Metagenome-based identification of functional traits of the black soldier fly gut microbiome associated with larval performance.

Background: The relationship between microbiomes and their hosts has been the subject of intensive study in recent years. For black soldier fly larvae (BSFL) (Hermetia illucens L., Diptera: Stratiomyidae), correlations between shifts in its microbial gut community composition and its health and performance suggest that the BSFL gut microbiome encodes important functions that complement the insect's own immune system and metabolism. To date, most BSFL microbiome studies have been based on 16S rRNA sequencing data. Because this approach derives a lot of information from very short sequencing reads, it was hypothesized that more insight into bacterial functionality could be generated using more extensive sequencing technologies. Here, whole genome shotgun (WGS) metagenomic sequencing was employed to investigate which microbiome-associated taxa and functions were associated with increased performance of larvae reared on a chicken feed (CF) or artificial supermarket food waste (SFW) based diet.

Results: Taxonomic and functional profiling of the BSFL gut microbiome revealed a significant shift in response to diet, where bacterial genes encoding specific metabolic functions, such as the metabolism of sorbitol, were significantly enriched in the microbiome of larvae reared on SFW-diet. This indicates that the nutritional composition of the substrate alters the gut bacterial composition by providing competitive benefits or new niches for specific bacteria that can utilise these compounds. Moreover, specific microbial functions, such as cobalamin synthesis, appear to be correlated with larval performance. Aside from metabolic functions, biosynthetic gene cluster analysis revealed potential antimicrobial competition and protective functions among bacterial species. Improved taxonomic resolution provided by WGS led to the identification of several metagenome assembled genomes (MAGs), including a potentially novel BSFL-associated Scrofimicrobium species. Furthermore, there were differences in larval performance between rearing diets, and larval growth was correlated with high abundance of several MAGs.

Conclusions: Variation in the nutritional and bacterial load of a diet can result in functional shifts in the gut microbiome of the larvae. Analysis of the BSFL metagenome identified several bacteria that are positively correlated with larval performance, which could potentially provide beneficial metabolic functions for the host that should be further explored.