Hundreds of amphibian species worldwide face extinction due to Chytridiomycosis. Chytridiomycosis is an infectious disease of amphibians that is caused by the aquatic fungal pathogen, Batrachochytrium dendrobatidis (Bd). Bd infects the keratin containing layers of the amphibian’s skin. This skin infection results in hyperketosis, causing the skin to become thick and fall off. When this happens, osmotic regulation fails, electrolyte levels drop, and the amphibian dies of cardiac arrest. Chytridiomycosis often leads to death unless the animal is treated in a lab. This disease is an immediate threat to amphibian biodiversity worldwide. There are, however, a few species of amphibians that are resistant to the Chytridiomycosis infection. This resistance is conferred by symbiotic bacteria present on their skin that produces antifungal metabolites.
Currently, amphibians with Chytridiomycosis must be treated in a lab environment and treating entire populations of amphibians is impossible. Laboratory treatments for Chytridiomycosis include killing Bd by heating the affected animal to 37 °C for 4 hours, but some amphibians are too sensitive to survive this treatment. Another treatment is with an antifungal ointment such as Itroconazole either through bathing or oral administration. More research is needed to know whether antifungal ointments are safe to use on certain amphibians.
The UFlorida iGEM team seeks to develop a new treatment for Chytridiomycosis based on the metabolites produced by the symbiotic bacteria of the amphibian populations resistant to the fungus. Tryptophol, a metabolite of a synergistic frog skin bacteria species, has been shown to be effective at combating the Batrachochytrium dendrobatidis fungus at low doses. Our team will modify E.coli with the genes comprising the pathway for tryptophol synthesis. The biosynthesis pathway for tryptophol begins with tryptophan as the substrate. Tryptophan is converted into (indol-3-yl)pyruvate by the enzyme Aromatic Amino Acid Aminotransferase II, which is coded for by the ARO9 gene in Saccharomyces cerevisiae. (Indole-3-yl)pyruvate is then converted into indole acetaldehyde by the enzyme 2-oxo Acid Decarboxylase, which is encoded by the ARO10 gene. Lastly, Indole acetaldehyde is converted into indole-3-ethanol (tryptophol) by the enzyme Alcohol Dehydrogenase I, which is encoded by the gene ADH1. In order to produce tryptophol from tryptophan, we will transform the ARO9, ARO10, and ADH1 genes into E.coli.
Our team chose to pursue this project because amphibian populations are an essential component to many ecosystems, especially as indicator species (which are known as early warning signs of declining health of the ecosystem). The amphibian populations in Florida are experiencing a steep decline, which could impact the rest of the ecosystem. In particular, amphibians help to control mosquito populations, which is a paramount issue in Florida. While the primary cause of the population decline is not known, Chytridiomycosis has been detected in Florida and remains a threat to the local amphibian populations.
References
Loudon, A. H., Holland, J. A., Umile, T. P., Burzynski, E. A., Minbiole, K. P., & Harris, R. N. (2014). Interactions between amphibians symbiotic bacteria cause the production of emergent anti-fungal metabolites. Frontiers in Microbiology, 5. doi:10.3389/fmicb.2014.00441
Muletz, C. R., Myers, J. M., Domangue, R. J., Herrick, J. B., & Harris, R. N. (2012). Soil bioaugmentation with amphibian cutaneous bacteria protects amphibian hosts from infection by Batrachochytrium dendrobatidis. Biological Conservation, 152, 119-126. doi:10.1016/j.biocon.2012.03.022
Fisher, M. C., Garner, T. W., & Walker, S. F. (2009). Global Emergence of Batrachochytrium dendrobatidis and Amphibian Chytridiomycosis in Space, Time, and Host. Annual Review of Microbiology, 63(1), 291-310. doi:10.1146/annurev.micro.091208.073435
Harris, R. N., Brucker, R. M., Walke, J. B., Becker, M. H., Schwantes, C. R., Flaherty, D. C., . . . Minbiole, K. P. (2009). Skin microbes on frogs prevent morbidity and mortality caused by a lethal skin fungus. The ISME Journal, 3(7), 818-824. doi:10.1038/ismej.2009.27
