Schistosome species, parasite development, and co-infection combinations determine microbiome dynamics in the snail Biomphalaria glabrata.

Background: Schistosomiasis is a snail-borne disease affecting over 200 million people worldwide. Despite dedicated control efforts and effective diagnostic tools, schistosomiasis remains prevalent. Novel and sustainable control measures are urgently needed. Bacteria might offer such a solution as links between bacteria, disease resistance and transmission potential of intermediate hosts have been established in other systems. To better understand the tripartite interaction potentially driving snail-schistosome compatibility patterns, microbial communities must be investigated throughout and across various parasite exposure conditions. Therefore, we studied Biomphalaria glabrata snails exposed to a high- and low-shedder population of Schistosoma mansoni and Schistosoma rodhaini in single and co-exposure experiments. Snails were sacrificed at different time points post-exposure and their bacterial communities and trematode (co-)infection status were determined through metabarcoding tools.

Results: Snails infected by low- and high-shedder S. mansoni populations were more likely to have bacterial community dysbiosis than those infected by S. rodhaini but this was also affected by miracidial load. Moreover, the single-infection hierarchical effect on the bacterial component of the microbiome is not maintained under co-infection with S. rodhaini, which appears to stabilize the snail's bacterial profile even after being outcompeted by high-shedder S. mansoni. Finally, alpha diversity differed significantly between infected and uninfected snails around the onset period of shedding at 30 days post-miracidial exposure.

Conclusion: The timing of this bacterial shift suggests an intricate parasite-snail interaction around key parasite development moments. Future studies investigating the tripartite interaction are advised to consider the effect of outcompeted or prepatent infections on the snail's microbiome.