Dynamic interactions within the host-associated microbiota cause tumor formation in the basal metazoan Hydra.

Author summary Here we follow up on our initial observation of tumor formation in the basal metazoan Hydra and demonstrate that tumor development in one of the evolutionary oldest animals is caused by a dynamic interplay between an environmental spirochete, the host-associated resident microbiota, and the tissue homeostasis within the animal. Unexpectedly, the pathogenicity of the environmental bacterium Turneriella is context-dependent: the virulence of this pathogen requires the presence of a member of Hydra's beneficial microbiome-the Pseudomonas bacterium. Dynamic interactions between two microbiota members have profound effects onto the host tissue homeostasis and fitness. Our data provide direct evidence for the important role of the resident microbiome in maintaining tissue homeostasis and pathogen defense, a fundamental process that is likely to take place in every tissue of every animal species. In summary, our study uncovers an evolutionary conserved role of the resident microbiome in guarding host's tissue homeostasis. The extent to which disturbances in the resident microbiota can compromise an animal's health is poorly understood. Hydra is one of the evolutionary oldest animals with naturally occurring tumors. Here, we found a causal relationship between an environmental spirochete (Turneriella spec.) and tumorigenesis in Hydra. Unexpectedly, virulence of this pathogen requires the presence of Pseudomonas spec., a member of Hydra ' s beneficial microbiome indicating that dynamic interactions between a resident bacterium and a pathogen cause tumor formation. The observation points to the crucial role of commensal bacteria in maintaining tissue homeostasis and adds support to the view that microbial community interactions are essential for disease. These findings in an organism that shares deep evolutionary connections with all animals have implications for our understanding of cancer.