Shifts in amphibian symbiotic bacterial communities are linked to epidemic spread of the fungal pathogen Batrachochytrium dendrobatidis

Background/Question/Methods

Microbial communities inhabiting metazoans are increasingly recognized to play important roles in disease resistance. Resident microflora can affect disease resistance by direct or host-mediated interactions with pathogens. Conversely, infection by certain pathogens can disturb resident microbial communities and alter their composition and function. We investigated the interaction between symbiotic bacterial communities inhabiting amphibian skin and the fungal pathogen Batrachochytrium dendrobatidis, the etiological agent of amphibian chytridiomycosis. We tested for associations between bacterial community composition and B. dendrobatidis infection in wild populations of a Sierra Nevada endemic frog, Rana sierrae. We present data from field surveys in which we simultaneously sampled skin-symbiotic microbial communities and B. dendrobatidis infection status from individual frogs from multiple study sites. Our sites span populations that vary in the presence and prevalence of B. dendrobatidis infection, as well as one population that transitioned from the uninfected to the epidemic state during the study period. We quantified B. dendrobatidis infection of each frog with pathogen-specific quantitative PCR, and characterized bacterial community similarity among frogs by fragment analysis and pyrosequencing of the 16S rRNA gene. We tested for correlations between bacterial community composition and pathogen infection intensity.

Results/Conclusions

Across multiple R. sierrae populations, the composition of bacterial communities on individual frogs was correlated with the intensity of B. dendrobatidis infection. Multivariate community similarity, as well as the relative abundances of several individual bacterial taxa, were strongly correlated with the severity of fungal infection. Furthermore, data from a frog population that experienced a B. dendrobatidis invasion and resulting epidemic and population crash reveal that frog-associated microbial communities undergo compositional shifts during the course of a B. dendrobatidis epidemic. Continuing research is experimentally investigating the causal relationships underlying these correlations in an effort to clarify whether pre-existing differences in bacterial communities affect host resistance to the pathogen, or conversely whether invasion by the pathogen alters the resident bacterial community.