The intestinal microbiota of Hermetia illucens larvae is affected by diet and shows a diverse composition in the different midgut regions

The larva of the black soldier fly (Hermetia illucens) has emerged as an efficient system for the bioconversion of organic waste. Although many research efforts are devoted to the optimization of rearing conditions to increase the yield of the bioconversion process, microbiological aspects related to this insect are still neglected. Here we describe the microbiota of the midgut of H. illucens larvae showing the effect of different diets and midgut regions in shaping microbial load and diversity. The bacterial communities residing in the three parts of the midgut, characterized by remarkable changes in luminal pH values, differed in terms of bacterial numbers and microbiota composition. The microbiota of the anterior part of the midgut showed the highest diversity that gradually decreased along the midgut, whereas bacterial load had an opposite trend, being maximal in the posterior region. The results also showed that the influence of the microbial content of ingested food was limited to the anterior part of the midgut and that the feeding activity of H. illucens larvae did not affect significantly the microbiota of the substrate. Moreover, a high protein content compared to other macronutrients in the feeding substrate seems to favor midgut dysbiosis. The overall data indicate the importance of taking into account the presence of different midgut structural and functional domains, as well as substrate microbiota, in any further study that aims at clarifying microbiological aspects concerning H. illucens larval midgut. IMPORTANCE The demand for food of animal origin is expected to increase by 2050. Since traditional protein sources for monogastric diets are failing to meet the increasing demand for additional feed production, there is an urgent need to find alternative protein sources. The larvae of Hermetia illucens emerge as efficient converters of low quality biomass into nutritionally valuable proteins. Many studies have been performed to optimize H. illucens mass rearing on a number of organic substrates and to maximize quantitatively and qualitatively the biomass yield. On the contrary, although insect microbiota can be fundamental for bioconversion processes and its characterization is mandatory also for safety aspects, this topic is largely overlooked. Here we provide an in depth study of the microbiota of H. illucens larval midgut taking into account pivotal aspects such as the midgut spatial and functional regionalization as well as microbiota and nutrient composition of the feeding substrate.