A coral peptide has bactericidal activity against a global marine pathogen, Vibrio coralliilyticus

Scleractinian corals and their associated microorganisms, including endosymbiotic dinoflagellates and bacteria, constitute coral holobionts. Rising seawater temperatures weaken coral symbiotic relationships, thereby increasing susceptibility to infectious diseases and leading to disease outbreaks and subsequent population declines. The temperature-dependent coral pathogen, Vibrio coralliilyticus, poses one of the greatest threats to coral reefs because of global warming. However, coral immune defenses against this pathogen are poorly understood. We previously identified coral genes responding to V. coralliilyticus exposure by transcriptomic analysis of the reef-building coral, Acropora digitifera. Here, bioinformatic analysis identified digitiferin, a coral antimicrobial peptide (AMP), in the genome of A. digitifera. Recombinant digitiferin showed antibacterial activity against both Gram-positive and -negative bacteria. A 3D structural model and direct microscopic observations revealed that digitiferin damaged bacterial cell membranes by forming pores; however, initial growth inhibition tests in 1.5 % (w/v) NaCl showed no effect on pathogens. Because we found that digitiferin is secreted from epithelial cells into mucus and shows salt sensitivity, we hypothesized that it is active against pathogens in low-salt environments. We then investigated whether it showed antibacterial activity against pathogens in low-salt, moderate-osmolality conditions, using mannitol for osmoregulation instead of NaCl. The results showed that digitiferin exhibits bactericidal activity against V. coralliilyticus under salt-free conditions. To the best of our knowledge, this is the first study to characterize a scleractinian AMP that kills V. coralliilyticus. These findings contribute to a better understanding of coral immunity and may facilitate development of techniques to overcome coral Vibrio disease. SignificanceHealthy coral reefs are one of the most important marine ecosystems, supporting [~]25 % of all marine organisms. Coral reef ecosystems are threatened by coral infectious diseases, as rising seawater temperatures exacerbate pathogen infectivity. Thus, understanding coral immune systems is becoming increasingly important to protect coral reefs. This study reports the discovery of an antimicrobial peptide, digitiferin, from the reef-building coral, Acropora digitifera. Digitiferin is effective against Gram-positive and -negative bacteria, including the major coral pathogen, Vibrio coralliilyticus, and is secreted into mucus from ectodermal granular epithelial cells. As the first antimicrobial peptide known to have bactericidal activity against V. coralliilyticus, this discovery enhances our understanding of coral immunology and coral-pathogen interactions.