Dynamics of Bacillus cereus during rearing of Hermetia illucens on authorized and unauthorized substrates under European legislation.

چکیده اصلی

In the context of sustainable waste valorization, we investigated microbiological safety hazards associated with spore-forming bacteria, focusing in particular on Bacillus cereus contamination and diversity during Hermetia illucens (Black Soldier Fly, BSF) bioconversion at different developmental stages. Larvae were reared on authorized (carrots, apricots, wheat bran) and unauthorized substrates (expired unpackaged retail food products and cafeteria waste). After 7 days of spontaneous fermentation of these substrates, total aerobic mesophilic bacterial counts were comparable across substrates; however, aerobic spore counts were significantly higher in unauthorized than authorized substrates. Notably, B. cereus was undetectable in authorized substrates but reached 5.2-6.0 log CFU/g in unauthorized waste. Despite substrate differences, BSF larvae and frass showed similar levels of total mesophilic and spore counts, with B. cereus consistently detected, indicating that vertical transmission may take place. Phylogenetic analysis revealed the persistence of potentially enterotoxigenic B. cereus group III strains harboring nheI and cytK genes throughout the bioconversion process. Additionally, group IV strains initially present in the substrate became dominant in larvae and frass by day 14. These findings demonstrate that BSF larvae are unable to significantly reduce B. cereus during entomoconversion. Frass and larvae from both authorized and unauthorized substrates showed similar contamination levels. Both types of substrates were suitable for larval rearing and did not lead to significant differences in B. cereus levels, however, the persistence of high loads in larvae and frass, regardless of the substrate, may represent a potential health hazard in the absence of further treatment.

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