Vibrio coralliilyticus induction of virulence toward larval oysters and corals at elevated seawater temperatures and potential mitigation strategies

Authors: Richards, Gary; USHIJIMA, BLAKE - Smithsonian Marine Station

Submitted to: Proceedings of US-Japan Natural Resources Panel on Aquaculture
Publication Type: Proceedings
Publication Acceptance Date: 5/20/2021
Publication Date: 11/17/2021
Citation: Richards, G.P., Ushijima, B. 2021. Vibrio coralliilyticus induction of virulence toward larval oysters and corals at elevated seawater temperatures and potential mitigation strategies. Proceedings of US-Japan Natural Resources Panel on Aquaculture. p. 104-110.

Interpretive Summary:

Technical Abstract: Recent research has advanced our understanding of the role pathogenic vibrios play in disease of various aquaculture species. Vibrio coralliilyticus, formerly thought to infect just corals, is now known to infect larval oysters, causing major losses in hatchery settings. The negative effect of V. coralliilyticus on wild-type oyster production remains uncertain, but circumstantial evidence suggests it may be substantial. We identified eight strains of V. coralliilyticus that infect both Eastern oyster (Crassostrea virginica) larvae and Pacific oyster (Crassostrea gigas) larvae in U.S. East and Pacific coast hatcheries. West Coast hatcheries have experienced major production losses over the years, due to the presence of V. coralliilyticus. Some of the largest hatcheries have reported losses of up to 80%, resulting in shortages in seed oysters needed for commercial oyster production. Losses can exceed 10 billion larvae per year in the larger hatcheries. Most U.S. East Coast hatcheries are considerably smaller, but also experience losses in Eastern oysters, possibly from V. coralliilyticus, but also from the known shellfish pathogen V. tubiashii. Interestingly, V. tubiashii is only able to infect Eastern oyster larvae, unlike V. coralliilyticus that causes acute mortalities with Eastern and Pacific species. The infection and death of corals by V. coralliilyticus is known to be enhanced at seawater temperatures greater than or equal to 27°C. In our studies, multiple strains of V. coralliilyticus produced high larval oyster mortalities with LD50’s ranging from 3.8 x 10e3 to 4 x 10e4 CFU/ml of seawater, depending on the V. coralliilyticus strain and the oyster species (Eastern vs. Pacific oysters). Studies with knockout mutations demonstrated that the transcriptional regulator ToxR and the outer membrane protein OmpU were important in larval oyster (and coral) infections caused by V. coralliilyticus. Gross pathological changes occur in V. coralliilyticus-infected larvae, initially in the velum and cilia and then in internal organs which become liquefied. Infection of larval oysters is likely induced by stress and a lowering of resistance to these pathogens. Stressors, particularly in hatcheries, likely include incorrectly adjusted seawater temperatures, salinities, pHs, and dissolved oxygen levels or inadequate nutrition, over stocking, high overall bacterial loads, etc. To begin to mitigate vibrios in hatcheries, we identified a variety of predatory bacteria which in nature, help to modulate Vibrio levels in seawater and/or shellfish. They include several predatory bacteria such as Pseudoalteromonas piscicida, which secrete digestive enzymes that inhibit and kill many kinds of bacteria including vibrios; and Halobacteriovorax, a Bdellovibrio and Like Organism (BALO), which infect Gram-negative bacteria, like vibrios, and replicate within them, killing the vibrios in the process. Together these bacteria may serve as probiotics in reducing hatchery mortalities and in killing vibrios and other bacterial pathogens in a variety of other aquaculture settings. Bacteriophages (phages) are another means to inactivate many undesirable pathogens in the environment and are increasingly being used in aquaculture. To date, we isolated multiple phages against V. coralliilyticus and V. tubiashii and showed the practical application of phage therapy to reduce or eliminate mortalities from V. coralliilyticus and V. tubiashii in larval oysters.