From Symbionts to Pathogens: Interactions within the Amphibian Skin Mucosome

摘要

The emerging fungal pathogens, Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal) cause the disease chytridiomycosis in many amphibians. As both are cutaneous fungal pathogens, skin and mucosal defenses against these pathogens may be critical. Potential defenses include secreted host defense compounds and microbiota. In this thesis, toxins produced in the skin of the boreal toad, Anaxyrus boreas, were identified and tested for antimicrobial properties against Bd. All three bufadienolides -- arenobufagin, gamabufotalin, and telocinobufagin -- were found to inhibit Bd at a similar level. Bufadienolide production is potentially a convergent trait similar to antimicrobial peptide defenses found on the skin of other amphibian species. Like boreal toads, spotted salamanders, Ambystoma maculatum, do not produce antimicrobial skin defense peptides. Spotted salamanders were investigated for their susceptibility to Bsal. Next generation sequencing was used to characterize the microbial communities associated with spotted salamanders. Several probiotics were also tested for their effectiveness. Overall, spotted salamanders were not susceptible to Bsal infection and presented no clinical signs of chytridiomycosis. Microbial communities on the skin showed high predicted antifungal function. Of the three probiotics tested, none were able to persist on the skin of the amphibians compared to controls and one, Penicillium, evoked a short-term corticosterone stress response. Bsal also caused a short-term stress response that may have helped activate host immune defenses against the pathogen. Addition of antifungal microbes and the fungal pathogen Bsal caused minor shifts in the skin microbiome. Stability and colonization resistance of the microbiome may have facilitated defense against Bsal. Life stage and time were the main factors influencing bacterial and fungal skin composition. As Bd and Bsal continue to spread, we show that skin and mucosal defenses are critical in defending against these pathogens and preventing or reducing the effects of chytridiomycosis.