Effects of Full Replacement of Dietary Fishmeal with Insect Meal from Tenebrio Molitor on Rainbow Trout Gut and Skin Microbiota.

Abstract

Background: Aquaculture must continue to reduce dependence on fishmeal (FM) and oil in feeds to ensure sustainable sector growth. Therefore, the use of novel aquaculture feed ingredients is growing. In this regard, insects can represent a new world of sustainable and protein-rich ingredients for farmed fish feeds. Accordingly, we investigated the effects of full replacement of FM with Tenebrio molitor (TM) larvae meal in the diet of rainbow trout (Oncorhynchus mykiss) on fish growth performance and gut and skin microbiota. Methods: A feeding trial was conducted with trout of about 80 g mean initial weight that were fed for 22 weeks with two isonitrogenous, isolipidic, and isoenergetic extruded experimental diets. Partially defatted TM meal was included in the diets to replace 0% (TM 0) and 100% (TM 100) of FM. To analyse the microbial communities, the Illumina MiSeq platform for sequencing of 16S rRNA gene and Qiime pipeline were used to identify bacteria in the gut and skin mucosa, and in the diets. Results: The data showed no major effects of full FM substitution with TM meal on bacterial species richness and diversity in both, gut mucosa- and skin mucus-associated microbiome. Skin microbiome was dominated by phylum Proteobacteria and especially by Gammaproteobacteria class that constituted approximately half of the bacterial taxa found. The two dietary fish groups did not display distinctive features, except for a decrease in the relative abundance of Deefgea genus (family Neisseriaceae) in trout fed with insect meal. The metagenomic analysis of the gut mucosa indicated that Tenericutes was the most abundant phylum, regardless of the diet. Specifically, within this phylum, the Mollicutes, mainly represented by Mycoplasmataceae family, were the dominant class. However, we observed only a weak dietary modulation of intestinal bacterial communities. The only changes due to full FM replacement with TM meal were a decreased number of Proteobacteria and a reduced number of taxa assigned to Ruminococcaceae and Neisseriaceae families. Conclusions: the data demonstrated that TM larvae meal is a valid alternative animal protein to replace FM in the aquafeeds. Only slight gut and skin microbiota changes occurred in rainbow trout after total FM replacement with insect meal. The mapping of the trout skin microbiota represents a novel contribution of the present study. Indeed, in contrast to the increasing knowledge on gut microbiota, the skin microbiota of major farmed fish species remains largely unmapped but it deserves thorough consideration.

Skin bacteria of rainbow trout antagonistic to the fish pathogen Flavobacterium psychrophilum

“…Sequences related to strains OX14 and GL7 were comprising of 1.60 and 0.17% of the skin microbial community. Furthermore, sequences related to strain OX14 was the dominant in the gill microbial community (61.4%) and also detected in rainbow trout used in previous studies 29 , 31 although their population was lower compared to the present study. These strains may play a role in preventing F. psychrophilum at the surface of fish, the first line of defense, and could also be applicable in future biological treatment for fish diseases.…”

“…The fraction of isolated strains in microbial communities of skin, gill, and the gut of rainbow trout, and surrounding water was estimated by high throughput sequencing analysis. We also compared our findings with published microbiome data of fish skin 29 , 31 , 32 to see whether similar sequences to the cultured isolates were found previously.…”

“…We compared our findings with two previous reports on the rainbow trout microbiome 29 , 31 . Proteobacteria was the dominant in skin microbiome of all of the studies.…”

“…While OTUs similar to 9 out of 20 isolates were detected at > 0.01% in skin microbial communities of rainbow trout in Terova et al 32 (0.54–0.88% in total), the proportions of sequences related to strain OX14 was low (0.004%), and those related to GL7 was not detected (Supplementary Table S5 ). We detected OTU having > 99% similarity with all of our isolates except MN5 and MN10 in the skin microbial community of Atlantic salmon 31 (0.39–10.79% in total, Supplementary Table S6 ). The ratios of OTUs similar to strains GL7 and OX14 in the skin microbial communities were < 0.09% and < 0.04%, respectively.…”

“…1 ), and this was one of the most frequently detected groups in the skin microbial community (0.63%, Table 2 ). Although this group was not detected in rainbow trout in Lowrey et al 29 , it was detected in skin of rainbow trout in Terova et al 31 , and was the most frequently found group in Atlantic salmon skin (< 7.06%, Supplementary Table S6 online), suggesting that this group may be suitable to reside on skin of these fish. All of our strains grew on FLP medium, whereas several did not grow on OXOID CM3 and LB media (Supplementary Table S2 online).…”