Culture and Isolation of Bacteria Associated with Mediterranean Corals

Abstract Corals harbor a diverse bacterial community that facilitates adaptation and sustains their health. In coral holobiont research, culture-independent approaches have transformed the existing paradigm. Molecular techniques, such as metabarcoding, revealed a high diversity of previously unrecognized bacterial symbionts. Coral microbiota characterization has relied on these techniques over the last decade, but relying solely on them does not provide a detailed understanding of the dynamics of the coral holobiont complex. Returning to classic microbiological methods and in vitro experimentation can yield novel insights into symbiont roles, physiology, and interactions within the holobiont. Under this premise, we aimed to isolate and culture bacteria from four Mediterranean coral species: Eunicella singularis , Cladocora caespitosa , Leptogorgia sarmentosa , and Paramuricea clavata . The recovery of 84 pure bacterial isolates and their initial classification based on the 16S rRNA gene revealed a diverse community of culturable coral-associated bacteria spanning four classes and 20 genera. Among these, six isolates may represent novel taxa (< 97% 16S rRNA identity to their closest described species) within Vibrio , Pseudoalteromonas , Alteromonas , Shewanella and Tritonibacter , underscoring the value of culture-based studies. The cultured bacterial diversity included taxa exhibiting traits potentially involved in host association, biofilm formation, surface colonization, and degradation of complex polysaccharides, suggesting functional roles in coral colonization and holobiont maintenance. Establishing this bacterial culture collection provides a foundation for investigating bacterial roles within the coral holobiont through detailed morphological and physiological characterization and experimental ecology studies.