Relationship between immune performance and the dominant intestinal microflora of turbot fed with different Bacillus species

Intestinal microorganisms have been shown to influence the health status of fish. Probiotics such as Bacillus can promote the healthy growth of marine organisms by balancing the structure of intestinal microflora. To study the effect of Bacillus as an effective probiotic on turbot in aquaculture, the growth performance, immune parameters, and intestinal microbiota of turbot were investigated in this study. The correlation between the dominant bacteria at the genus level and immune/antioxidant parameters was also analyzed. The experimental fish were categorized into the treated groups B1, B2, and B3, which were fed diets supplemented with Bacillus subtilis SMF1, Bacillus licheniformis LMF1, and Bacillus siamensis DL3, respectively; the control group (T0) was given a diet without any probiotics. The results showed that the growth performance as well as the immune and antioxidant enzyme activities of turbot were significantly (P < 0.05) improved when the fish were fed with different Bacillus species, and the order of growth performance and feed effect of turbot in each group was: B2 > B1 > B3 > T0. The specific growth rate of group B2 was 1.4 times as that of the group T0. Furthermore, the relative abundance of Bacillus in group B2 was the highest (24.18%), B. licheniformis LMF1 was more conducive to Bacillus colonization in the intestine of turbot than B. subtilis SMF1 and B. siamensis DL3. The immune and antioxidant activities of turbots in groups B2 and B3 were higher than those in group B1, owing to fewer probiotics being induced by B. subtilis SMF1. The RT-PCR analysis of immune-related genes also verified the effects of different Bacillus species. Spearman's correlation analysis at the genus level revealed that most of the immune and antioxidant parameters were significantly positively correlated with intestinal microorganisms such as Bifidobacterium, Lactobacillus, Ralstoni, and Bacillus, while being negatively correlated with some pathogenic intestinal bacteria such as Bacteroides, Parabacteroides, and Paeniclostridium. These results could guide the rational use of different Bacillus species in aquaculture.