Amphibians face a multitude of threats and therefore make a prime example of the current biodiversity crisis. Multiple amphibian stressors in anthropogenic landscapes include infectious diseases and agrochemicals. Synergic effects between these stressors may increase the negative impact of infections on amphibian health. In a 56-day trial, we assessed the impact of co-exposure to the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) and the pathogenic fungus Batrachochytrium salamandrivorans (Bsal) on infection parameters (infection intensity and disease severity) and health (body condition and telomere length) in Italian crested newts (Triturus carnifex). Twenty days post-inoculation with Bsal, newts were either exposed to 2,4-D for 12 days or not exposed (control). Most newts developed high infection loads that steadily increased towards the end of the trial. While body condition was negatively correlated with pathogen burden, only one out of 23 animals died. Telomere length remained unaffected by the pesticide and the pathogen. The 2,4-D treatment did not exacerbate Bsal infection. Most newts survived almost two months with significant pathogen loads; thus, even in a pesticide-infested environment, T. carnifex could be an important long-term Bsal reservoir for co-occurring species on the Italian peninsula, a urodele diversity hotspot.

Background: Amidst the current biodiversity crisis, amphibians are particularly endangered by the emergence of infectious diseases. The skin disease chytridiomycosis is caused by the fungi Batrachochytrium dendrobatidis (Bd) and B. salamandrivorans (Bsal) , which may interact with bacterial symbionts present on the amphibian epidermis. Extensive research has explored the interactions between the amphibian microbiota and Bd ; yet, little is known about its interactions with Bsal . In this paper, we used the ribbed newt ( Pleurodeles waltl ), a model species displaying pronounced among-individual variation in response to Bsal, to (1) determine whether susceptibility to Bsal and individual microbiota vary between source groups; (2) test whether susceptibility to Bsal can be predicted from skin microbiota before exposure and (3) quantify microbiota volatility over time to determine whether Bsal infection intensity and chytridiomycosis severity correlate with the magnitude of shifts in bacterial communities caused by Bsal exposure. Results: Our results demonstrate that newts of different origin harbor distinct microbiota even under uniform rearing conditions. We show that Bsal infection intensity and disease severity cannot be predicted from the diversity, structure, or composition of the skin microbiota of P. waltl. Instead, a strong relation between newts’ source group and their response to Bsal suggests that other factors might underpin among-individual variation in Bsal susceptibility in this species. Moreover, our results indicate that the intensity of early Bsal infection and longer-term severity of chytridiomycosis do not correlate with the magnitude of microbiota change following Bsal exposure. Conclusion: These results demonstrate a limited involvement of the microbiota in Bsal dynamics in P. waltl, suggesting that other mechanisms contribute to individual Bsal susceptibility. Further research on the relation between chytrid pathogens and their amphibian hosts will be instrumental to improve the conservation of the most endangered vertebrate class on earth. Graphical