Can the genetic background modulate the effects of feed additives? Answers from gut microbiome and transcriptome interactions in farmed gilthead sea bream (Sparus aurata) fed with a mix of phytogenics, organic acids or probiotics

The synergies between selective breeding and feed additives remain under-explored in farmed fish, despite their sustainability. Reference (REF) and selected gilthead sea bream for growth (GS) were fed with the control (CTRL) diet during 14 days. CTRL diet was oil-coated with three functional additives (PHY: phytogenic based on garlic and medium chain fatty acid; OA: organic acid mixture with a 70% of butyric acid sodium salt; PROB: probiotic based on Bacillus subtilis, pumillus and licheniformes species). These experimental diets were then sequentially administered at high (PHY/OA = 7.5 g/kg, PROB = 2 × 1011 CFU/kg; 2 weeks) and low (PHY = 5 g/kg, OA = 3 g/kg, PROB = 4 × 1010 CFU/kg; 10 weeks) additive doses. The capacity of a given genotype and additive to modify the fish growth performance, gut health and the host interaction with its anterior intestine (AI) microbiota was evaluated as a whole population or individually (9 fish/diet/genetics). GS fish showed a better growth and feed conversion ratio, linked to a reduced individual variability of gut microbial composition. The PHY additive had a major impact upon the intestinal transcriptome of GS-PHY fish, with the up-regulation of markers of epithelial integrity, sphingolipid and cholesterol/bile salt metabolism. With the OA additive, impaired growth performance, reduced AI goblet cell area and enhanced AI granulocyte infiltration were concomitant with a down-regulation of neutrophil degranulation markers associated with a decrease of pathogenic genera (Staphylococcus/Streptococcus/Neisseria), and an over-representation of acetone/butanol/ethanol fermentation and vitamin K biosynthesis inferred pathways. Bacillus establishment and lack of AI inflammation were parallel in PROB fish of both genetic backgrounds. However, GS fish grew and utilized feed better with the additive, whereas a worsening appeared in REF fish. This amelioration was related with a higher abundance of the nitrate-reducer Kocuria, an up-regulation of markers of epithelial cell maintenance and proliferation, and a down-regulation of microbiota-correlated protein synthesis and ubiquitination markers, supporting a reduced epithelial turnover and improved intestinal barrier function. Overall, the success of nutritional innovations in gilthead sea bream is largely dependent on the host genome predisposition, but also on the intestinal microbiota according to the hologenome theory.