Fermentation of Agricultural By-products and Zinc Supplementation: A Synergistic Approach to Mealworm Microbiome Optimization

The increasing world population has put pressure on food industry to swiftly come up with initiatives that can sustain an increase in the demand for food, which have led to environmentally unsustainable practices. Insects, such as Tenebrio molitor (TM) larvae, have been widely proposed as sustainable alternative sources of protein with less environmental footprint. Insects can be reared with diets featuring low-cost agricultural side-streams, promoting circular economical practices, which can also be fermented to promote the development of probiotic microbiota. The inclusion of probiotics in the diets of insects during rearing has recently been explored to mitigate potential exposure to pathogens, potentially serving as alternatives to antibiotics. The focus of this study was the fermentation of spinach to naturally develop complex probiotic populations to enrich the diet of mealworms, promoting host health, thus resulting in a more nutritious food/feed source, featuring a safer microbiome profile. Sequencing revealed that fermented spinach had increased lactic acid bacteria (LAB), while repressing Pseudomonas. LAB species have been acknowledged and used as probiotics, whereas Pseudomonas populations are often associated with opportunistic resistant infections. Moreover, zinc is an essential element for bacterial microbiomes of many organisms, including insects. As the effects of dietary zinc on insect gut microbiome have been scarcely investigated, we have supplemented our formulated diets with this trace element and fed it to TM larvae for 35 days. While various diet compositions did not affect mealworm mortality, larvae fed spinach-based and zinc-rich diets gained less weight than that of control. Furthermore, higher dosage of zinc-supplementation resulted in significantly higher accumulation in TM larvae and a lower bioaccumulation factor. Sequencing of larvae fed diets supplemented with fermented spinach revealed increased presence of probiotic-associated genera, Lactococcus and Weissella. It was also found that supplementing diets with spinach and zinc significantly modulated the microbiome of larvae.