Microbiological Quality and Bacterial Diversity of Protaetia brevitarsis (White-Spotted Flower Chafer) Larvae Reared on Oak Sawdust and Spent Mushroom Substrate

In this study, we investigated the microbial quality and bacterial diversity in P. brevitarsis larvae reared on different substrates (spent mushrooms and oak sawdust). The microbiological quality of the rearing substrates differed significantly. Total viable counts (TVC) did not differ between oak sawdust and mushroom-based substrates, whereas coliforms (4.3 vs. 3.0 log CFU/g) and Bacillus cereus group (3.9 vs. 3.0 log CFU/g) were lower in the mushroom-based substrate. The drying process markedly improved microbial quality, reducing total viable counts from 9.4 to 9.1 to 7.3–4.0 log CFU/g and lowering B. cereus group, coliform, and fungi to near or below detection levels (<1 log CFU/g). Major pathogens were not detected in dried larvae. Comparative analyses at the phylum, genus, and species levels revealed distinct microbial profiles between substrates and larval guts. Substrate microbiota were dominated by environmental taxa, whereas larval guts were enriched in host-associated genera such as Citrobacter, Dysgonomonas, Bacteroides, and Lactococcus, and species including Proteiniphilum acetatigenes and Massilibacteroides vaginae. These patterns indicate a host-driven selection of gut microbiota during larval development, consistent across both oak sawdust and mushroom-based substrates. Both mushroom-based and oak sawdust substrates proved microbiologically safe for rearing P. brevitarsis larvae, with slightly lower microbial loads observed in the mushroom-based group. In addition, mushroom-based substrates promoted beneficial gut bacteria and offered sustainable reuse of agricultural byproducts. These findings highlight the potential of substrate selection to improve edible insect safety and support circular economy practices, reinforcing the role of P. brevitarsis as a sustainable protein source.