Minor impact of probiotic bacteria and egg white on Tenebrio molitor growth, microbial composition, and pathogen infection.

The industrial rearing of the yellow mealworm (Tenebrio molitor) for feed and food purposes on agricultural by-products may expose larvae and adults to entomopathogens used as biocontrol agents in crop production. Bacterial spores/toxins or fungal conidia from species such as Bacillus thuringiensis or Metarhizium brunneum could affect the survival and growth of insects. Therefore, the aim of this study was to investigate the potential benefits of a wheat bran diet supplemented with probiotic bacteria and dried egg white on larval development and survival and its effects on the gut microbiome composition. Two probiotic bacterial species, Pediococcus pentosaceus KVL B19-01 and Lactiplantibacillus plantarum WJB, were added to wheat bran feed with and without dried egg white, as an additional protein source, directly from neonate larval hatching until reaching a body mass of 20 mg. Subsequently, larvae from the various diets were exposed for 72 h to B. thuringiensis, M. brunneum, or their combination. Larval survival and growth were recorded for 14 days, and the bacterial microbiota composition was analyzed using 16S rDNA sequencing prior to pathogen exposure and on days 3 and 11 after inoculation with the pathogens. The results showed increased survival for T. molitor larvae reared on feed supplemented with P. pentosaceus in the case of co-infection. Larval growth was also impacted in the co-infection treatment. No significant impact of egg white or of P. pentosaceus on larval growth was recorded, while the addition of Lb. plantarum resulted in a minor increase in individual mass gain compared with infected larvae without the latter probiotic. On day 14, B. thuringiensis was no longer detected and the overall bacterial community composition of the larvae was similar in all treatments. On the other hand, the relative operational taxonomic unit (OTU) abundance was dependent on day, diet, and probiotic. Interestingly, P. pentosaceus was present throughout the experiments, while Lb. plantarum was not found at a detectable level, although its transient presence slightly improved larval performance. Overall, this study confirms the potential benefits of some probiotics during the development of T. molitor while underlining the complexity of the relationship between the host and its microbiome.

Effect of CO2 Concentrations on Entomopathogen Fitness and Insect-Pathogen Interactions.

Numerous insect species and their associated microbial pathogens are exposed to elevated CO2 concentrations in both artificial and natural environments. However, the impacts of elevated CO2 on the fitness of these pathogens and the susceptibility of insects to pathogen infections are not well understood. The yellow mealworm, Tenebrio molitor, is commonly produced for food and feed purposes in mass-rearing systems, which increases risk of pathogen infections. Additionally, entomopathogens are used to control T. molitor, which is also a pest of stored grains. It is therefore important to understand how elevated CO2 may affect both the pathogen directly and impact on host-pathogen interactions. We demonstrate that elevated CO2 concentrations reduced the viability and persistence of the spores of the bacterial pathogen Bacillus thuringiensis. In contrast, conidia of the fungal pathogen Metarhizium brunneum germinated faster under elevated CO2. Pre-exposure of the two pathogens to elevated CO2 prior to host infection did not affect the survival probability of T. molitor larvae. However, larvae reared at elevated CO2 concentrations were less susceptible to both pathogens compared to larvae reared at ambient CO2 concentrations. Our findings indicate that whilst elevated CO2 concentrations may be beneficial in reducing host susceptibility in mass-rearing systems, they may potentially reduce the efficacy of the tested entomopathogens when used as biological control agents of T. molitor larvae. We conclude that CO2 concentrations should be carefully selected and monitored as an additional environmental factor in laboratory experiments investigating insect-pathogen interactions.

Effect of the rearing diet on gene expression of antimicrobial peptides in Hermetia illucens (Diptera: Stratiomyidae).

Insect proteins have been proposed for human and animal food production. Safeguarding the health status of insects in mass rearing allows to obtain high-quality products and to avoid severe economic losses due to entomopathogens. Therefore, new strategies for preserving insect health must be implemented. Modulation of the insect immune system through the diet is one such strategy. We evaluated gene expression of two antimicrobial peptides (one defensin and one cecropin) in Hermetia illucens (L.) (Diptera: Stratiomyidae) reared on different diets. Analyses were performed on prepupae and 10-day-old larvae reared on cereal- and municipal organic waste-based diets and on only prepupae reared on a cereal-based diet supplemented with sunflower, corn, or soybean oil. The inclusion of sunflower oil at different points in the cereal-based diet was also evaluated. Moreover, diet-driven differences in the inhibitory activity of the hemolymph were tested against Escherichia coli DH5α and Micrococcus yunnanensis HI55 using diffusion assays in solid media. Results showed that a municipal organic waste-based diet produced a significant overexpression of antimicrobial peptides only in prepupae. Inclusion of vegetable oils caused an upregulation of at least one peptide, except for the corn oil. Higher expression of both genes was observed when sunflower oil was added 5 days before pupation. All hemolymph samples showed an inhibitory activity against bacteria colonies. Our results suggest that municipal organic waste-based diet and vegetable oil-added diet may successfully impact the immune system of H. illucens. Such alternatives may also exist for other species of economic interest.

Bugs in Bugs: The Role of Probiotics and Prebiotics in Maintenance of Health in Mass-Reared Insects.

Interactions between insects and their microbiota affect insect behaviour and evolution. When specific microorganisms are provided as a dietary supplement, insect reproduction, food conversion and growth are enhanced and health is improved in cases of nutritional deficiency or pathogen infection. The purpose of this review is to provide an overview of insect-microbiota interactions, to review the role of probiotics, their general use in insects reared for food and feed, and their interactions with the host microbiota. We review how bacterial strains have been selected for insect species reared for food and feed and discuss methods used to isolate and measure the effectiveness of a probiotic. We outline future perspectives on probiotic applications in mass-reared insects.