CONSIDERING MICROBIAL PESTICIDES AS A DISEASE MITIGATION STRATEGY FOR THE AMPHIBIAN FUNGAL DISEASE, CHYTRIDIOMYCOSIS

Denita Mychele Weeks

Data is provided by the student.

Abstract

Chytridiomycosis is an infectious amphibian disease caused by the pathogenic fungus Batrachochytrium dendrobatidis (Bd). Bd attaches to keratin on the epidermis of amphibians, 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. Through agricultural use, these bacteria likely increase in environmental abundance and provide added opportunity for amphibian exposure. These bacteria are known to produce antifungal metabolites that inhibit growth of fungal plant pathogens. 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. I determined 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. I assessed toxicological effects of a commercial biopesticide on Southern Leopard Frog (Lithobates sphenocephalus) larvae. In vitro, B. thuringiensis significantly inhibited the growth of Bd. In vivo, adult L. sphenocephalus exposed to B. thuringiensis prior to Bd experienced a trend toward lower disease prevalence and lower infection loads than the group only exposed to Bd. Furthermore, in environmentally relevant doses of a Bt-biopesticide, embryos and larvae of L. sphenocephalus do not experience changes to developmental rate, post-metamorphic size, or mortality. These data suggest B. thuringiensis is safe, colonize the skin of L. sphenocephalus, and warrant further studies to investigate their anti-Bd potential.