Spatial organization of cutaneous microbiomes reveals putative microbial contributions to host chemical defenses in the American toad.

Chemical defenses are widely evolved throughout the tree of life. Animals can exploit mutualisms with toxin-producing symbionts as a mechanism of chemical defense. However, this has only begun to be explored in depth, and how these mutualisms may relate to how animals synthesize or acquire their toxins has been even less studied. True toads synthesize their own toxins and offer a novel system to study the interplay between the cutaneous skin microbiome and how it may contribute to toxin synthesis or biotransformation. In this study, we investigated whether the cutaneous microbiome of the American toad (Anaxyrus americanus) was spatially structured across body surfaces in relation to toxin storage and secretion and assessed whether microbial communities exhibit distinctive bacterial taxa involved in toxin-related biochemical pathways. To do this, we used 16S rRNA gene sequencing, diversity metrics, differential abundance comparisons, functional pathway predictions, and ecological interaction networks. Our results indicate that the dorsal and ventral cutaneous surfaces harbor distinct bacterial assemblages, with the dorsal surface being enriched for bacterial taxa associated with the predicted potential to degrade or transform structurally complex organic compounds. This study provides insights into how the toad skin microbiome may contribute to the chemical defenses of toads and could reveal novel aspects of host-microbiome interactions in amphibians.