Skin Microbiome Variation Among Hosts and Batrachochytrium dendrobatidis Infection in Hyla meridionalis and Pelophylax perezi.

Chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis (Bd), is a key driver of global amphibian declines. The amphibian skin microbiome, which may include Bd-inhibitory bacteria, plays a crucial role in defence against infection, influencing host susceptibility and disease outcome. In this study, we investigated the skin microbiota and Bd infection in two frog species, Hyla meridionalis (Hm) and Pelophylax perezi (Pp), from northeastern Spain using full-length 16S rRNA gene sequencing. We found that microbiota composition differed significantly between frog species, with Pp harbouring greater bacterial richness and a community composition more similar to the aquatic environment than Hm. Asymptomatic Bd infection did not significantly alter microbiota diversity or community composition in either species. Nonetheless, differential abundance analyses revealed distinct bacterial taxa associated with host species and, to a lesser extent, with Bd infection. The composition of putative Bd-inhibitory bacteria also differed between frog species but was not influenced by Bd, suggesting that the microbiome's protective role may not be straightforward. Lower pathogen loads in Pp suggest a potential link between microbial richness and disease resistance. Our findings indicate that amphibian skin microbiota do not necessarily shift in response to Bd infection in asymptomatic hosts but may play a role in species-specific mechanisms of tolerance and resistance. Additionally, we detected potentially pathogenic bacteria of public and animal health concern on amphibian skin, highlighting amphibians as potential reservoirs and sentinels of ecosystem and public health. Overall, our findings indicate that amphibian skin microbiomes are shaped primarily by host species identity rather than asymptomatic Bd infection, emphasizing the importance of baseline microbiome variation in understanding host-pathogen dynamics and informing probiotic bioaugmentation strategies.