Insights Into the Immune Response of the Black Soldier Fly Larvae to Bacteria

Abstract

In insects, a complex and effective immune system that can be rapidly activated by a plethora of stimuli has evolved. Although the main cellular and humoral mechanisms and their activation pathways are highly conserved across insects, the timing and the efficacy of triggered immune responses can differ among different species. In this scenario, an insect deserving particular attention is the black soldier fly (BSF), Hermetia illucens (Diptera: Stratiomyidae). Indeed, BSF larvae can be reared on a wide range of decaying organic substrates and, thanks to their high protein and lipid content, they represent a valuable source of macromolecules useful for different applications (e.g., production of feedstuff, bioplastics, and biodiesel), thus contributing to the development of circular economy supply chains for waste valorization. However, decaying substrates bring the larvae into contact with different potential pathogens that can challenge their health status and growth. Although these life strategies have presumably contributed to shape the evolution of a sophisticated and efficient immune system in this dipteran, knowledge about its functional features is still fragmentary. In the present study, we investigated the processes underpinning the immune response to bacteria in H. illucens larvae and characterized their reaction times. Our data demonstrate that the cellular and humoral responses in this insect show different kinetics: phagocytosis and encapsulation are rapidly triggered after the immune challenge, while the humoral components intervene later. Moreover, although both Gram-positive and Gram-negative bacteria are completely removed from the insect body within a few hours after injection, Gram-positive bacteria persist in the hemolymph longer than do Gram-negative bacteria. Finally, the activity of two key actors of the humoral response, i.e., lysozyme and phenoloxidase, show unusual dynamics as compared to other insects. This study represents the first detailed characterization of the immune response to bacteria of H. illucens larvae, expanding knowledge on the defense mechanisms of this insect among Diptera. This information is a prerequisite to manipulating the larval immune response by nutritional and environmental factors to increase resistance to pathogens and optimize health status during mass rearing.

Immune Responses of the Black Soldier Fly Hermetia illucens (L.) (Diptera: Stratiomyidae) Reared on Catering Waste

“…Both AMPs’ transcripts were significantly up-regulated in prepupae fed a dairy product-added diet showing a 2-fold upper transcript level as compared with insects fed an FVBM, and 5-fold and a 2-fold upper transcript levels when compared to an FVB diet for defensin and cecropin, respectively. As previously shown, a higher microbial load of the substrate may lead to the major expression of AMP-encoding genes [ 56 , 57 ], and the bacteria present in the dairy products may act as probiotics.…”

Genotype-by-Diet Interactions for Larval Performance and Body Composition Traits in the Black Soldier Fly, Hermetia illucens

“…Immune counter-defences, not only in insects, are generally considered costly and governed by trade-offs [ 114 , 120 , 121 , 122 ]. The BSF possesses a remarkable repertoire of immune genes [ 52 ], permitting complex response cascades [ 114 , 123 , 124 ], among which antimicrobial peptides are of transdisciplinary interest [ 6 , 125 ]. The latter were indeed previously shown to be alternatively expressed on different diets, including upregulation on protein and lipid-rich feed [ 126 ].…”

A Bombyx mori Infection Model for Screening Antibiotics against Staphylococcus epidermidis

“…In insects, proPO leads to melanin formation by generating quinones and other reactive intermediates, with the final aim of trophically isolating and killing pathogens and parasites [ 54 ]. The lack of activation of this enzymatic cascade might depend on the bacterial load in the hemolymph, since it is known that proPO system is induced only over a certain threshold concentration of bacteria [ 33 ] to avoid the unneeded production of melanin, which is toxic for the insect itself [ 55 ]. An alternative possible explanation might involve the ability of some specific pathogens to produce serpin-type inhibitors or non-proteinaceous factors (e.g., polyphenol derivatives), which specifically interfere with the proteolytic activation of proPO, increasing their survival in the hosts [ 54 , 56 ].…”

“…After two centrifugations at 250× g for 5 min and one at 1600× g for 10 min at 4 °C, the supernatant was collected and diluted 1:10 with sterile phosphate-buffered saline (PBS: 1370 mM NaCl, 27 mM KCl, 100 mM Na 2 HPO 4 × 12H 2 O, 19.8 mM KH 2 PO 4 ). The activity of lysozyme was determined according to Bruno et al [ 33 ]. Briefly, 100 µL of diluted hemolymph were added to 150 µL of 0.45 mg/mL lyophilized Micrococcus lysodeikticus (Sigma-Aldrich) in 30 mM phosphate buffer (38 mM KH 2 PO 4 , 61.4 mM K 2 HPO 4 , pH 7.2) (OD 600nm of 0.6–0.7).…”