Coral diseases are one of the greatest contributors to the
global decline of coral reefs. Anthropogenic stressors including increasing sea
surface temperatures, ocean acidification, pollution, and sedimentation may
compromise the immune system of coral polyps, upset zooxanthellae symbiotic associations
and lead to increased incidence of disease. Although phage therapy has been
utilised to treat viral infections in human medicine for decades, it is only
recently that aquatic applications for bacteriophages e.g. aquaculture (see
Jack Jones post “An end to antibiotic resistance”) have been pursued. Atad et al. (2012) investigated the in vivo effectiveness of phage therapy
in treating and preventing transmission of white plague-like disease (etiologic
agent- Thalassomonas loyana) of the scleractinian coral Favia
favus in the Red Sea.
Healthy F. favus colonies were fixed to two
purpose made stands (fixed 60 m apart) sitting above the seafloor and placed at
a depth of 3-4 m (8-10 colonies per stand). After 1 week acclimation, 2-3
obviously diseased colonies were added to the stand. A plastic lid was then
placed over the stand (figure 1) and T.
loyana Bacteriophage BA3 -previously isolated and cultivated- was injected
into the plastic box to a final concentration of 104 PFU per ml (the
second box acted as a no-phage control). After 48 hours the lid was removed and
the coral health monitored for both stands at intervals for 7 weeks. This
experiment was carried out in September 2009, and repeated in September 2011.
At the end of the experiment the corals were transferred to aquaria and BA3
phage concentrations enumerated in the aquaria water (after 2 days incubation).
The presence of T. loyana was assessed
in coral tissue by 16S rRNA analysis using T.
loyana specific primers.
Figure 1. The heath robinson coral stand with associated plastic lid and attached syringe.
At the end of
the in vivo experiment the originally
diseased corals that were subsequently phage treated suffered a further tissue
loss of only 5 and 13 % (2009/2011), whilst the control suffered 65 and 57 %
further tissue loss (2009/2011). The presence of T. loyana was assessed in coral tissue by 16S rRNA analysis using T. loyana specific primers and found to
be present only at the disease edge tissue and not from healthy tissue,
confirming T. loyana as the etiologic
agent for the disease in this study. White plague-like disease transmission to
the healthy corals was significantly reduced by BA3 phage addition.
Transmission to healthy corals in the control treatments was 62 and 60 %
(2009/2011), whilst in the BA3 phage treatments only 11 % of healthy corals in
2009 and none of the healthy corals in 2011 contracted the disease. Interestingly
the aquarium water that contained healthy corals which received phage treatment
at the end of the experiment still contained 1.3 x 103 PFU mL of
phages demonstrating that the corals were acting as a reservoir of phages even
after >7 weeks inoculation.
I thought that
this was a fascinating experiment, genius in its conception, and simplicity. The
progression of white plague-like disease as well as transmission was
effectively halted in the experimentally treated corals after administration of
a single inoculum. The authors speculate that 10 litres of a 1016
phage inoculum would be enough to treat 10 m2 of coral reef to a
depth of 10 m with a final concentration of 106. The benefits of
phage antibacterial treatment is a high degree of host specificity (unlikely to
affect environmental bacteria), the ability for self-replication as long as the
host bacterium is present, and therefore unlike antibiotics safe to use in an
open system. The longevity of the protective phage action as evidenced by
retention of BA3 phages after 7 weeks in this study is encouraging. The
potential application for this method in protecting keystone coral species
facing a known etiologic bacterial disease e.g. Acropora palmata in the Caribbean, is an exciting avenue that
should be explored further.
Reference:
Atad, I., Zvuloni, A., Loya, Y., & Rosenberg, E. (2012). Phage therapy of the white plague-like disease of Favia favus in the Red Sea. Coral reefs, 31(3), 665-670.
Hi Matt, interesting research, if a method can be implemented to help corals fight diseases then this will definitely be a step in reducing the stress on these environments. I feel only 10 L may be a bit optimistic, how did they come to this value? There is also the question of how this will scale over a whole reef system, the costs involved in implementing this strategy could be quite large in my opinion.
ReplyDeleteCarrying on from Ben's comment, in the study they used a semi enclosed space to deliver the phages (corals covered by a lid) and I was wondering if they took into account how the inoculated solution would be carried around by currents if it were to be deployed over a reef. Also what would be the best method to deploy this solution?
ReplyDeleteHi Ben, yes exciting times in the fight against coral disease. The authors said that they could reach 10 to the 12 phages per mL in a laboratory fermentor, then I suppose it is just a case of concentrating a samples to reach higher concentrations. 10 to the 16 is a lot of phages, and the beauty of phage therapy is that they will continue to replicate as long as their host is present. Yes I think the ability to treat a whole reef is unrealistic. However if you have a coral species which is critically threatened for example, phage therapy may help to conserve it in a particular area until a pandemic has subsided and it can recolonise. Cheers
ReplyDeleteHi Maria, yes this is a very good point. The delivery application when considering larger scales is something that requires further research. Whether it is simply a case of highly concentrating a sample to make delivery easier, or perhaps some kind of injection may work, but would be impractical on a larger scale. Currents would need to be considered as you say, and treating at slack water on a neap tide may also help to concentrate the sample. From the evidence of this study once the phages become associated with the coral they seem to persist for a long time suggesting a longevity of protection, so repeated applications may not be necessary. One of the main limitations I can see with phage therapy is that coral disease may not be caused by an etiologic agent but simply by an opportunistic pathogen taking advantage of a stressed coral. Therefore phage therapy will be of most use in treating the -few- coral diseases where a known bacterium is responsible. However, the option of using a cocktail of phages perhaps specific to a few known coral pathogens may act to reduce disease more generally. This is a good reference considering cocktail phage therapy against vibrios in aquaculture:
ReplyDeleteMateus, L., Costa, L., Silva, Y. J., Pereira, C., Cunha, A., & Almeida, A. (2014). Efficiency of phage cocktails in the inactivation of Vibrio in aquaculture. Aquaculture, 424, 167-173.