Chytridiomycosis is an infectious amphibian disease caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd). Bd attaches to keratin on the dermis, invades skin cells, and may lead to pathogenesis in susceptible individuals. However, susceptibility varies within and among species. While this is due to many factors, the skin microbial community is a significant contributor to disease resistance. Amphibians form symbiotic relationships with environmental microbes on the skin surface, some of which produce antifungal agents that inhibit Bd. Interestingly, many agricultural biopesticides utilize the common soil-dwelling bacteria, Bacillus thuringiensis. These bacteria are known to produce antifungal metabolites and volatile organic compounds that inhibit growth of fungal plant pathogens. Through agricultural use, these bacteria likely increase in environmental abundance and provide added opportunity for amphibian exposure. Additionally, Bacillus spp. appear in amphibian skin microflora, some of which inhibit Bd. Yet, B. thuringiensis has never been considered as a biological control agent for Bd. Here we determine the anti-Bd potential of B. thuringiensis in vitro and in vivo. Furthermore, while the bacteria alone may be beneficial, the toxicity of commercial formulations has been scarcely tested on amphibians. We assessed toxicological effects of a commercial biopesticide on Lithobates sphenocephalus larvae.
Results/Conclusions
In vitro, B. thuringiensis significantly inhibited the growth of Bd. In vivo, adult frogs exposed to B. thuringiensis prior to Bd experienced less disease prevalence and lower infection loads than the group only exposed to Bd. However, individuals exposed to B. thuringiensis that became infected did not experience a significant mitigation in infection loads as they continued to increase over time. Many factors may contribute to this including an ineffective concentration of B. thuringiensis exposure. Current analyses are underway to assess the potential for a B. thuringiensis biopesticide to clear L. sphenocephalus larvae of Bd infection in mesocosm environments. In toxicology tests, we found that eggs and premetamorphic tadpoles of L. sphenocephalus are sensitive to high doses of one commercial formulation but do not experience mortality at environmentally relevant doses. These data suggest B. thuringiensis have anti-Bd potential and colonize the skin of L. sphenocephalus, but further studies are needed to determine if these bacteria would be effective as a Bd disease mitigation strategy. Ongoing research will assess the utility of these biopesticides in the microbial immunity of amphibians.