Beyond a single species: Mapping virulence traits across the redefined Fusobacterium nucleatum complex

Fusobacterium nucleatum was long regarded as a single species and later subdivided into four subspecies (nucleatum, polymorphum, animalis, vincentii/fusiforme). In 2022, these subspecies were validated as separate species and further members of the F. nucleatum complex have been proposed (F. watanabei, F. paranimalis sp. nov.). Given the increasing evidence linking F. nucleatum to various diseases, identifying (sub-)species-specific virulence factors has become essential. Infection in mammalian hosts depends on virulence factors that can be surface-exposed, released into the extracellular environment, or injected directly into host cells. This narrative review aims to address the different pathogenic potentials of each former subspecies. These differences range from adhesin diversity and metabolic adaptations, the repertoire of ABC transporters, lyases and type IV conjugative pili to the capability to invade tissues, evade the immune system and form biofilms and outer membrane vesicles.