Are amphibian skin associated microbial communities regulated by the host, the external environment, or both?

Background/Question/Methods

Skin associated microorganisms and their multicellular hosts interact in numerous ways; however, it remains unclear if composition of skin-associated microbial communities is driven primarily by the host or by the external environment. Amphibians are an excellent model system to examine microbial community regulation because amphibians undergo large physiological and environmental changes between life stages, both adults and larvae harbor diverse skin associated microbial communities, and it is unclear whether amphibian skin associated microbial communities are shared or differ across life stages and environments. Two key environmental variables of the pre and post-metamorphic amphibian environment, pH and canopy cover, are known to affect amphibian survival, growth, and development, yet it is unknown if these environmental variables affect skin microbial communities. Using the Bullfrog (Rana catesbeiana) as our model, we tested if small changes to larval habitat pH (7 to 6) and presence or absence of canopy cover could affect microbial communities of Bullfrog skin without altering Bullfrog survival, growth or development.  Further, we tested if microbial community structure changed with ontogeny.  Microbial community analyses were conducted utilizing a terminal restriction fragment length polymorphism technique coupled to sequencing of the 16S rDNA bacterial gene region and analyzed with nonmetric-multidimensional scaling.

Results/Conclusions

In agreement with previous studies, canopy cover significantly altered time to metamorphosis; but surprisingly juvenile mass was not affected in our study. Additionally, canopy cover did not significantly affect microbial community structure; but in contrast, pH  significantly altered the structure of the microbial community living on the larvae’s skin, without affecting time to metamorphosis or juvenile mass. Larval microbial communities differed significantly from post-metamorphic juvenile microbial communities, although there were no effects of larval environment on post-metamorphic skin community composition.   Clone library comparisons highlighted large differences in skin associated microbiota between larvae and post-metamorphic juveniles with a notable shift from a Bacteriodetes dominated (73%) larval flora to a Betaproteobacteria dominated (83%) flora in juveniles. Genera of both Bacteriodetes and Betaproteobacteria have been found to inhibit Bd (Batrachochytrium dendrobatidis), a globally distributed pathogen known as a leading cause of amphibian declines. Post-metamorphic traits of the host (i.e. antimicrobial peptides) may exert differential regulating pressures on these taxa, potentially explaining the difference between microbial communities across developmental stages. Together, our results indicate amphibian skin associated microbial community structure can be influenced by both the host (ontogenetically) and the external environment (in this case, a small change in habitat pH).