Vibrio shiloi successfully inhibits free radicals and causes coral bleaching

This paper looks into more detail at how Vibrio shiloi, the most well studied bacterium known to cause coral bleaching, is able to become a successful virulent at raised temperatures. The genes which code for some of the virulent factors are only expressed at high temperatures explaining why the coral and algae are able to co-exist with the bacteria during lower temperatures of 15-20^oC. In temperatures between 26^oC and 31^oC the bacteria is thought to adhere to methyl β-D-galacto pyranoside receptors of the coral mucus and penetrates into the host cell, where the corals symbiotic zooxanthellae live. It then produces a toxin that transports ammonia, which the coral has produced, into the algal cells in order to inhibit photosynthesis and lyse algal cells, which causes the coral bleaching. The zooxanthellae produce a high concentration of free radicals as a by-product of photosynthesis. These are extremely toxic to bacteria and act as a defence for the coral. The V.shiloi must have some sort of mechanism for overcoming the free radicals and this paper looks at how Superoxide Dismutase (SOD), a free radical inhibitor, may play an important role in the bacterial infection of the coral.

The researchers grew V.shiloi at 23^oC and at 30^oC and tested for the SOD activity at both temperatures as well as the type of SOD which was produced by the bacteria. The findings showed that V.shiloi is better adapted at surviving and growing at higher temperatures (30^oC rather than 23^oC). They also showed that there was an increase in SOD activity in V.shiloi grown at a higher temperature. Their results clearly showed that the V.shiloi grown at 23^oC was not as good at surviving in a stressful environment as V.shiloi grown at 30^oC as number of viable cells was significantly fewer.

They also tested for the specific SOD that V.shiloi produces and found it to be a Manganese SOD (Mn-SOD) which converts superoxide anion radicals into hydrogen peroxide. They came to this conclusion by testing whether the SOD was inhibited by either H₂O₂ or KCN. If it were Fe–SOD it would have been inhibited by H₂O₂ and if it were CuZn–SOD KCN would have inhibited it but neither H₂O₂ nor KCN inhibited SOD so it must have been Mn-SOD.

In conclusion, one of the reasons that V.shiloi becomes virulent at high temperatures is that they have increased expression of genes that code for Mn-SOD, which allows it to overcome the corals natural defences and penetrate the coral by inhibiting the free radicals produced by the symbiotic zooxanthellae.

Reference: Murali, M, Raja, S, & Devaraj, S 2010, 'Neutralization of radical toxicity by temperature-dependent modulation of extracellular SOD activity in coral bleaching pathogen Vibrio shiloi and its role as a virulence factor', Archives Of Microbiology, 192, 8, pp. 619-623