Not so sterile after all: The endomicrobiome of plerocercoids of the cestode parasiteSchistocephalus solidusand changes to the microbiome of its Threespine Stickleback host

Despite the growing recognition of the role of bacteria in animal biology, the microbiome of parasites remains largely unexplored. In particular, the presence of bacteria in tapeworms has never been investigated and parasites that exit the intestine would be considered sterile. We characterized for the first time the microbiome of a tapeworm. Schistocephalus solidus plerocercoids, collected from the body cavity of its stickleback host, were found to harbor a complex microbiome. The most abundant and the most prevalent bacteria was Polynucleobacter sp .. In addition, S. solidus infection was associated significant changes in the stickleback host gut microbiome with an increase in microbial load and changes in diversity and composition. Finally, the same bacteria were often found in S. solidus and the stomach and intestine of the corresponding hosts, a result that highlights the importance of characterizing the microbiome of host tissues and parasites from the same individuals to assess the potential for horizontal transmission of microbes. This study clearly emphasizes the need for further characterization of the microbiome of a broad range of parasites and for studies to determine the ecological, evolutionary and functional role that microbes play in host-parasite interactions.