Positive effects of dietary postbiotics on growth, intestinal health, immunity, and ammonia nitrogen tolerance in hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂)

Postbiotics, as an emerging feed additive in aquaculture, have received increasing attention. This study investigated the effects of dietary postbiotics products (Dyvon Lumensa) derived from Lactobacillus acidophilus and Saccharomyces cerevisiae via multi-phase fermentation, on growth performance, immunity, intestinal health, and ammonia nitrogen stress resistance in hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂; initial weight: 31.30 ± 0.64 g). Four experimental diets were formulated by incorporating postbiotics into basal diets at 0 (S0, control), 0.25 (S250), 0.75 (S750), and 2.00 mL/kg (S2000). Fish were fed twice daily (2-3 % body weight) for 8 weeks, followed by a 1-week ammonia nitrogen challenge (5 mg/L). Results demonstrated that postbiotic supplementation significantly enhanced weight gain rate and reduced feed conversion ratio, with optimal growth observed in the S750 group (P < 0.05). There was a significant increase in protease activity and a significant decrease in lipase activity in the S250 group (P<0.05), while the opposite was true for the S2000 group (P<0.05). The lipase activity in the S750 group was significantly higher than the other groups (P<0.05). Dietary postbiotics at 0.75 mL/kg increases muscularis thickness and villus surface area in the midgut (P<0.05). Besides, dietary postbiotics at 0.25–0.75 mL/kg suppress pathogenic genera (Vibrio, Pseudoalteromonas, and Aeromonas) while enriching beneficial taxa (Bacillus, Bacteroides, and Faecalibacterium). Besides, postbiotics at 0.25–0.75 mL/kg enhanced hepatic activities of superoxide dismutase, catalase, acid phosphatase, and alkaline phosphatase, while reducing malondialdehyde levels. Additionally, under ammonia stress, dietary postbiotics improved survival rates, with optimal survival in the S2000 group. Meanwhile, dietary postbiotics mitigated hepatic tissue damage, and down-regulated pro-inflammatory gene expression (il-1, il-6, nfκb), while up-regulated anti-inflammatory gene tgf-β expression (P<0.05). In conclusion, 0.75 mL/kg postbiotics optimally enhanced growth, immunity, and intestinal health, whereas given its superior anti-inflammatory and gut-protective properties, a dosage of 0.75-2.00 mL/kg proves most suitable in ammonia nitrogen-stressed environments.