Cutaneous microbiota as an innate immune defense in amphibians

Background/Question/Methods   As infectious diseases emerge at an increasing rate, they threaten global biodiversity and human health. The disease chytridiomycosis, which is caused by the chytrid fungus, Batrachochytrium dendrobatidis (Bd), affects amphibians and is implicated in population declines and extinctions worldwide. Studying the microbial ecology of amphibian skin, specifically the interaction of the dispersing Bd zoospores and the resident microbial community, is critical for understanding and eventually controlling this disease. In vitro agar plate challenge assays, laboratory experiments, and field surveys suggest that amphibians' skin microbiota plays an important role in disease susceptibility. However, at this time, only a few species have been examined for these relationships. The objective of this study was to examine the role of skin microbiota in the innate immunity of amphibians. Specifically, we characterized the bacterial communities of several amphibian species and the ability of these bacteria to inhibit pathogen growth.

Results/Conclusions   A preliminary survey indicates a diverse skin microbiota on Eastern newts, bullfrogs, and pickerel frogs. In summer 2008, five juvenile bullfrogs and three pickerel frogs from a single pond were sampled for skin microbes capable of inhibiting Bd. Bullfrogs harbored an average of 20 distinct bacterial isolates, 10% of which had anti-Bd properties. Based on sequencing of the 16S rRNA gene, these anti-Bd isolates are related to bacteria in the following genera: Chryseobacterium, Cytophaga, Delftia, Haloanella, Pseudomonas, and Serratia. Eighty percent (4 of 5) of bullfrogs and 100% (3 of 3) of pickerel frogs had at least one anti-Bd isolate. In summer 2009, 16 bullfrogs and 30 Eastern newts from three sites were sampled for skin microbiota. The average number of bullfrog and newt isolates was 23 and 19, respectively, and these microbes are currently being assayed for anti-Bd activity. Investigating host-microbe interactions is important, as using anti-Bd bacteria to combat infection may be the only tool available for the in situ conservation of many at-risk amphibian populations.