Microbial safety of industrially reared <i>Hermetia illucens </i>larvae and frass: bacterial dynamics and prevalence of antibiotic resistance genes

The larvae of the black soldier fly (BSFL) can efficiently convert food waste into valuable biomass. They are also an alternative source of fat, protein, and chitin, but little is known about the total microbiota of the whole BSFL and its impact on the microbiological safety of food and feed. This study was conducted to determine bacterial microbiota dynamics of the whole BSFL and frass residues during the industrial rearing process, including the effects of thermal treatment, counts and identification of cultivable bacteria, and the presence of antibiotic resistance genes (ARGs). The second and the fourth instar larvae, frozen and dried fourth instar larvae, and frass samples were examined. The composition of the total bacterial microbiota in BSFL samples was similar and dominated by Proteobacteria while in frass Firmicutes prevailed. The lowest diversity was observed in the second instar larvae and the highest in the frass. The samples showed a relatively low bacterial community diversity. In agreement with the analysis of the total microbiota, the isolated cultivable bacterial strains were members of <i>Proteus, Providencia, Morganella, Staphylococcus, Klebsiella, Enterococcus,</i> and <i>Bacillus</i> genera. The counts of cultivable bacteria increased during the growth of larvae and were similar in the fourth instar larvae and frass residues. Dried larvae had the lowest number of viable counts and were dominated by spore-forming bacteria. The determined viable aerobic counts meet the criteria for edible insects. ARGs conferring resistance to aminoglycosides (<i>aac-aph</i>), β-lactams (<i>blaZ</i>), erythromycin (<i>ermA</i>), tetracycline (<i>tetM, tetW</i>), and vancomycin (<i>vanA, vanB</i>) were detected by PCR, with the highest diversity and detection rate in frass. The gene <i>tetM</i> was the most widespread and detected in all groups of the tested samples. The results of this work extend the scarce knowledge about the dynamics of microorganisms and ARGs in the industrial-scale food waste upcycling process by BSFL.