Impacts of chytridomycosis on microbial communities associated with Hellbender skin

Background/Question/Methods

Host-associated microbial communities might have important functions, including mutualistic defense against pathogens and other enemies. On the other hand, pathogens can disrupt normal host-microbe interactions, drive community reorganization, and potentially leave hosts more vulnerable to secondary infections. The pathogen Batrachochytrium dendrobatidis (Bd) is a major conservation concern because it is associated with amphibian declines worldwide. Bd infects the skin, and recent research has attempted to find beneficial skin symbionts that might be used as probiotics. Other work has found that Bd infection alters skin-associated communities, raising doubts about the resilience (and potential defensive capabilities) of normal host-associated flora. Here we evaluated effects of Bd infection on the microbial ecology of Hellbender (Cryptobranchus alleganiensis) skin. We used plate counts and next generation sequencing of 16s rRNA amplicons to compare the abundance and diversity of bacteria from infected and uninfected Hellbenders to environmental samples. We compared the antibacterial function of slime from infected and uninfected Hellbender skin using mixed plating experiments.

Results/Conclusions

Uninfected Hellbenders supported about 10x fewer viable bacteria (colony-forming-units in R2A medium) than the surfaces of rocks in the same habitat. The composition of bacterial taxa (phylotypes at 97% similarity) was similar, but relative abundances differed significantly, confirming that normal Hellbender skin does not simply contain a random sample of bacteria from the environment. Samples from the skin of Bd-infected Hellbenders closely resembled environmental samples from rocks in both the total abundance and relative abundance of bacteria. Mixed plating assays indicated that slime from uninfected Hellbender skin generally inhibits bacterial growth, but this effect is lost or reduced in Bd-infected individuals. It appears that Bd infection reduces the selective properties of normal Hellbender skin, leaving the salamanders more subject to colonization by environmental microbes. These impacts have the potential to increase vulnerability to secondary infections of the skin and might interfere with mothers’ ability to use skin secretions to protect their eggs from natural microbial enemies.