Effects of Timing of Microbial Exposure on Microbiome Assembly and Amphibian Immune Development.

Critical periods of development are time points that are especially sensitive to disruptions. Critical periods are important for microbiome assembly and the development of an effective immune system and, therefore, can have large and lasting impacts on host health, even through later life stages. Here, we investigated how a disruption to the microbiome of amphibians (treatment with a cocktail of six antibiotics and one antifungal compound) during early life and subsequent introduction to microbes at different developmental stages influences microbiome assembly and the development of the immune system (lymphoid tissues thymus and spleen). We found that antimicrobial treatments and introduction to microbes altered microbiome assembly (total microbial richness, antifungal microbial richness, composition, and relative abundances) and these changes were dependent on the timing of microbial introduction. Tadpoles treated with antimicrobials and then introduced to microbes at different developmental stages also had higher scaled abundances of bacteria in the phylum Actinobacteriota. However, after 7.5 weeks of tadpole development, we found no effects of treatment on lymphoid organ (thymus and spleen) size or on lymphoid cell counts. Overall, these results suggest that a disruption to the microbiome during early development, and more specifically, the length of the disruption and timing of reintroduction to microbes, can have significant impacts on microbiome assembly, potentially leading to long term impacts on host health.