Fate of Salmonella enterica Typhimurium and Listeria monocytogenes in Black Soldier Fly (Hermetia illucens) Larvae Reared on Two Artificial Diets

Black Soldier Fly (Hermetia illucens) Microbiome and Microbe Interactions: A Scoping Review

Black soldier fly (Hermetia illucens, BSF) is farmed worldwide to convert organic waste into usable biomaterials. Studies on the larval microbiome have been carried out to check for symbiotic or pathogenic microbes and their respective functions and fates. Some studies tested these microbes for industrial applications, while others tested the effects of exogenous microbes as probiotics or for substrate pre-processing to improve larval fitness, bioconversion rates, or nutritional qualities. This review examined all peer-reviewed literature on these topics to consolidate many disparate findings together. It followed the PRISMA guidelines for scoping reviews. The results found no evidence of globally conserved core microbes, as diet strongly correlated with gut microbiome, but some genera appeared most frequently in BSF larval guts worldwide regardless of diet. The gut microbes undoubtably assist in digestion, including pathogen suppression, and so microbial probiotics show promise for future investigations. However, the common gut microbes have not been explored as probiotics themselves, which would be a promising direction for future work. The impacts of BSF bioconversion on pathogens varied, so each rearing facility should investigate and manage their pathogen risks independently. The data summarized in this study provide useful reference points for future investigations into BSF-microbe interactions.

Microbial dynamics and vertical transmission of Escherichia coli across consecutive life stages of the black soldier fly (Hermetia illucens)

BACKGROUND

The black soldier fly (BSF, Hermetia illucens L.) is one of the most promising insects for bioconversion of organic waste, which often carry a high microbial load with potential foodborne pathogens. Although horizontal transmission (from rearing substrate to larvae) has been extensively studied, less is known about vertical transmission of microorganisms, and particularly of foodborne pathogens, across different BSF life stages.

RESULTS

This study investigated the microbial dynamics and vertical transmission of Escherichia coli across different life stages (larvae, prepupae, pupae and adults) of one BSF life cycle and its associated substrate (chicken feed) and frass, based on a combination of general microbial counts (based on culture-dependent techniques) and the bacterial community composition (based on 16S rRNA gene sequencing). Multiple interactions between the microbiota of the substrate, frass and BSF larvae were affirmed. The larvae showed relative consistency among both the microbial counts and bacterial community composition. Diversification of the bacterial communities started during the pupal stage, while most notable changes of the microbial counts and bacterial community compositions occurred during metamorphosis to adults. Furthermore, vertical transmission of E. coli was investigated after substrate inoculation with approximately 7.0 log cfu/g of kanamycin-resistant E. coli, and monitoring E. coli counts from larval to adult stage. Although the frass still contained substantial levels of E. coli (> 4.5 log cfu/g) and E. coli was taken up by the larvae, limited vertical transmission of E. coli was observed with a decreasing trend until the prepupal stage. E. coli counts were below the detection limit (1.0 log cfu/g) for all BSF samples from the end of the pupal stage and the adult stage. Additionally, substrate inoculation of E. coli did not have a substantial impact on the bacterial community composition of the substrate, frass or different BSF life stages.

CONCLUSIONS

The fluctuating microbial counts and bacterial community composition underscored the dynamic character of the microbiota of BSF life stages. Additionally, vertical transmission throughout one BSF life cycle was not observed for E. coli. Hence, these findings paved the way for future case studies on vertical transmission of foodborne pathogens across consecutive BSF life stages or other insect species.

Microbiota is structured by gut regions, life stage, and diet in the Black Soldier Fly (Hermetia illucens)

The larvae of the Black Soldier Fly (Hermetia illucens) provide numerous ecological benefits, leading to significant commercial advancements. These benefits include the bioconversion of low-value waste into high-value feed and soil amendments. Understanding how the bacterial and eukaryotic microbiota communities affect host performance becomes vital for the optimization and specialization of industrial-scale rearing. This study investigates H. illucens-associated microbiota taxonomic composition and dynamics across the developmental cycle (eggs, neonates, larvae, prepupae, and imago X0 to second generation X1) when reared on two substrates: (i) plant-based (Housefly Gainesville diet) and (ii) animal-based (poultry hatchery waste). By using the 16S gene amplicon metataxonomic approach, we found that the results revealed that bacterial microbiota inherited from parents reared on a different substrate may have induced dysbiosis in the progeny. Specifically, the interaction networks of individuals reared on hatchery waste showed a high prevalence of negative interactions and low connectivity. Proteobacteria (39-92%), Firmicutes (4-39%), Bacteroidota (1-38%), and Actinobacteria (1-33%). In animal feed-reared individuals, Firmicutes reached the highest relative abundance (10-80%), followed by Proteobacteria (6-55%), Actinobacteria (1-31%), and Bacteroidota (0-22%). The rearing substrate was the main driver of microbiota composition, while the developmental stage influenced only the whole individual's bacterial microbiota composition. Gut regions were associated with distinct bacterial composition and richness, with diversity decreasing along the digestive tract. For the first time, microeukaryotes of the microbiota other than Fungi were investigated using 18S genetic marker amplicon sequencing with novel blocking primers specific to the Black Soldier Fly. Microeukaryotes are a neglected part of multitrophic microbiota communities that can have similar effects on their hosts as bacterial microbiota. Microeukaryotes from seven orders were identified in black soldier flies, including potential pathogens (e.g., Aplicomplexa group). Nucletmycea were the dominant class throughout development, followed by Holozoa and Stramenophiles. The eukaryote microbiota was structured by developmental stages but not by gut regions. Insights from this study are a stepping stone toward the microbiological optimization of black soldier flies for industrial rearing, highlighting how a synthetic microbiota assembly should be tailored to the rearing environment of the larvae at a targeted developmental stage.

Edible Insects for Humans and Animals: Nutritional Composition and an Option for Mitigating Environmental Damage

Every day, there is an increase in environmental damage on the planet regarding human action. One of the causes is food production. Edible insects are presented as an option to mitigate the environmental damage generated by the production of conventional food for humans and animals. The objective of this study was to investigate the main nutritional aspects of insects and how they can provide a nutritional and sustainable alternative to the planet. As the main results, the nine orders of insects that are most consumed on the planet are presented: Blattodea, Coleoptera, Diptera, Hemiptera, Hymenoptera, Isoptera, Lepidoptera, Odonata, and Orthoptera. Their main macro- and micronutrient aspects as well as their bioavailable and bioaccessibility proteins and essential amino acids, monounsaturated fatty acids, minerals, vitamins, and fiber (chitin) are also explored. Additionally, some of the species that are used for animal food processing and the possible risks that insects can present when used as food are discussed. With this, edible insects are established as a real option to mitigate climate change being an important nutritional source for the development of food for humans and animals.