The coral pathogen Vibrio coralliilyticus kills non-pathogenic holobiont competitors by triggering prophage induction

Competition among species in the coral microbiome has important outcomes in terms of coral health, but little is known about the mechanisms allowing pathogens to gain a competitive advantage. Here the authors show that the pathogen Vibrio coralliilyticus outcompetes commensal bacteria by inducing prophages. The coral reef microbiome is central to reef health and resilience. Competitive interactions between opportunistic coral pathogens and other commensal microbes affect the health of coral. Despite great advances over the years in sequencing-based microbial profiling of healthy and diseased coral, the molecular mechanism underlying colonization competition has been much less explored. In this study, by examining the culturable bacteria inhabiting the gastric cavity of healthy Galaxea fascicularis, a scleractinian coral, we found that temperate phages played a major role in mediating colonization competition in the coral microbiota. Specifically, the non-toxigenic Vibrio sp. inhabiting the healthy coral had a much higher colonization capacity than the coral pathogen Vibrio coralliilyticus, yet this advantage was diminished by the latter killing the former. Pathogen-encoded LodAB, which produces hydrogen peroxide, triggers the lytic cycle of prophage in the non-toxicogenic Vibrio sp. Importantly, V. coralliilyticus could outcompete other coral symbiotic bacteria (for example, Endozoicomonas sp.) through LodAB-dependent prophage induction. Overall, we reveal that LodAB can be used by pathogens as an important weapon to gain a competitive advantage over lysogenic competitors when colonizing corals.

Automated summary

Competition in the coral microbiome plays a significant role in coral health, and a study has found that a pathogen called Vibrio coralliilyticus outcompetes commensal bacteria by inducing prophages. This discovery reveals the molecular mechanism underlying colonization competition in corals and highlights the importance of certain genes used by pathogens to gain a competitive advantage.