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Variable harlequin frogs Atelopus varius have declined significantly throughout their range as a result of infection with the fungal pathogen Batrachochytrium dendrobatidis (Bd). The Panama Amphibian Rescue and Conservation Project maintains an ex situ population of this Critically Endangered species. We conducted a release trial with surplus captive-bred A. varius individuals to improve our ability to monitor frog populations post-release, observe dispersal patterns after freeing them into the wild and learn about threats to released frogs, as well as to determine whether natural skin toxin defences of frogs could be restored inside mesocosms in the wild and to compare Bd dynamics in natural amphibian communities at the release site vs a non-release site. The 458 released frogs dispersed rapidly and were difficult to re-encounter unless they carried a radio transmitter. No frog was seen after 36 days following release. Thirty frogs were fitted with radio transmitters and only half were trackable by day 10. Tetrodotoxin was not detected in the skins of the frogs inside mesocosms for up to 79 days. Bd loads in other species present at sites were high prior to release and decreased over time in a pattern probably driven by weather. No differences were observed in Bd prevalence between the release and non-release sites. This trial showed that refinements of our methods and approaches are required to study captive Atelopus frogs released into wild conditions. We recommend continuing release trials of captive-bred frogs with post-release monitoring methods, using an adaptive management framework to advance the field of amphibian reintroduction ecology.

The emerging fungal pathogen Batrachochytrium salamandrivorans (Bsal) threatens the diversity of amphibians, particularly in North America where it is projected to invade. Amphibian skin defenses include a mucosal layer containing microorganisms that can potentially modulate host response to pathogens such as Bsal. In this study, we focused on the composition of the skin microbiome across life stages of spotted salamanders (Ambystoma maculatum). We also evaluated the stress hormone corticosterone and skin microbiome response to inoculations with Bsal and probiotics at both the larval and juvenile developmental stages, and the response to different environmental conditions. Results indicated that both bacterial and fungal communities found on the skin significantly differed in structure and diversity between life stages of A. maculatum. Exposure to three different probiotics (Bacillus thuringiensis, Chryseobacterium rhizoplanae, and Penicillium sp.) and Bsal evoked shifts in the microbiome of larvae and juveniles, and the metabolite profile of the larval mucosal layer of A. maculatum. Despite changes in the microbiome, all tested probiotics and Bsal were unable to persist on the skin. Larval bacterial microbiomes shifted in response to Bsal and B. thuringiensis with no significant impacts on antifungal function or bacteria richness, however fungi strongly responded to Bsal and B. thuringiensis application. This indicates that developmental shifts in the microbiome can be initiated by microbial applications such as B. thuringiensis, a widely used mosquito larvicide. Overall, experimental results indicate that life stage, growth and development, and environmental conditions appeared to be the main factors driving changes in the amphibian skin microbiome and potential anti-Batrachochytrium function.

Abstract  Disease has spurred declines in global amphibian populations. In particular, the fungal pathogen Batrachochytrium dendrobatidis has decimated amphibian diversity in some areas unaffected by habitat loss. However, there is little evidence to explain how some amphibian species persist despite infection or even clear the pathogen beyond detection. One hypothesis is that certain bacterial symbionts on the skin of amphibians inhibit the growth of the pathogen. An antifungal strain of Janthinobacterium lividum, isolated from the skin of the red-backed salamander Plethodon cinereus, produces antifungal metabolites at concentrations lethal to B. dendrobatidis. Antifungal metabolites were identified by using reversed phase high performance liquid chromatography, high resolution mass spectrometry, nuclear magnetic resonance, and UV-Vis spectroscopy and tested for efficacy of inhibiting the pathogen. Two metabolites, indole-3-carboxaldehyde and violacein, inhibited the pathogen’s growth at relatively low concentrations (68.9 and 1.82 μM, respectively). Analysis of fresh salamander skin confirmed the presence of J. lividum and its metabolites on the skin of host salamanders in concentrations high enough to hinder or kill the pathogen (51 and 207 μM, respectively). These results support the hypothesis that cutaneous, mutualistic bacteria play a role in amphibian resistance to fungal disease. Exploitation of this biological process may provide long-term resistance to B. dendrobatidis for vulnerable amphibians and serve as a model for managing future emerging diseases in wildlife populations. [ABSTRACT FROM AUTHOR]