Microbiota of the black soldier fly

The BSF is associated with an elevated microbiological risk related to the diversity of the feeding conditions. To our knowledge, only three studies investigated the microbiota of the BSF, and only one is mycobiota. The mycobiota diversity is influenced by the diet and can be reduced by a prolonged feeding time. According to the authors, the majority of the yeasts detected on the BSF fed on several substrates are producers of antimicrobial compounds. The impact of the feed on the microbiota has been evaluated on the larvae’s entire digestive tract and specific parts of the midgut. The microbiota of all BSF stages has also been tracked when fed the Gainesville diet, a reference diet for Diptera (50% wheat bran, 30% alfalfa, and 20% corn meal). The microbiota of the whole larva contained 54% Bacteroidetes, 20% Firmicutes, 28% Proteobacteria, and 9% Actinobacteria, similar to its midgut microbiota when fed on the Gainesville diet. It seems that the bacteria taxa of the BSF provided with a balanced diet are dominated by Bacteroidetes (i.e., peptidoglycan degraders) and contains 9–20% Firmicutes and 16–28% Proteobacteria. Hence, when fed an unbalanced diet such as 100% fish meal and 100% cocked rice, the microbial diversity completely changes. Indeed, the larva digestive tract does not contain Bacteroidetes anymore but is now colonized by Proteobacteria (54–56%) and Firmicutes (43–47%), which may be problematic since most food-poisoning microorganisms are part of these taxa [37-38]. However, it is not necessarily problematic since lactic acid bacteria also belong to the Firmicutes.

Proteobacteria is a group of gram-negative bacteria, meaning that they possess an external membrane of lipopolysaccharides and a thin layer of peptidoglycan, including some essential pathogen bacteria. Indeed, Escherichia coli, which are known to induce gastroenteritis, Salmonella spp., responsible for salmonellosis, and Shigella spp., which provokes shigellosis in humans, are members of the Enterobacteriaceae family (Gammaproteobacteria) which are Proteobacteria. Another well-known Gammaproteobacteria is Pseudomonas aeruginosa, which is an opportunist pathogen. Campylobacter spp. are also Proteobacteria (Epsilonbacteria), one of the most often reported etiologic agents of gastroenteritis (i.e., campylobacteriosis).

Firmicutes are a group of gram-positive bacteria that possess a thick layer of peptidoglycan, allowing them to increase their resistance to physical disruption, heat, and desiccation but are more vulnerable to antibiotics and other chemical antimicrobial compounds than gram-negative bacteria. Firmicutes also include major pathogens, which are mainly in two classes, Clostridia and Bacilli. Clostridia, including Clostridium perfringens, a sporulated food-spoilage bacteria, are often found in vacuum-packed food or food poorly refrigerated. Other important food-poisoning microorganisms are Listeria monocytogenes, Bacillus cereus, and coagulase-positive Staphylococci (Bacilli). 

In addition to the feeding substrates that greatly influence the BSF larvae’s microbial load, the harvest stage is also essential. Indeed, when fed the Gainesville diet, it has been shown that the microbial community of prepupae contains 20% fewer Bacteroidetes, 10% fewer Firmicutes, and 25% more Proteobacteria than larvae. As a result, the prepupae may include more pathogens and should be further investigated. For some microorganisms, even the presence of a few bacteria is enough to induce toxicity, while for others, it requires higher counts. It is therefore critical to quantify the presence of these pathogens per gram to establish the risk they represent. Table 1.3 reports the only published microbial load data on the BSF larvae or prepupae until now. We know that Listeria spp. and Shigella spp. have not been detected in BSF larvae, but B. cereus, E. coli, Salmonella spp., Staphylococcus spp., Pseudomonas spp. and Clostridia have been. Further processing steps must focus on reducing the microbial load to ensure product safety.  

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