The Coral Host Microbiome Modulates the Virulence of the Bacterial Pathogen <scp> <i>Vibrio coralliilyticus</i> </scp>

Coral disease outbreaks pose a major threat to reef ecosystems, often leading to widespread mortality and declines in coral cover. A key factor predicted in disease susceptibility is the coral microbiome, which is thought to protect corals from pathogens like Vibrio coralliilyticus. However, this protective function has not been empirically demonstrated with live coral, nor is it well understood how these microbiome-pathogen interactions contribute to the observed variation in virulence among different V. coralliilyticus strains. This study investigated the role of the Hawaiian rice coral (Montipora capitata) microbiome in susceptibility to infection by endemic and non-endemic strains of V. coralliilyticus. Laboratory infection experiments revealed that antibiotic-induced dysbiosis generally increased host susceptibility, though infection rates varied between strains. Notably, the type of M. capitata colour morph did not influence infection outcomes, and induced dysbiosis did not affect susceptibility to another pathogen, Pseudoalteromonas piratica. Additionally, the quorum sensing genes vcpR and aphA were examined for their roles in V. coralliilyticus pathogenicity in relation to the host microbiome. These findings underscore the protective function of the coral microbiome and highlight the complexity of host-pathogen interactions, contributing to a deeper understanding of coral disease dynamics and informing future mitigation strategies.