Effect of diets containing full-fat Hermetia illucens on rainbow trout microbiota: A dual cultivation-independent approach with DGGE and NGS

Given the drift to improve economic and ecological sustainability of the aquaculture sector, novel ingredients fulfilling these requirements are sought. Hermetia illucens, commonly called black soldier fly, (Diptera: Stratiomydae; H) is a promising dietary protein source but its effect on fish gut microbiota is still to be clarified. The aim of the present study was to increase the knowledge of the effect of dietary full-fat H meal on rainbow trout (Oncorhynchus mykiss) microbiota and, in particular, on intestinal mucosa-adherent microbiota by applying a dual approach based on polymerase chain reaction-denaturing gradient gel electrophoresis and high-throughput sequencing of the 16S rRNA gene. Rainbow trout (initial body weight of 137.3 ± 10.5 g) was fed for 98 days with a control diet (H0) containing fishmeal and protein-rich vegetable ingredients and an experimental diet (H50) where 50% of the fishmeal had been replaced by full-fat H meal rich in saturated fatty acids. Proteobacteria, Firmicutes and Actinobacteria were generally present in all samples, although the core microbiota (relative prevalence higher than or equal to 80% in all samples) only consisted of the proteobacteria Caulobacter, Delftia, Agrobacterium and Ochrobactrum. In addition, Streptococcus infantis and a member of the Cytophagaceae family were part of the core taxa of mucosa samples. Tenericutes were abundant in pyloric caeca samples and, among them, Mycoplasmataceae seemed to increase in the group fed the high saturated fatty acid diet containing H meal; a consideration about the connection between this bacterial group and the dietary lipid content must be considered. Dietary treatment did not clearly affect alpha-diversity metrics, but mucosa samples tended to be more resilient to dietary changes than content samples. Permutational analysis of variance showed significantly different β-diversities between diets (p < 0.05) but principal coordinates analysis did not confirm this result. Diets for rainbow trout containing full-fat H meal determined interesting modifications in the gut microbiota with patterns similar to the ones found in the literature. The dietary lipids can exert an effect on microbiota. Nonetheless, research data on this topic are still scarce and further studies are highly encouraged.