Gut microbiome following probiotic supplementation in barramundi
Health promoting effects of Bacillus subtilis probiotics were investigated in juvenile barramundi Lates calcarifer following two different administration routes. The probiotics were given with feed 2 times per day or dispensed into the water column 1 time per day for 60 days. Fish were examined for growth rate throughout the experiment, and were evaluated for cortisol levels following physical stress, expression of immune-related and growth performance genes, gut microbial communities, and challenge test against Streptococcus iniae at the end of the trials. The result found a significant increase in average weight gain, and relative growth rate in both oral and immersion groups compared to the control group (p < 0.05). No significant difference in serum cortisol levels following physical stress stimulation among treatments. Significant upregulation of immune-related genes (MHC I, CCL4, IL-1B) and a growth factor gene (Occ) were observed in the intestine of oral and immersion groups compared to the control group (p < 0.05). In terms of the gut microbiome, the Shannon index showed no significant difference among the group of treatment; on the contrary, Bray-Curtis distance significantly differed in the oral group compared to the other two groups (p < 0.05). Fish belonging to oral and immersion groups showed an increased abundance of Ruegeria spp., Lactobacillus spp., Rubritalea spp., and Nautella spp. Relative percentage survival (RPS) at 7 days following intraperitoneal injection with S. iniae of oral and immersion groups was 12%. In summary, B. subtilis supplementation to barramundi via oral and immersion routes enhanced the growth of the fish, upregulated the intestinal gene expression, and altered the gut microbial community increasing the richness and evenness of microorganisms. However, the inconsistent effects of immersed probiotics in this study may derive from indirect ingestion through the water column. Therefore, oral supplementation should be applied to sustain the maximum benefit of Bacillus probiotics in barramundi.