Motile zoospores of Batrachochytrium dendrobatidis move away from antifungal metabolites produced by amphibian skin bacteria.

Chytridiomycosis is an amphibian skin disease that threatens amphibian biodiversity worldwide. The fungal agent of chytridiomycosis is Batrachochytrium dendrobatidis. There is considerable variation in disease outcomes such that some individuals and populations co-exist with the fungus and others quickly succumb to disease. Amphibians in populations that co-exist with the B. dendrobatidis have sublethal infections on their skins. Symbiotic skin bacteria have been shown in experiments and surveys to play a role in protecting amphibians from chytridiomycosis. Little is known about the mechanisms that antifungal skin bacteria use to ameliorate the effects of B. dendrobatidis. In this study, we identified that B. dendrobatidis isolate JEL 310 zoospores display chemotaxis, in the presence of two bacterially-produced metabolites (2,4-diacetylphloroglucinol and indole-3-carboxaldehyde). In the presence of either metabolite, B. dendrobatidis zoospores move more frequently away from the metabolite. Using parameters estimated from this study, a simple stochastic model of a random walk on a lattice was evaluated. The model shows that these individual behaviors over short time-scales directly lead to population behaviors over long time-scales, such that most zoospores will escape, or not infect a tryptone substrate containing the bacterially-produced metabolite, whereas many zoospores will infect the tryptone substrate containing no metabolite. These results suggest that amphibians that have skin bacteria produce antifungal metabolites that might be able to keep B. dendrobatidis infections below the lethal threshold and thus are able to co-exist with the fungus.