Prebiotics and probiotics have been long investigated with aquatic species to improve health and diminish the usage of antibiotics. In the present study, the potential synergism between the beneficial bacteria contained in probiotics with the fermentable complex carbohydrates (or prebiotics) and their metabolites were explored in red drum using the commercial products BactocellTM and Grobiotic®-A. Four experimental diets were formulated and supplemented with either 1 g kg-1 of Bactocell , 20 g kg-1 of Grobiotic® -A, or their combination. A non-supplemented basal diet served as the negative control. Groups of 12 juvenile red drum with an average weight of 5.5 g/fish were distributed in 16, 38-L aquaria . Each of the four experimental diets were assigned to four replicate tanks for 8 weeks with daily rations divided into two feedings and adjusted weekly according to total biomass. At the end of the feeding trial, production performance, whole-body proximate composition, plasma immunological responses, and intestinal microbial community were evaluated. Data were analyzed as a mixed model, having a 2 × 2 factorial design (absence or presence of probiotic or prebiotic, as the main factors) and the disposition of the aquaria was used as a statistical block.
Red drum fed diets supplemented with probiotics had better growth performance than those fed the non-supplemented diets, and a higher protein content in their whole-body composition. No differences were observed for feed efficiency, survival, whole-body lipid and ash, or protein conversion efficiency. Fish fed diets supplemented with the prebiotic had a lower concentration of circulating protein in plasma. The i ntestinal microbiome was assessed with the denaturing gradient gel electrophoresis (DGGE) method and next-generation sequencing (NGS) targeting the V4 region of the 16S rRNA gene . The bacterial community of fish fed diets containing the probiotic, and the combination of probiotic and prebiotic, were 91.5% simil ar to the other two experimental diets, according to the disposition of the DNA amplicons in the DGGE gel . NGS data indicated alpha and beta diversity were significantly affected by dietary treatments. A higher relative abundance of the lactic acid genus Pediococcus sp. was observed for fish fed diets supplemented with the prebiotic. Differences were also detected in the predicted functions of the microbiota, where the relative abundance of 52 pathways were significantly higher for fish treated with the prebiotic and 18 pathways for fish fed the probiotic. Many of these pathways involved the biosynthesis of essential amino acids like lysine and methionine and the biosynthesis of nucleotides. Even though no potential synergistic effect was observed for the supplementation of both commercial products , the individual inclusion of the prebiotic and probiotic positively affected growth performance and intestinal health of red drum.