Abstract
The objective of this study was to investigate the effects of incorporating Lactobacillus paracasei N1115 (LP N1115) into the diet on growth, hepatic antioxidant and immune biomarkers, and intestinal microbiota and histology of hybrid sturgeon. Fish with an initial body weight of 12.07 ± 0.16 g were fed four experimental diets containing 0%, 0.03%, 0.30%, and 3.0% of LP N1115 (termed as control, 0.03% LP, 0.30% LP, and 3.0% LP, respectively) twice a day (9:00 and 16:00) until apparent satiation for 56 days. The four isonitrogenous (38.72% crude protein) and isolipidic (9.89% crude lipid) diets contained viable bacteria concentrations of 0, 1.41 × 10(7), 1.33 × 10(8), and 1.02 × 10(9) cfu/g diet, respectively. Each diet was randomly distributed into three tanks with 20 fish per tank (capacity: 312 L). At the end of feeding trial, whole fish was collected for body composition, and liver at postprandial 6 h was sampled for antioxidant and immune biomarkers. Mid-duodenum was excised for morphological observation and intestinal digesta was gently squeezed for microbiota analysis. The remaining fish were exposed to 50 mg/L ammonia for 96 h post feeding trial; they were monitored every 12 h for mortality. The results indicated a significant increase in final body weight (FBW), weight gain rate (WGR), and specific growth rate (SGR) of 0.03% LP and 0.30% LP groups, along with a significant reduction in feed conversion ratio in the 0.03% LP group relative to control group (p < 0.05). Besides, dietary addition of 0.03% LP N1115 significantly improved hepatic activities of glutathione transferase, total superoxide dismutase (T-SOD), and glutathione content, as well as markedly decreased hepatic contents of H(2)O(2) and malonaldehyde. Furthermore, mRNA levels of nuclear factor-erythroid 2-related factor 2 (nrf2) and glutathione peroxidase (gpx) and NRF2 protein expression in the liver were significantly increased in the 0.03% LP group compared to control group (p < 0.05). Both villi height and muscularis thickness in the duodenum reached their maxima in the 0.03% LP group and declined at dietary probiotic levels beyond 0.03% (p < 0.05). Moreover, the composition of the intestinal microbiota was altered by the addition of 0.03% LP and characterized by an increase in the proportions of beneficial microbes (Lactobacillus, Candidatus Arthromitus, and Bifidobacterium) and a reduction in harmful bacteria (Achromobacter) at the genus level. Fish fed 0.03% LP and 0.30% LP diets had significantly higher survival rates at 96 h under ammonia stress compared to the control group (77.78% and 66.67% versus 55.56%) (p < 0.05). In conclusion, dietary LP N1115 supplementation at the dose of 0.03% (1.41 × 10(7) cfu/g diet) could improve growth performance, hepatic antioxidant ability, ammonia-resistant ability, and duodenal histology in juvenile sturgeon partly through altering gut microbiota.