Vibrio coralliilyticus

by Megan Sawatzky and Emily Byrnes

Introduction

Vibrio coralliilyticus is a rod-shaped, Gram-negative bacterium found globally in the marine environment. Gram-negative bacteria have an outer lipid membrane that can help in resisting host and environmental defenses, thus making them more difficult to destroy. V. coralliilyticus is primarily pathogenic to several genera of tropical corals. Corals benefit human communities by supplying food and shelter for fish and other marine animals that are a principal source of food and income and providing coastal protection. Coral reefs act as a buffer between the open ocean and shorelines; without them, shorelines are at risk of erosion and coastal communities are vulnerable to storms.

V. coralliilyticus is a temperature-dependent, opportunistic pathogen. An opportunistic pathogen is one that, while harmless in healthy organisms, may be threatening to those that are weakened by other factors. For example, increasing sea temperatures decrease chlorophyll concentration in the coral’s algal symbionts, which subsequently limits the coral’s food supply and induces stress. This cascade of events makes the coral more susceptible to infection by V. coralliilyticus (Figure 1). V. coralliilyticus directly benefits from the warmer temperatures as well, as there is an upregulation in the pathogen’s motility, resistance, and virulence.

Figure 1. Factors contributing to increased host susceptibility to, and virulence of, V. coralliilyticus. Source: Megan Sawatzky, 2021.

Disease

V. coralliilyticus is associated with disease in a variety of marine organisms, including oyster and mussel larvae, rainbow trout, and most significantly, several genera of corals. The symptoms of disease vary depending on the species/organism that is infected. The disease progression in V. coralliilyticus infections has been principally studied in the coral, Pocillopora damicornis. In P. damicornis, V. coralliilyticus causes temperature-induced bleaching and tissue lysis. Bleaching is the disruption in the obligate symbiotic relationship between the coral and the dinoflagellates, i.e., algae, living in their tissues. This relationship can be damaged from environmental stress, or by a direct attack on the algae by V. coralliilyticus. The algae are the coral’s primary energy source, and when their relationship is damaged, it leaves the coral highly susceptible to other diseases.

When V. coralliilyticus enters the coral host, P. damicornis, it grows in the mucus and tissue. Next, bleaching occurs readily at temperatures between 25-29°C. Coral tissue lysis, promoted by extracellular bacterial proteases (enzymes that break down proteins), is apparent three to five days post-infection and reaches completion after two weeks at temperatures above 27°C. Bleaching and tissue lysis are slowed at temperatures below 25°C, and virulence is significantly impaired at temperatures below 22°C.

In other coral species, V. coralliilyticus is the primary cause of White syndrome (WS). WS, or White Band Disease (WBD), is a term used to describe a class of coral tissue loss diseases characterized by acute and/or rapid tissue lysis leaving the coral as a bare skeleton. In non-coral hosts, V. coralliilyticus infections cause widespread mortalities. V. coralliilyticus is a highly infectious bacterial pathogen that can be transmitted through direct contact between the tissue of an infected and uninfected host. In corals, once contact is made, the bacterium can enter through the mouth or tears in the tissue.

Epidemiology

V. coralliilyticus is commonly located throughout the Indo-Pacific Ocean, as well as the Red and Mediterranean Seas (Figure 2). In the Red Sea, V. coralliilyticus was first shown to infect the coral P. damicornis in 2002. Since then, the bacterium has been the causative agent of numerous outbreaks of bleaching in the species. In the fall of 2006, there was an outbreak in the North-west Mediterranean Sea where V. coralliilyticus caused tissue loss in the coral, Paramuricea clavata. The minimum infectious dose, which is the minimum number of bacteria needed to cause fifty percent tissue lysis in infected corals, was 104 bacterial cells per gram of tissue.

In the Indo-Pacific Ocean, major outbreaks have been reported in Kāne’ohe Bay, Hawai’i, and the West Coast of the United States. In Kāne’ohe Bay, Hawai’i, V. coralliilyticus was a prominent agent in an outbreak of Monitpora WS in the coral, Montipora capitata, from 2010 to 2011. The minimum infectious dose in this outbreak was between 107 and 108 bacterial cells per gram of tissue, however, this was exceeded in many of the infected coral. Outbreaks of V. coralliilyticus have also been observed in non-coral species, such as oysters, causing mass mortalities. In hatcheries on the West coast of the U.S, oyster larvae are frequently killed by this pathogen. Only hatcheries face this threat because V. coralliilyticus is only infectious to the oyster larva in their stages before they stop being free-swimming, which is only the first two to three weeks. In the past couple of years, two hatcheries on the West coast have lost over 80% of their larvae due to V. coralliilyticus.

