The role of symbiotic microbes on amphibian disease prevention

Background/Question/Methods   Recent declines of amphibian populations are associated with the disease chytridiomycosis, which is caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd). Cutaneous bacteria isolated from the amphibian species Plethodon cinereus and Rana muscosa inhibit the growth of Bd in vitro. In this study, the cutaneous bacterial communities present on the salamander P. cinereus and the endangered frog R. muscosa were investigated to determine if bacteria can provide protection to amphibians from the lethal and sub-lethal effects of chytridiomycosis.

Results/Conclusions   In a “bacteria-removal” experiment, the skin bacteria of P. cinereus were reduced with antibiotics to determine if the absence of skin bacteria affects the health of the amphibians when exposed to Bd. Infected salamanders with a reduced bacterial community experienced a greater decrease in body mass, which is a symptom of the disease, when compared to infected individuals with an unmanipulated skin community. In a separate “bacteria-addition”, we tested the hypothesis that disease severity of infected R. muscosa individuals decreases when the microbial community of frogs is augmented with the anti-Bd bacterial species Janthinobacterium lividum. J. lividum produces the anti-Bd metabolite violacein and we have isolated this species from skins of R. muscosa and P. cinereus. The results of this experiment showed that bio-augmentation of frog skin with J. lividum prevented morbidity and mortality caused by Bd. Overall, the results of these studies demonstrate that mutualistic microbes associated with amphibian species appear to be an important component of their innate skin defenses, and alteration to this community can change disease resistance. An understanding of the community ecology of amphibian skin is a key component of attempts to control this disease. Future directions of this work include determining how amphibians acquire their cutaneous microbial community and what impact disturbances have on this complex microbial community.