Bacteriocin-producing Lactococcus lactis subsp. lactis A7 enhances health status, immunity and gut microbiota in Nile tilapia (Oreochromis niloticus)

This study investigated the properties of bacteriocin produced by Lactococcus lactis subsp. lactis A7, and evaluated its probiotic potential in promoting the health, immunity, and disease resistance of Nile tilapia. The bacteriocin exhibited inhibitory activity against Streptococcus agalactiae , with an average inhibition zone of 8.80 ± 0.20 mm. An eight-week feeding trial was conducted with four dietary treatments: a control (no supplementation) and three probiotic groups receiving L. lactis subsp. lactis A7 at 2.18 × 10 8 CFU/kg diet (LLactis-8), 2.18 × 10 10 CFU/kg diet (LLactis-10), and 2.18 × 10 12 CFU/kg diet (LLactis-12). Growth performance did not differ significantly among groups ( P > 0.05). However, LLactis-10 significantly enhanced immune parameters, including reduced serum ALT levels and increased total protein, lysozyme activity, and lymphocyte counts ( P < 0.05). Digestive enzyme activities were significantly enhanced by probiotic supplementation, with protease and lipase activities peaking in the LLactis-12 group, while amylase activity was significantly increased in both LLactis-10 and LLactis-12 compared to the control ( P < 0.05). Gene expression analysis showed a significant downregulation of IL-1β and upregulation of IFN-γ in LLactis-10 ( P < 0.05), while other immune- and growth-related genes showed no consistent differences, except for reduced IGF-1 expression in LLactis-8. Additionally, microbiota profiling revealed greater microbial diversity and a higher abundance of beneficial Lactobacillales, along with a significant reduction in Aeromonadales ( P < 0.05), particularly in LLactis-10. Upon challenge with S. agalactiae , the LLactis-10 group demonstrated a significantly higher survival rate compared to the control ( P < 0.05). These findings suggest that dietary supplementation with L. lactis subsp. lactis A7 at 2.18 × 10 10 CFU/kg diet can enhance immune function, modulate gut microbiota, and improve disease resistance in Nile tilapia, supporting its potential application as a probiotic in aquaculture.