Other outbreaks have been reported in Nelly Bay, Great Barrier Reef; Majuro Atoll, Republic of the Marshall Islands; and Nikko Bay, Republic of Palau, also all in the Indo-Pacific. Since this is a newly studied pathogen, outbreaks of V. coralliilyticus are still being discovered with different symptoms of infection.

Figure 2. Outbreaks of V. coralliilyticus and corresponding diseases. Source: iStock by Getty Images (map), Emily Byrnes, 2021 (legend).

Virulence

The virulence of a pathogen refers to its ability to cause damage and/or disease in the host; virulence factors are structures, molecules, and/or cell systems that help it do so. Very few virulence factors have been identified in V. coralliilyticus; those that have, demonstrate the temperature-dependent nature of the pathogen. A key temperature-dependent virulence factor of V. coralliilyticus is the flagellum. Flagella are “whip-like” surface structures used by motile bacteria to facilitate chemotaxis, adhesion, and invasion of host surfaces. Chemotaxis is cell/bacterium movement in response to the concentration of certain chemicals in the extracellular environment. V. coralliilyticus, and other Vibrio species, have a single flagellum at one end, known as a polar flagellum.

The first step in any bacterial infection is contact between the pathogen and the host. V. coralliilyticus uses its flagellum to follow a chemical gradient, i.e., move from areas of low to high chemical concentration, and facilitate contact with the coral host. Corals secrete copious amounts of mucus which covers their epidermis (outer “skin” layer) to help protect against surrounding pathogens. Ironically, the mucus of P. damicornis provides the chemical gradient that attracts V. coralliilyticus. With no flagellum, the bacterium is unable to adhere to/invade the coral host and cause disease.

Figure 3. V. coralliilyticus flagellum as a key virulence factor. Source: Megan Sawatzky, 2021.

Treatment

The main treatment plan currently being studied for organisms infected by V. coralliilyticus is bacteriophage, or phage, therapy. In bacteriophage therapy, specific viruses are used to kill a target bacterial pathogen. For V. coralliilyticus infected corals, bacteriophages stop tissue degradation and prevent the dissociation of the coral’s algal symbionts, thus preventing mortality. Many bacteriophages can infect V. coralliilyticus, and bacteriophages belonging to the Myoviridae family, including bacteriophage YC and vB_VcorM-GR28A, are the most effective.

Bacteriophage YC was discovered in Nelly Bay, Great Barrier Reef. It is an effective lytic phage in V. coralliilyticus, meaning it takes over the bacterium and destroys it. The other bacteriophage, vB_VcorM-GR28A, is a lytic phage that has been used for treatment in oyster larvae. Bacteriophage therapy is the main source of treatment against infections caused by V. coralliilyticus at this time.

References

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Ben-Haim, Y., Thompson, F. L., Thompson, C. C., Cnockaert, M. C., Hoste, B., Swings, J., & Rosenberg, E. (2003). Vibrio coralliilyticus sp. nov., a temperature-dependent pathogen of the coral Pocillopora damicornis. International Journal of Systematic and Evolutionary Microbiology, 53(1), 309-315. doi:https://doi.org/10.1099/ijs.0.02402-0

Ben-Haim, Y., Zicherman-Keren, M., & Rosenberg, E. (2003). Temperature-Regulated Bleaching and Lysis of the Coral Pocillopora damicornis by the Novel Pathogen Vibrio coralliilyticus. Applied and Environmental Microbiology, 69(7), 4236-4242. doi:10.1128/AEM.69.7.4236-4242.2003

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Richards, G. P., Watson, M. A., Madison, D., Soffer, N., Needleman, D. S., Soroka, D. S., . . Elkins, C. A. (2021). Bacteriophages against Vibrio coralliilyticus and Vibrio tubiashii: Isolation, Characterization, and Remediation of Larval Oyster Mortalities. Applied and Environmental Microbiology, 87(10), e00008-00021. doi:10.1128/AEM.00008-21

Richards, G. P., Watson, M. A., Needleman, D. S., Church, K. M., Häse, C. C., & Griffiths, M. W. (2015). Mortalities of Eastern and Pacific Oyster Larvae Caused by the Pathogens Vibrio coralliilyticus and Vibrio tubiashii. Applied and Environmental Microbiology, 81(1), 292-297. doi:10.1128/AEM.02930-14

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Ushijima, B., Videau, P., Burger, A. H., Shore-Maggio, A., Runyon, C. M., Sudek, M., . . . Wommack, K. E. (2014). Vibrio coralliilyticus Strain OCN008 Is an Etiological Agent of Acute Montipora White Syndrome. Applied and Environmental Microbiology, 80(7), 2102-2109. doi:10.1128/AEM.03463-13

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