Halobacteriovorax halts disease progression in endangered Caribbean corals.

Predation is a top-down regulator of ecosystem integrity and a key driver of community structure and evolution in plants and animals. Despite our awareness of these dynamics, our understanding of microbial top-down control by bacterial predators remains limited. Predatory Halobacteriovorax bacteria are common, low abundance members of many marine and estuarine microbiomes and are considered generalists with less specific prey ranges than most viruses, yet more selective targets than antibiotics. This "Goldilocks" prey range has huge potential to treat polymicrobial infections, particularly in complex microbiomes; however, few studies employing Halobacteriovorax as a tool to manipulate dysbiotic microbiomes have been pursued. We developed a single-pathogen disease mitigation model in the critically endangered Caribbean coral, Acropora cervicornis. We employed a strain of the highly versatile Vibrio coralliilyticus as our pathogen, which causes rapid tissue loss and death in stony corals and mortality in oyster larvae. To demonstrate that predatory bacteria can alter disease dynamics in corals we infected A. cervicornis with virulent V. coralliilyticus and upon the first signs of disease, treated corals with Halobacteriovorax cultures. Without predators, 100% of corals were bleached by 48 h and 86% displayed tissue loss within five days; however with Halobacteriovorax, 57% of corals did not bleach beyond the inoculation site and no tissue loss was observed. This living probiotic successfully halted Vibrio-induced disease progression in A. cervicornis, suggesting that predatory bacteria broadly function as top-down regulators of community dynamics in eukaryotic microbiomes and microbial predators are a promising coral disease therapy.