Inhibitory bacteria reduce fungi on early life stages of endangered Colorado boreal toads (Anaxyrus boreas)

Abstract

Increasingly, host-associated microbiota are recognized to mediate pathogen establishment, providing new ecological perspectives on health and disease. Amphibian skin-associated microbiota interact with the fungal pathogen, Batrachochytrium dendrobatidis (Bd), but little is known about microbial turnover during host development and associations with host immune function. We surveyed skin microbiota of Colorado's endangered boreal toads (Anaxyrus boreas), sampling 181 toads across four life stages (tadpoles, metamorphs, subadults and adults). Our goals were to (1) understand variation in microbial community structure among individuals and sites, (2) characterize shifts in communities during development and (3) examine the prevalence and abundance of known Bd-inhibitory bacteria. We used high-throughput 16S and 18S rRNA gene sequencing (Illumina MiSeq) to characterize bacteria and microeukaryotes, respectively. Life stage had the largest effect on the toad skin microbial community, and site and Bd presence also contributed. Proteobacteria dominated tadpole microbial communities, but were later replaced by Actinobacteria. Microeukaryotes on tadpoles were dominated by the classes Alveolata and Stramenopiles, while fungal groups replaced these groups after metamorphosis. Using a novel database of Bd-inhibitory bacteria, we found fewer Bd-inhibitory bacteria in post-metamorphic stages correlated with increased skin fungi, suggesting that bacteria have a strong role in early developmental stages and reduce skinassociated fungi.

Antifungal Bacteria on Woodland Salamander Skin Exhibit High Taxonomic Diversity and Geographic Variability

“…dendrobatidis bacteria likely have general antimicrobial properties (52) and may provide a general defensive benefit against pathogens to the host (9,13,23,42). Therefore, our results may also be useful if the deadly, closely related fungal pathogen Batrachochytrium salamandrivorans (43) reaches this salamander biodiversity hot spot (44).…”

“…An active line of research has been to identify bacteria that inhibit B. dendrobatidis growth, here referred to as anti-B. dendrobatidis bacteria, and to use their geographic distribution to predict fungal disease outcome in the field (11,22,23) or to use them in trials of efficiency of bioaugmentation to mitigate B. dendrobatidis-associated disease symptoms (24)(25)(26)(27). To date, roughly 255 anti-B.…”

Mitigating amphibian chytridiomycoses in nature

“…Since then bacteria that inhibit Bd have been isolated from amphibians from across the Americas, Africa, Europe and Australia. Field studies of amphibian microbiomes indicate that the bacterial community on amphibian skin changes with amphibian life-history stage, with fewer Bd-inhibitory species in later life stages, suggesting that targets for field intervention may be age-specific [34]. An expanding research programme is underway to ascertain if resistance to or limitation of infection can be enhanced by augmenting amphibian skin microbiomes with inhibitory bacteria.…”

Direct and Indirect Horizontal Transmission of the Antifungal Probiotic Bacterium Janthinobacterium lividum on Green Frog (Lithobates clamitans) Tadpoles

“…Larval skin communities differ from their aquatic environment (45) and are generally dominated by one taxon (45,46,70). Species like Ambystoma tigrinum, Pseudacris triseriata, and Bufo boreas boreas are dominated by Betaproteobacteria (46,70), whereas Rana cascadae tadpoles are dominated by Pseudomonas (45).…”

“…Colonization by a probiotic bacterium in the larval stage may be more effective than colonization in the adult stage, since the larval skin community has a lower diversity than the adult stage (45,46), and the host immune system is not yet fully developed (15). We hypothesized that transmission of the probiotic would be greatest when both direct and indirect horizontal transmission (DIH) was possible.…”