Within and between pond variation in symbiotic microbial assemblages on the skin of bullfrogs and eastern newts

Background/Question/Methods

Amphibians host diverse microbial assemblages on their skin, and some of these microbes may play a role in disease resistance. The fungus Batrachochytrium dendrobatidis (Bd) infects amphibian skin, and is responsible for many global amphibian extinctions. Recent research examining the role of cutaneous microbial symbionts in preventing Bd infection is becoming increasingly common.  However, little is known about variation in amphibian skin microbiota at the individual and species levels, or whether these skin microbes are simply environmental isolates or if they represent unique host symbionts. In a single pond, we examined the bacterial community structure of juvenile bullfrogs, Rana catesbeiana (N=12), and co-habiting adult eastern newts, Notophthalmus viridescens (N=10), and how their communities relate to those of their pond environment (water, N=3; substrate, N=3). In addition, to assess population-level variation in microbiota, newts (N=10) from two additional sites were also sampled. Barcoded 454 pyrosequencing of the 16S rRNA gene was used to assess the microbial communities.

Results/Conclusions

Based on a 97% similarity cut-off for OTU delineation, bullfrogs had five OTUs that were present on >90% of individuals at the pond, representing the core microbial community. For newts the core microbial community consisted of 8, 12, and 16 OTUs at each of the three sites respectively. Based on Principal Component Analysis (PCA), the microbial communities of bullfrogs and newts at the same pond clustered separately. However, there was some overlap in particular OTUs across species and sites. Two core OTUs (in the families Sanguibacteraceae and Pseudomonadaceae) were present on both bullfrogs and newts in the same pond, and were also present on newts at all three sites. Our results suggest that amphibian species host unique assemblages of bacteria, but that some of the dominant members appear to be common across species. At the pond where we sampled both amphibian species and the substrate and water, there was only a single OTU that was present in the core microbiota of the amphibians and in the substrate samples.  There were no core OTUs that overlapped with the water samples. This suggests that amphibian skin is not simply colonized by bacteria from their environment.