed increased intestinal villus height and width in encapsulated-fed groups, while liver architecture remained normal across treatments. Multivariate analyses demonstrated strong positive associations among growth performance, gut microbiota enrichment, and immune gene expression. Resistance to <i>Aeromonas hydrophila</i> infection was significantly improved at higher dietary doses, with post-challenge survival reaching 61.67-75% in supplemented groups versus 45.00% in controls. These findings demonstrate that latex-derived <i>Lysinibacillus</i> sp. PWR01 acts as a strain-specific immunomodulatory probiotic that enhances growth, antioxidant capacity, microbial community balance, and disease resistance to Nile tilapia. Future studies should investigate the role of the Nrf2 antioxidant pathway, clarify TLR2-mediated immune signaling, and assess gut microbiota-immune system interactions within commercial-scale production systems.