Effects of dietary supplementation of Bacillus, β-glucooligosaccharide and their synbiotic on the growth, digestion, immunity, and gut microbiota profile of abalone, Haliotis discus hannai

Probiotics, prebiotics, and synbiotics are known to improve aquatic animals' growth and health conditions. In this study, we investigated the effect of Bacillus sp. KRF-7 (KRF-7), Bacillus sp. PM8313 (PM8313), beta-glucooligosaccharide (BGO), and their synbiotics on the growth, digestive physiology, immunity, and gut microbiota of abalone, Haliotis discus hannai. The control diet (CON) was sprayed with 10(8) CFU/g KRF-7 (PRO1) or PM8313 (PRO2) as probiotics, 0.10% BGO (PRE) as prebiotic, and their combinations (SYN1: PRO1 + BGO; SYN2: PRO2+ BGO) as synbiotic treatments. These diets (triplicates) were given once daily to the abalone (initial body weight: 1.32 +/- 0.05 g, initial shell length: 21. 69 +/- 0.19 mm) for 12 weeks. The results showed that the PRE diet induced a significant 15% shell length and body weight increase against the CON (SLGR: 45.77% vs. 53.04%; WGR: 182.77% vs. 210.22%), the highest among dietary supplements (P < 0.05). The level of IGF and IGFBP in the cell-free hemolymph, protein expression of IGF signaling, and gene expression of myosin heavy chain in the adductor muscle increased significantly (P < 0.05) following dietary supplementation. Correspondingly, PRE-, PRO1-, and PRO2-fed abalone had the highest trypsin, lipase, and amylase secretion in the hepatopancreas (P < 0.05) and increased histomorphometry (stomach epithelial thickness and intestinal villi height) (P < 0.05). Regarding antioxidative capacity and immunity, the hepatopancreatic expression of SOD, CAT, IRAK-4, NF-Kb, TNF-alpha, and beta-defensin were significantly upregulated in abalone fed with dietary additives compared to the CON (P < 0.05). In terms of gut microbiota profile, PRE-fed abalone had a significant reduction in microbial richness (P < 0.05). The phylum level was similarly occupied, while a diet-specific core microbiota at the genus level was apparent. The findings illustrated that BGO, KRF-7, and PM8313 can be used in abalone aquaculture as growth-promoting and immune-stimulating supplements.