Saving coral reefs with phage therapy from Vibrio coralliilyticus?

Coral reefs build large underwater ecosystems. In the last decades they have to struggle more and more with the consequences of global warming as the spread of coral disease often related with a pathogen of the Vibrio genus. One of these pathogens is Vibrio coralliilyticus which was isolated from different geographical regions and though its influence on corals is an important point to investigate. With respect to the fact that coral disease cannot be treated with antibiotics and that corals do not have an adaptive immune system phage therapy might be a reasonable way to prevent coral disease which was investigated by Cohen et al. (2013). 

In this study the V. coralliilyticus strain P1 was used, so was a purified bacteriophage termed YC isolated from infected corals. Both were found on infected corals and in the seawater around at the Great Barrier Reef, Australia. 

This study was split up in three main parts. The first one was the identification of the phage. The phage was isolated from seawater of the Great Barrier Reef and observed with TEM to determine its morphology. That shows that it belongs to the Myoviridae family. Purification of the phage nucleic acid and gel electrophoresis were used to show that the phage is a DNA phage with a small genome size of 11 kb. Furthermore Cohan et al. observed that the absorption rate amount 500 phage particles per bacteria cell per minute. The phage was termed bacteriophage YC. 

The second part was the investigation of the effect of V. coralliilyticus strain P1 on the corals’ endosymbiont Symbiodinium. The photosynthetic activity was measured by using an iPAM fluorometer. A decrease of the photosynthetic activity was observed in the treatments with P1. After 24 h Cohen et al. measured a reduction of 90 % in the photosynthetic activity. There was no effect detected when P1 was treated with Phage YC before adding to Symbiodinium. 

To examine the efficiency on living corals 6 month-old juvenile corals were observed. They were raised from larvae and then brought out in the reef water of the Great Barrier Reef. At age of 6 months they were recollected and positioned in aquaria. They activity was indirectly measured by investigating their photosynthetic activity using an iPAM fluorometer as before. Changes in morphology were observed by using a dissecting microscope and a digital camera. When the juvenile corals were treated with P1 they immediately retracted their polyps and after 1 h they began to expel their symbionts. After 9 h only the skeleton remained. The addition of Phage YC to P1 before adding to the juvenile corals changed the results. No changes were detected in the 12 h experimentation period. After 12 h a slight increase of photosystem inactivation and also expulsion of Symbiodinium and tissue lesions were observed. 

The study suggest that the addition of Phage YC leads to a lysis of V. coralliilyticus P1 what result in an inactivation of the Zn-metalloprotease of the bacteria that normally leads to an inhibition of the photosystems in Symbiodinium. All in all Cohen et al. conclude that Phage YC is a good treatment to prevent or stop spread of coral disease triggered by Vibrio coralliilyticus but that further research is needed to examine whether this phage treatment is feasible and possible in open coral reef systems. 

This paper can be an approach on preventing spread of coral disease. Having said this, it needs much more research on the long-time effect of phage therapy. It is interesting that phage treatment has a positive effect for a short-time period of 12 h as investigated in this study but slightly negative effects were shown after 12 h in the juvenile corals. This study does not seem complete because in their conclusion Cohen et al. do not mention these negative effects. This gives the impression that the researches have not thought on long-time effects and they wanted to publish as soon as possible without finishing their work. As a conclusion I would say that this study is a base for further studies on long-term effects of phage treatment where it can be shown that phage treatment might not be as effective as suggested by Cohen et al.

Reviewed paper:

Cohen, Y., Joseph Pollock, F., Rosenberg, E., & Bourne, D. G. (2013). Phage therapy treatment of the coral pathogen Vibrio coralliilyticus. Microbiologyopen, 2(1), 64-74.