Effects of increasing level of dietary organic acids and nature-identical compounds on growth, intestinal immune parameters and gut bacterial communities of European sea bass Dicentrarchus labrax juveniles

Despite the known beneficial effects of dietary organic acids (OA) and plant derivates in improving the performance and health of terrestrial livestock, limited comprehensive research has been conducted to evaluate their efficacy in aquatic animals and few data are available in European sea bass (Dicentrarchus labrax). A study was undertaken to assess the effects of increasing dietary levels of a microencapsulated mixture of citric acid, sorbic acid and nature-identical compounds (NIC – thymol and vanillin), on growth, intestinal immune response and gut bacterial communities of sea bass juveniles.
Four isonitrogenous and isolipidic extruded commercial diets containing the mixture of OA and NIC (MIX) at 0.0, 0.25, 0.50 and 1.0 g kg-‘ were fed to triplicate fish groups of 60 individuals (initial weight 13 g) to apparent satiation over 71 days. At the end of the trial fish were also exposed to suboptimal environmental conditions for 7 days.
No significant differences (P > 0.05) among diets for final body weight, specific growth rate, feed intake and feed conversion rate were detected as well as for viscerosomatic and hepatosomatic indexes. No significant differences among treatments were detected in the general pattern of cytokines expression from the distal intestine (IL-1b, IL-8, IL-6, IL-10, TGFb and TNFa) before and after the environmental changes, even if a slight increment of some transcripts at increasing dietary level of the MIX was observed. Gut bacterial community was performed by next generation sequencing. The sea bass gut bacterial ecosystem was dominated by Lactobacillaceae (24%), Leuconostocaceae (20%), Streptococcaceae (16%) Bacillaceae (8%), Streptophyta (8%) and
Corynebacteriacee (7%). Even if no significant differences among these groups were detected, PCoA based on the Euclidean distances between samples showed at the end of the trial a significant (P < 0.05) separation among the treatments. Gut bacterial community of fish fed with the MIX after the environmental changes showed a tendency in reducing biodiversity indices increase when comparing to control diet.
In conclusion, although growth and intestinal immune-system parameters were not significantly affected at this stage, dietary inclusion of the MIX slightly modulated the gut bacterial community indicating a possible ability to contain the bloom of opportunistic bacterial species that can proliferate under conditions of environmental stress.

Acknowledgements: This research was funded by Vetagro S.p.A. and POR-FESR 2014-2020 Emilia Romagna grant.

For further information: technicalservice@vetagro.com