Gut microbiota changes in insect-fed monogastric species: state-of-the-art and future perspectives

The intestinal microbiome can be defined as “the genes and genomes of the gut microbiota, as well as their products and the host environment” (Berg et al., 2020). Therefore, three major microbiome components can easily be unearthed, as the term “microbiome” does not only comprise the “assemblage of living microorganisms present in the intestinal environment” (the so-called “microbiota”), but also its “theatre of activity”, which is the “collection of their genomes and genes” (the so-called “metagenome”) and the “whole spectrum of molecules produced by them, including their structural elements, metabolites, and molecules produced by coexisting hosts and structured by the surrounding gut conditions” (the so-called “metabolome”) (Berg et al., 2020). One of the main factors affecting the intestinal microbiome in either humans or animals is the diet, as feed nature and characteristics exert a significant influence on nutrient specificity and availability for microbiome members, thus, in turn, selecting/excluding taxa that are adept at/deficient in processing the available biomolecules. In particular, the relationship between diet and gut microbiome seems to have a key role in the animal production systems, as they represent two of the main components involved in the establishment and maintenance of a proper health status of the intestine, which is of vital importance to animal health and growth performance (Biasato et al., 2018). Nowadays, when choosing a diet for monogastric species, insects (especially Hermetia illucens [HI] and Tenebrio molitor [TM]) cannot be taken out of the picture, not only for their excellent nutritional profile, but also for their interesting nutraceutical components (i.e., chitin, antimicrobial peptides [AMPs], and lauric acid), which have recently been suggested to exert a primary influence on animal gut microbiota (Biasato et al., 2022). So far, most of the research dealing with the binomial “insects-animal gut” focused on the intestinal microbiota (on the bacterial composition only), with very few studies had started exploring the metagenome potential and metabolome as well. Microbiome study (applying various -omics technologies, such as metataxonomics, metagenomics, metaproteomics, metabolomics, and metatranscriptomics) can be used for an in-depth characterization of the complexity of the microbial ecosystems to highlight any shift related to dietary modifications. Indeed, the gut microbiome has an enormous functional potential for the host, as changes in dietary nutrients play a fundamental role in shaping the structure of the gut microbiome and, in turn, determining the inter-relationship between the latter and host. The present review aims to critically summarize—for the first time—the current knowledge about the intestinal microbiota (and microbiome, when available) of monogastric species intended for production purposes (poultry, fish, pigs, and rabbits) fed diets including or supplemented with insect-based products (meals, fats, and live larvae). In particular, a focus on the mode of action of insect-based products and the different species-specific response is herein provided, with final remarks about the future challenges and perspectives as well.