Extracellular polymeric substances drive Curvibacter host adaptation and cospeciation with Hydra.

A symbiont-specific extracellular polymeric substances (EPS) operon enables Curvibacter host colonization in Hydra, supporting host-specific adaptation and stability of the symbiosis.

• Why it matters: Host–microbe coevolution depends on how microbes adapt to distinct host environments, but the molecular factors driving Curvibacter specialization within Hydra remain unclear.
• What they did: The study combined phylogenetic congruence across Hydra species with preferential recolonization of germ-free Hydra by native Curvibacter, then used comparative genomics and functional experiments to test EPS roles.
• The result: Host-associated Curvibacter strains show enriched pathways for host glycan interactions, EPS, DNA metabolism, ABC transport, sugar metabolism, and chemotaxis, with some signatures detectable only in the Hydra vulgaris-associated subgroup.

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