Staphylococcus aureus in Substrates for Black Soldier Fly Larvae (Hermetia illucens) and Its Dynamics during Rearing.

Black soldier fly larvae (BSFL; Hermetia illucens) are promising insects for the conversion of organic waste streams into valuable biomolecules. Such waste streams can contain foodborne pathogens. To assess this risk factor, this study evaluated the presence of Staphylococcus aureus in waste streams as a substrate ingredient for BSFL production as well as in the rearing process. First, the general microbiological quality and the occurrence of S. aureus were investigated for different waste streams. Staphylococcus aureus was abundantly present. Control of pH and water activity should avoid pathogens, which cannot grow in single-substrate ingredients, redeveloping when mixing streams for optimal substrate conditions for BSFL production. Next, it was investigated whether S. aureus present in the substrate was ingested and/or eradicated by BSFL. In inoculation trials, with S. aureus added to chicken feed as the substrate at 3 or 7 log CFU/g, the larvae showed a reducing effect on S. aureus. After 6 days, S. aureus counts were below the detection limit (2.0 log CFU/g) in all larvae samples and decreased in the substrate to <2.0 and <3.1 log CFU/g for inoculation levels of 3 and 7 log CFU/g, respectively. While this is promising, it is still recommended to monitor and control this pathogen in BSFL rearing. Intriguingly, screening of antimicrobial activity of dominant microorganisms associated with BSFL showed a clear activity of Trichosporon isolates against S. aureus. Future research should explore whether Trichosporon, which is frequently observed in BSFL, plays a role in controlling specific microorganisms, such as S. aureus. IMPORTANCE Given the increasing need for (more sustainable) methods to upcycle organic waste streams, the interest to rear insects, like black soldier fly larvae (BSFL), on such streams is increasing. This study reveals that S. aureus is abundantly present in such waste streams, which might be a point of attention for insect producers. At the same time, it reveals that when S. aureus was inoculated in chicken feed as the substrate, it was not detected in the larvae and was reduced in the substrate after 6 days. Future inoculation trials should investigate whether this reduction is substrate dependent or not. Toward the future, the role of the BSFL microbiota in controlling intestinal bacterial community homeostasis should be explored, because one of the dominant microorganisms associated with BSFL, Trichosporon spp., showed clear activity against S. aureus.

Dynamics of Salmonella inoculated during rearing of black soldier fly larvae (Hermetia illucens).

The black soldier fly is currently the most produced edible insect on industrial scale, with its larval stage being processed into animal feed as the main application. As this insect species enters the feed and food chain, good hygiene and monitoring practices are needed to avoid the entrance of foodborne pathogens via the larvae. However, insufficient data on the risk of such introductions via industrial larvae production are available. To address this gap, a range of rearing trials were conducted in which the substrate, chicken feed, was inoculated with different levels of Salmonella and in which total viable counts and Salmonella counts were determined during the following days. The outgrowth of Salmonella was slower in those experiments with a lower initial contamination level than in experiments with a higher level. No significant reducing effect originating from the larvae on the substrate Salmonella counts was observed, in contrast to previous studies using other substrates. Our study also revealed that airborne transmission of Salmonella is possible under rearing conditions corresponding to those applied at industrial production sites. Based on our results, we recommend insect producers to use substrate ingredients free of Salmonella, and not to count on the antimicrobial activities that BSFL may exert in some situations towards food pathogens. More inoculation studies using other Salmonella serotypes, other zoonotic bacteria, other substrates, larvae of other ages and including variations on rearing protocols are needed in order to obtain a general view on the dynamics of food pathogens in this insect species and to support comprehensive risk assessments.