Background: Prebiotics are known to have a positive impact on fish health and growth rate, and beta-glucans are among the most used prebiotics on the market. In this study, rainbow trout (Oncorhynchus mykiss) were treated with a beta-1,3;1,6-glucan dietary supplement (at a dose of 0 g, 1 g, 10 g, and 50 g beta-glucan per kg of feed). After 6 weeks, the effect of the beta-glucan was evaluated by determining the changes in the microbiota and the blood serum metabolites in the fish. The impact of beta-glucan on the immune system was evaluated through a challenge experiment with the bacterial fish pathogen Yersinia rucked.

Results: The microbiota showed a significant change in terms of composition following beta-glucan treatment, notably an increase in the relative abundance of members of the genus Aurantimicrobium, associated with a decreased abundance of the genera Carnobacterium and Deefgea. Furthermore, analysis of more than 200 metabolites revealed that the relative levels of 53 metabolites, in particular compounds related to phosphatidylcholines, were up- or downregulated in response to the dietary supplementation, this included the amino acid alanine that was significantly upregulated in the fish that had received the highest dose of beta-glucan. Meanwhile, no strong effect could be detected on the resistance of the fish to the bacterial infection.

Conclusions: The present study illustrates the ability of beta-glucans to modify the gut microbiota of fish, resulting in alteration of the metabolome and affecting fish health through the lipidome of rainbow trout.