Next-generation sequencing characterization of the gut bacterial community of gilthead sea bream (Sparus aurata, L.) fed low fishmeal based diets with increasing soybean meal levels

- Increasing soybean meal levels in low fishmeal-based diets was tested in sea bream.
- A deep analysis of the gut bacterial community during the on-growing was successfully obtained.
- A minimal impact of soybean meal levels on the overall gut bacterial community was observed.
- Soybean meal seems to promote gut bacterial community components with beneficial functions.
- Soybean meal did not negatively affect overall growth performance and gut histology.

The present study was carried out to evaluate growth, gut histology and gut bacterial community of gilthead sea bream (Sparus aurata) fed with increasing dietary soybean meal (SBM) levels in a low fishmeal (FM) based diet, in comparison with a control diet. Five isoproteic and isolipidic experimental diets were formulated to contain increasing levels of SBM (0, 100, 200, and 300 g kg−1 named S0, S10, S20 and S30, respectively) with 150 g kg−1 of FM, and one control diet (C) without SBM and containing 350 g kg−1 of FM. Sixty sea bream (initial body weight 75.9 ± 1.9 g, n = 900) per tank were reared in a recirculation system at 23.0 ± 1.0 °C and fed to satiation. The trial was run in triplicate and lasted 100 days. At the end of the trial fish fed the S30 diet showed a higher (P ≤ 0.05) specific growth rate (SGR) compared to S0 (SGR, 1.17 ± 0.03, 1.20 ± 0.01, 1.22 ± 0.01, 1.25 ± 0.01 and 1.21 ± 0.04 for S0, S10, S20, S30 and C, respectively), and a higher feed intake (FI) compared to S0, S10 and S20. Sea bream fed the C diet had a higher (P ≤ 0.05) FI compared to S0 (FI, 1.40 ± 0.01, 1.45 ± 0.01, 1.44 ± 0.03, 1.51 ± 0.03 and 1.46 ± 0.02 for S0, S10, S20, S30 and C, respectively). No significant differences in feed conversion rate, protein efficiency ratio, gross protein efficiency and gross lipid efficiency among the treatments were detected. No specific histopathological changes indicative of soy-induced enteritis were observed in the intestine of any fish examined. Gut bacterial community of the distal intestine content was analyzed by Next-Generation Sequencing. At the phylum level, the gut bacterial community was dominated by Firmicutes (relative abundance 71%), while the most represented family was Lactobacillaceae (26%). Even if no significant differences (P ≤ 0.05) in the gut bacterial community α and β-diversity according to the different diets were detected, Cyanobacteria and Lactobacillaceae progressively increased from diet C to diet S30. In conclusion results of growth, nutrient utilization, gut histology and gut bacterial community indicate that SBM can be successfully incorporated up to a level of 300 g kg−1 with the inclusion of 150 g kg−1 of FM, without any deleterious effects on growth, protein utilization and gut health during the on-growing of sea bream.