The Effects of Dietary Insect Meal from Hermetia illucens Prepupae on Autochthonous Gut Microbiota of Rainbow Trout (Oncorhynchus mykiss)

Simple Summary The composition of fish-gut microbial communities has been demonstrated to adapt when the host is fed different ingredients. Fishmeal and soy are the conventional protein sources used in aquafeeds. However, these feed options are not sustainable anymore due to the progressive depletion of wild marine fish stocks and the considerable environmental cost of protein-rich terrestrial plant cultivation. In this perspective, insects could be a promising source of protein and may help aquaculture to cope with the increasing global demand for new protein sources, representing the idea of waste into feed bioconversion. In this perspective, we evaluated the effects of dietary insect meal from Hermetia illucens (Hi) larvae on autochthonous gut microbiota of rainbow trout (Oncorhynchus mykiss). Hi larvae were grown on leftover fruit and vegetables provided by a local wholesale market. Three diets, with increasing levels of insect meal inclusion (10%, 20%, and 30%) and a control diet without insect meal were tested in a 3-month fish feeding trial. The data showed that feeding insects influences the intestinal bacterial communities, thus improving fish gut health. In our opinion, these findings represent a precious tool for future research on salmonid's microbial communities and their interaction with diet and the host. Abstract This study evaluated the effects of dietary insect meal from Hermetia illucens larvae on autochthonous gut microbiota of rainbow trout (Oncorhynchus mykiss). Three diets, with increasing levels of insect meal inclusion (10%, 20%, and 30%) and a control diet without insect meal were tested in a 12-week feeding trial. To analyze the resident intestinal microbial communities, the Illumina MiSeq platform for sequencing of 16S rRNA gene and QIIME pipeline were used. The number of reads taxonomically classified according to the Greengenes database was 1,514,155. Seventy-four Operational Taxonomic Units (OTUs) at 97% identity were identified. The core of adhered intestinal microbiota, i.e., OTUs present in at least 80% of mucosal samples and shared regardless of the diet, was constituted by three OTUs assigned to Propiobacterinae, Shewanella, and Mycoplasma genera, respectively. Fish fed the insect-based diets showed higher bacterial diversity with a reduction in Proteobacteria in comparison to fish fed the fishmeal diet. Insect-meal inclusion in the diet increased the gut abundance of Mycoplasma, which was attributed the ability to produce lactic and acetic acid as final products of its fermentation. We believe that the observed variations on the autochthonous intestinal microbiota composition of trout are principally due to the prebiotic properties of fermentable chitin.