Extracellular polymeric substances drive Curvibacter host adaptation and cospeciation with Hydra

Abstract The interactions between hosts and their microbial symbionts play a crucial role in shaping biological diversity and ecosystem function. Bacteria can adapt to specific host environments over evolutionary timescales, leading to co-speciation and the formation of specialized host-microbe relationships. Understanding the molecular mechanisms underlying these adaptations provides key insights into the evolution of symbiosis and the stability of microbial communities. This study investigates the co-speciation and molecular adaptations of Curvibacter to its host Hydra , a well-established model for the study of host-microbe interactions. We provide strong evidence of co-speciation, as demonstrated by phylogenetic congruence between different Hydra species and their corresponding Curvibacter symbionts, along with preferential recolonization of germ-free Hydra by their native Curvibacter strains. Comparative genomic analyses reveal that host-associated Curvibacter strains exhibit distinct metabolic and biosynthetic adaptations compared to their free-living relatives. Specifically, the enrichment of proteins involved in sugar metabolism and transport, as well as the selective purification of proteins linked to macromolecule biosynthesis, highlights the specialization of Curvibacter symbionts within the Hydra glycocalyx. Functional experiments identify a symbiont-specific extracellular polymeric substances (EPS) operon as key factor for microbial adhesion and host colonization, underscoring its role in facilitating symbiont specificity and stability. These findings provide insights into the molecular mechanisms driving host-microbe co-evolution and highlight the evolutionary forces shaping microbial specialization within host-symbiont relationships. Significant Statement The vast majority of animal and plant species are associated with microbial organisms that offer a wide range of benefits for development, physiology, and health. Understanding the evolutionary forces that shape these symbiotic interactions is crucial for elucidating the mechanisms through which organisms mitigate environmental stress and enhance their resilience, particularly in response to rapidly changing environmental conditions. Using advanced bioinformatic, bioanalytical, and genetic approaches, this study demonstrates that Curvibacter symbionts have evolved specific extracellular polymeric substances to enhance their ability to form a specific symbiosis with Hydra .