Effects of dietary Bacillus subtilis DSM 32315 supplementation on the growth, immunity and intestinal morphology, microbiota and inflammatory response of juvenile largemouth bass Micropterus salmoides

Abstract

This study aimed at evaluating the effects of dietary inclusion of Bacillus subtilis DSM 32315 on the growth, immunity, intestinal architecture and gut microbiota of juvenile largemouth bass Micropterus salmoides (LMB). B. subtilis DSM 32315 was supplemented in the diets at the levels of 0 (control, CON), 1.5 (BS1.5, 3 × 106 CFU/g feed) and 3 g/kg (BS3, 6 × 106 CFU/g feed). Each diet was assigned to four replicate tanks of LMB juveniles (initial mean body weight, 8.32 ± 0.06 g) with 20 fish per tank for 9 weeks. The results showed that the growth and feed utilization of LMB were not sensitive to dietary modifications (p > .05). Compared with the CON fish, dietary BS supplementation did not affect the haematological parameters (white and red blood cell counts and haematocrit value) of LMB (p > .05), but the BS1.5 fish performed better than the BS3 fish. Serum innate immunity indicators (lysozyme activity and complement C3 and C4 levels) were improved with dietary BS supplementation at both doses, while higher serum IgM level was only recorded at 1.5 g/kg feed (p < .05). All the sampled fish exhibited intact intestines, but the intestinal architecture (villus height and muscular layer thickness) still benefited from dietary BS inclusion at both doses (p < .05). Although intestinal anti-inflammatory response (mRNA levels of IL-10 and TGF-β1) did not differ between the BS supplemented fish (p > .05), the BS1.5 fish obtained superior pro-inflammatory response (mRNA levels of IL-1β, IL-8, and TNF-α) over the BS3 fish (p < .05). Compared with the CON fish, the abundance of the harmful Plesiomonas shigelloides decreased in the gut microbiota of the BS1.5 fish (p < .05). Despite the BS3 fish obtained higher diversity of gut microbiota, the abundances of potential beneficial (Cetobacterium somerae and Shewanella xiamenensis) and harmful bacteria (Acinetobacter johnsonii and P. shigelloides) increased or decreased simultaneously (p < .05). Taken together, it was concluded that dietary BS DSM 32315 inclusion presented some positive effects on LMB. Although the BS1.5 fish performed better than the BS3 fish, the best supplemental dose need to be further studied.