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

Abstract

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.

Keywords: Hermetia illucens; Staphylococcus aureus; inoculation trial; organic side streams; rearing.

FIG 1

Mean diamonds plots representing the Staphylococcus aureus counts in log CFU/g of the waste streams sampled in periods 1 and 2. The symbols ● and ▴ represent the replicates in period 1 (P1; June to September) and period 2 (P2; January to February in the year after sampling period 1), respectively. The top and bottom of each diamond represent the 100% confidence interval for each period, and the horizontal line in the middle of each diamond represents the mean for each period. The 95% confidence marks are indicated as lines above and below the mean. The horizontal black lines represent the mean plate count of all replicates of both sampling periods for each waste stream. The thick horizontal black line represents the detection limit (2.0 log CFU/g). Mean plate counts from different periods of the same waste stream that share a letter (a and b) are not statistically different (P ≥ 0.05). For strawberry leaves and tomato leaves, only period 1 data are available.

FIG 2

Inhibition zone illustrating the antimicrobial activity of a Trichosporon species isolate against Staphylococcus aureus (left) and a negative control (isolate without activity against S. aureus) (right).

FIG 3

Relation between the Enterobacteriaceae/lactic acid bacteria ratio and the pH for all waste streams with a pH below 7.0.

FIG 4

Experimental setup of the inoculation trial (CF, chicken feed; P, pathogen Staphylococcus aureus; L, larvae).