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.

人们对常驻微生物群的扰动会损害动物健康的程度了解甚少。九头蛇是具有自然发生的肿瘤的进化最古老的动物之一。在这里，我们发现了一种环境螺旋体（Turneriella spec。）与九头蛇中肿瘤发生之间的因果关系。出乎意料的是，这种病原体的毒力要求存在九头蛇属的假单胞菌。有益微生物组，表明常驻细菌与病原体之间的动态相互作用会导致肿瘤形成。该观察结果指出了共生细菌在维持组织动态平衡中的关键作用，并为微生物群落相互作用对于疾病必不可少的观点提供了支持。与所有动物有着深厚的进化联系的生物体中的这些发现对我们对癌症的理解具有影响。