A recent study by Vogel et al. (2018)  showed that the black larvae of the black soldier fly can produce a wide range of antimicrobial peptides depending on the substrates on which they are grown.
The black soldier fly is an economically important agricultural insect whose proteins have been used in animal feed for many years. In addition, black soldier fly larvae are able to recycle organic waste, such as animal manure. Growing larvae on organic waste, however, can lead to their infection by potential pathogens, which, of course, is a serious problem for the feed additive industry.
In their study, scientists fed 10 grams of larvae with 10 grams of various substrates within 72 hours. The control substrate consisted of feed for laying hens, the second group consisted of 5 grams of feed for laying hens and 5 grams of either sulfonated lignin, or cellulose powder, or chitin, brewer pellets, and sunflower oil. The third group of substrates consisted of 10 grams of feed for laying hens with the addition of 1.25 grams of either Bacillus subtilis or Micrococcus luteus, Escherichia coli BL21 (DE3) or Pseudomonas fluorescens BL915. RNA of the black soldier flies were then analyzed for the expression of genes associated with a protective response.
Effect on gene expression
In general, 53 genes were differentially expressed in the black soldier fly, which are responsible for the synthesis of atomic microbial peptides. All of these peptides showed activity against gram-positive and gram-negative bacteria. Some peptidoglycan recognition proteins (PGRPs) and binding proteins (GNBPs) of gram-negative bacteria have also been identified. These two types of proteins are receptors for pattern recognition (standard molecular structures) that are involved in the recognition of pathogens (Dziarski, 2004; Kim et al., 2000). In addition, several phenol oxidases have been found. These are important enzymes in the defense against bacteria, fungi and viruses in insects.
Results by Substrate Type
The study showed that the cultivation of larvae on protein-rich substrates with the addition of brewer pallets and sunflower oil has led to the highest content of antimicrobial peptides (AMP) in comparison with the control group. The smallest number of genes encoding AMP was found in larvae grown on chicken feed with the addition of cellulose. This suggests that protein substrates can induce stronger immune responses in black soldier fly larvae. The researchers also suggested that phytosterols, such as sitosterol in sunflower oil, may have immunomodulating effects.
Cultivating larvae on feed with high bacterial load showed that they produce several types of AMP. This coincides with the previous assumption about the initiation of an immune response due to the presence of E. coli and M. luteus in the substrate (Zdybicka-Barabas et al., 2017). However, it was not investigated whether the high content of bacteria in the substrate has a detrimental effect on the health and growth of the larvae, which calls into question the significance of these findings.
An assessment of the inhibition of bacterial growth showed that the composition of the nutrient substrate affects the antimicrobial activity against B. subtilis, M. luteus, E. coli and P. fluorescens. However, the effectiveness of growth inhibition depends on the specific types of bacteria. Nevertheless, the data indicate an increased antibacterial activity in the larvae when they are grown on a protein substrate. This may be due to the substrate-dependent production of AMP.
B. subtilis bacteria are gram-negative and usually non-pathogenic, but some strains have been used to produce insect-specific toxins. M. luteus is a gram-positive non-pathogenic bacterium that is commonly used in the analysis of bacteriolysis (Kaaya, 1993). Gram-negative E. coli BL21 (DE3) is a non-pathogenic research model that is commonly used in natural science laboratories and in the biotechnology industry (Jeong et al., 2009). P. fluorescens is a gram-negative bacterium that has been found to cause disease in immunocompromised patients and causes sepsis after transfusion of infected blood (Gershman et al., 2008; Morduchowicz et al., 1991).
The disadvantage of the study was its relatively short growth period (72 hours) of the black soldier fly larvae on the respective substrates before the extraction of RNA for the analysis of gene expression. Further studies can be directed to the study of protective reactions associated with gene expression at all stages of development of the larvae.
 Vogel H. et al. Nutritional immunology: diversification and diet-dependent expression of antimicrobial peptides in the black soldier fly Hermetia illucens //Developmental & Comparative Immunology. – 2018. – T. 78. – p. 141-148.