Black Soldier Fly larvae microbiome has become the target of an increasing number of academic journal publications. These papers are always well received by us insect nerds at Nutrition Technologies because BSFL microbial ecology is central to our process and our research.
Today I will focus on one recent publication by Klammsteiner and co-authors titled: “ The Core Gut Microbiome of Black Soldier Fly (Hermetia illucens) Larvae Raised on Low-Bioburden Diets.” The paper was published in Frontier in Microbiology on May 21st 2020 (full-text Link). This is by no means the first publication to look at how feed affects the BSFL microbiome, but I found that it provides a set of valuable insights.
The paper examines the relationship between distinct diets (chicken feed, grass cuttings, and fruit and vegetables) and the BSFL microbiome. The authors ask whether the BSFL microbiome consists of a stable in-born microbiome or is driven by bacteria present in the feed (as suggested by Boccazzi et al. 2017). The question is worth asking because the BSFL microbiome is likely pivotal to their ability to consume a wide range of organic material, decrease pathogens in their environment, and degrade pharmaceutical products.
Klammsteiner and co-authors raised the BSFL neonates on a standard Chicken Feed diet for 6 days before transferring them to their respective feeds. The larvae were regularly sampled and dissected to remove their guts for DNA extraction and sequencing. The microbial community of the feed was also analyzed, although unfortunately these authors only sampled the fresh feed and did not track changes in the microbial community over time.
The study found that the microbial community differed between diets, with the grass cutting diet showing a distinct Bacilli-dominated composition compared to the other two. However, the authors could not find evidence for substantial time- or diet-associated changes in the BSFL microbiome. In this case the evidence points to a core microbiome that remains relatively stable across feeds and over the larvae development time. This core microbial community represented 44% of all sequences and consisted of Actinomyces sp., Dysgonomonas sp., and Enterococcus sp. and an unclassified Actinomycetales.
Figure 1. Relative abundance of bacterial classes from BSFL gut samples collected throughout larval development.
Image from Klammsteiner et al. 2020.
It is very tempting to guess, as the authors did, at the potential role that those genera of bacteria play in BSFL metabolism and health. Actinomyces sp. are known to break down organic material that includes lignin and chitin, and will often product antibiotics to inhibit other bacteria. Dysogonomonas is even more intriguing: they play a key role in termite guts to degrade lignocellulose and maybe contribute to the degradation of pharmaceutical products like ciprofloxacin. Lastly Enterococcus is often dominant in insect microbiomes and is associated with increasing nutrient availability as well as providing immunity-related antimicrobial peptides.
As exciting (and potentially useful) as the star players of this ‘core microbiome’ might be, does this mean that the BSFL microbiome is always fixed and cannot adapt to different feedstocks? Not necessarily. For one thing, the finding is not consistent with several other papers where the BSFL microbiome was significantly different across feedstocks (e.g. Boccazzi et al. 2017, Zhan et al. 2019). One potential reason for this is that other authors used feedstocks such as manure with high richness and diversity of bacteria compared to the low bioburden feeds used in this study.
Klammsteiner et al. also make an excellent observation that I have reason to believe is right on the money. All treatments used seed larvae that had been raised on chicken feed for 6 days prior to exposure to the new diets, however this common practice in academic studies could be a major flaw in the experimental design because the larvae may experience a “priming effect” where a resilient microbial community is established early in the lifecycle and is then not susceptible to microbial colonization from diet changes. As inconvenient as it may be, future academic trials should endeavor to use un-fed neonates directly transferred onto the target feed, rather than using 6-day old seed larvae that have been fed a standard diet.
In general, this paper provides us with a fascinating view of what a core BSFL microbiome might look like, and the broad functional competences of the core genera. While providing a counterpoint to other similar studies, the paper illustrates the importance of neonatal nutrition when measuring BSFL microbial composition.
Boccazzi, I. V., Ottoboni, M., Martin, E., Comandatore, F., Vallone, L., Spranghers, T., et al. (2017). A survey of the mycobiota associated with larvae of the black soldier fly (Hermetia illucens) reared for feed production. PLoS One 12:e0182533. doi: 10.1371/journal.pone.0182533
Klammsteiner, Thomas & Walter, Andreas & Bogataj, Tajda & Heussler, Carina & Stres, Blaz & Steiner, Florian & Schlick-Steiner, Birgit & Arthofer, Wolfgang & Insam, Heribert. (2020). The Core Gut Microbiome of Black Soldier Fly (Hermetia illucens) Larvae Raised on Low-Bioburden Diets. Frontiers in Microbiology. 11. 10.3389/fmicb.2020.00993.
Zhan, S., Fang, G., Cai, M., Kou, Z., Xu, J., Cao, Y., et al. (2019). Genomic landscape and genetic manipulation of the black soldier fly Hermetia illucens, a natural waste recycler. Cell Res. 30, 50–60. doi: 10.1038/s41422-019-0252-6