Modulation of gut microbiota, blood metabolites, and disease resistance by dietary ß-glucan in rainbow trout (Oncorhynchus mykiss).

BACKGROUND: Prebiotics are known to have a positive impact on fish health and growth rate, and ß-glucans are among the most used prebiotics on the market. In this study, rainbow trout (Oncorhynchus mykiss) were treated with a ß-1,3;1,6-glucan dietary supplement (at a dose of 0 g, 1 g, 10 g, and 50 g ß-glucan per kg of feed). After 6 weeks, the effect of the ß-glucan was evaluated by determining the changes in the microbiota and the blood serum metabolites in the fish. The impact of ß-glucan on the immune system was evaluated through a challenge experiment with the bacterial fish pathogen Yersinia ruckeri. RESULTS: The microbiota showed a significant change in terms of composition following ß-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 ß-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 ß-glucans to modify the gut microbiota of fish, resulting in alteration of the metabolome and affecting fish health through the lipidome of rainbow trout.

Impact of water quality parameters on geosmin levels and geosmin producers in European recirculating aquaculture systems.

AIMS: Geosmin is associated with off-flavour problems in recirculating aquaculture systems (RAS) and represents an economic problem for the aquaculture industry. This study aims at investigating factors influencing the composition of the bacterial microbiota, in particular the presence of geosmin producers and the environmental and farming factors favouring geosmin accumulation. METHODS AND RESULTS: Several water quality parameters were correlated to the composition of the microbiota with special emphasis on the presence of geosmin producers within 26 different RAS from four European countries. Three novel groups of geosmin-producing bacteria were quantified to identify potential correlations with geosmin concentration. CONCLUSIONS: The microbiome differed significantly between systems. However, phosphate levels, calcium levels and redox potential correlated to geosmin concentration in the water and the presence of the Actinomycetales geosmin-producers but not with the presence of other groups of geosmin-producing bacteria. Oxygen levels and conductivity were found to negatively correlate with geosmin concentration. A large proportion of the detected geosmin producers represented novel taxonomic groups not previously linked with this activity. SIGNIFICANCE AND IMPACT OF THE STUDY: These results improve our understanding of the diversity of microbiota in RAS and the water quality parameters favouring the populations of geosmin-producing bacteria and the production of geosmin.

Identification of the Scopularide Biosynthetic Gene Cluster in Scopulariopsis brevicaulis.

Scopularide A is a promising potent anticancer lipopeptide isolated from a marine derived Scopulariopsis brevicaulis strain. The compound consists of a reduced carbon chain (3-hydroxy-methyldecanoyl) attached to five amino acids (glycine, l-valine, d-leucine, l-alanine, and l-phenylalanine). Using the newly sequenced S. brevicaulis genome we were able to identify the putative biosynthetic gene cluster using genetic information from the structurally related emericellamide A from Aspergillus nidulans and W493-B from Fusarium pseudograminearum. The scopularide A gene cluster includes a nonribosomal peptide synthetase (NRPS1), a polyketide synthase (PKS2), a CoA ligase, an acyltransferase, and a transcription factor. Homologous recombination was low in S. brevicaulis so the local transcription factor was integrated randomly under a constitutive promoter, which led to a three to four-fold increase in scopularide A production. This indirectly verifies the identity of the proposed biosynthetic gene cluster.

Identification of the biosynthetic gene clusters for the lipopeptides fusaristatin A and W493 B in Fusarium graminearum and F. pseudograminearum.

The closely related species Fusarium graminearum and Fusarium pseudograminearum differ in that each contains a gene cluster with a polyketide synthase (PKS) and a nonribosomal peptide synthetase (NRPS) that is not present in the other species. To identify their products, we deleted PKS6 and NRPS7 in F. graminearum and NRPS32 in F. pseudograminearum. By comparing the secondary metabolite profiles of the strains we identified the resulting product in F. graminearum as fusaristatin A, and as W493 A and B in F. pseudograminearum. These lipopeptides have previously been isolated from unidentified Fusarium species. On the basis of genes in the putative gene clusters we propose a model for biosynthesis where the polyketide product is shuttled to the NPRS via a CoA ligase and a thioesterase in F. pseudograminearum. In F. graminearum the polyketide is proposed to be directly assimilated by the NRPS.