Structural and Functional Characterization of Hermetia illucens Larval Midgut

Abstract

The larvae of Hermetia illucens are among the most promising agents for the bioconversion of low-quality biomass, such as organic waste, into sustainable and nutritionally valuable proteins for the production of animal feed. Despite the great interest in this insect, the current literature provides information limited to the optimization of rearing methods for H. illucens larvae, with particular focus on their efficiency in transforming different types of waste and their nutritional composition in terms of suitability for feed production. Surprisingly, H. illucens biology has been neglected and a deep understanding of the morphofunctional properties of the larval midgut, the key organ that determines the extraordinary dietary plasticity of this insect, has been completely overlooked. The present study aims to fill this gap of knowledge. Our results demonstrate that the larval midgut is composed of distinct anatomical regions with different luminal pH and specific morphofunctional features. The midgut epithelium is formed by different cell types that are involved in nutrient digestion and absorption, acidification of the lumen of the middle region, endocrine regulation, and growth of the epithelium. A detailed characterization of the activity of enzymes involved in nutrient digestion and their mRNA expression levels reveals that protein, carbohydrate, and lipid digestion is associated to specific regions of this organ. Moreover, a significant lysozyme activity in the lumen of the anterior and middle regions of the midgut was detected. This enzyme, together with the strong acidic luminal pH of middle tract, may play an important role in killing pathogenic microorganisms ingested with the feeding substrate. The evidence collected led us to propose a detailed functional model of the larval midgut of H. illucens in which each region is characterized by peculiar features to accomplish specific functions. This platform of knowledge sets the stage for developing rearing protocols to optimize the bioconversion ability of this insect and its biotechnological applications.

Hydrolytic Profile of the Culturable Gut Bacterial Community Associated With Hermetia illucens

“…Degradation of urea (which could derive, in turn, from uric acid utilization, Martin et al, 2018) with ammonia production was carried out by 32.6% of the strains of our collection, while the majority of the isolates (90.7%) were able to release ammonia from peptone. Protease activity, and specifically the one exerted by serine proteases, was mainly retrieved in the posterior part of the midgut, which reached a pH value of 8.3 (Bonelli et al, 2019). In our collection, 16.6% of the isolates are able to degrade proteins (i.e., milk proteins in our experiments), representing also in this case an extra enzymatic activity source for the insect.…”

“…strains (Supplementary Table 1). The hydrolytic profiles obtained for BSF strains might suggest a primary role of the bacterial partners for the host nutrient supplementation (Engel and Moran, 2013;Gold et al, 2018), specifically contributing to the high levels of lipases and proteases that have been characterized in BSF gut content (Kim et al, 2011b;Bonelli et al, 2019). Taking into account its hydrolytic degradation abilities on complex substrates, BSF could be considered a potential source of enzymes with important industrial applications: recently polymer-degrading enzymes, such as cellulases and serine proteases, have been indeed characterized from H. illucens holobiont (Kim et al, 2011a;Lee et al, 2014).…”

“…Taking into account its hydrolytic degradation abilities on complex substrates, BSF could be considered a potential source of enzymes with important industrial applications: recently polymer-degrading enzymes, such as cellulases and serine proteases, have been indeed characterized from H. illucens holobiont (Kim et al, 2011a;Lee et al, 2014). However, since strong variations of pH levels, ranging from slightly acidic conditions in the anterior part, to strong acidic values in the middle portion, and to alkaline values in the posterior region, have been reported in BSF midgut (Bonelli et al, 2019;Bruno et al, 2019) and also considering that hypoxic and anoxic conditions might occur in BSF gut (Engel and Moran, 2013;Chouaia et al, 2019), further quantitative verifications of the metabolic activities of the isolates (and of their combination) should be performed under different oxygen concentrations and pH levels, especially regarding the strain B. licheniformis HI169, which showed the best performances in in vivo tests.…”

“…Regarding carbohydrate metabolism, several isolates were able to release amylolytic and cellulolytic enzymes. Degradation of carbohydrates (e.g., starch) occurs mainly in the anterior midgut and to a lesser extent in the posterior midgut (Bonelli et al, 2019): this could underline a major role exerted by insect, which led to a small number of active amylolytic and cellulolytic isolates. Conversely, further studies are necessary to characterize cellulolytic bacteria in the insect.…”

Salmonella Typhimurium Level in Mealworms (Tenebrio molitor) After Exposure to Contaminated Substrate

“…The pH is 5.6 in the anterior and middle midgut of T. molitor larvae and 7.9 in the posterior midgut, i.e., there is no significant gastric acid barrier in the larvae acting on Salmonella that tolerate pH down to 4 (Moreira et al, 2017). This is in contrast to pH values as low as 2.0 and 3.1 in the middle midgut of larvae of black soldier flies (Hermetica illucens) and house flies (Musca domestica), respectively, assumingly reducing the change of Salmonella surviving passage through these fly larvae (Terra and Regel, 1995;Bonelli et al, 2019). Still, in T. molitor as in other insects, are antimicrobial peptides (AMP) reported to act in the defense against bacterial infections (Wu et al, 2018;Jo et al, 2019;Keshavarz et al, 2020).…”