Phage Therapy: hero or zero?!

 Scleractinian  corals are host to a diverse microbial consortia that has been found to play a number of roles from mutualistic symbionts to vectors of disease. In fact, the coral microbiota is thought to be a major part of the holobiont and so has repeatedly been posited to play a pivotal role in coral response to changing global environments.

 Unfortunately, the diversity of the coral microbiome also comes hand in hand with its complexity and therefore, in spite of the fact that there have been over three dozen coral diseases described, the aetiological agent behind the majority is still unkown. This phenomenon has largely been attributed to the ‘compromised host’ hypothesis which suggests that many coral diseases are causes by an accumulation of factors that lead to stress-induced vulnerability to virulence factors. In order to understand the mechanisms driving this, the bacterial composition of the holobiont has been studied across all coral health states. However, what has yet to be examined is the bacteriophage community associated with corals, along with its effect on coral health.

Therefore, in a study by Soffer et al, the coral associated bacteria and bacteriophages are simultaneously examined in order to further understand their role in coral disease. But why look at phages? Well, in addition to the fact that lytic phages are thought to cause around 10²⁸ infections, some lysogenic phages have been implicated to play a beneficial role in fighting off infection to coral disease. Because of this many have put forward the idea of ‘phage therapy’ as a means for treatment of coral diseases. And, although it makes sense to use phages to target disease-casing microbes, the interactions involved in this top-down means of control are largely underexplored. The study therefore uses a combination of 16S rRNA gene amplicon and shotgun metagenomics analyses to investigate in situ variances in phage and bacterial populations across coral health states, in order to identify possible candidates for use in phage therapy.

The study identified six phage strains that exclusively targeted two bacteria which are enriched in white plague disease which implicates them as potential candidates for phage therapy. In addition to this, it was discovered that coral mucus contains receptors for bacteriophages and therefore may retain beneficial phages, a condition which may be interrupted in a diseased state. All of this indicates potential for the use of phage-therapy as a means of controlling disease-associated bacteria in cases where host immunity has been compromised.

It is my opinion, however, that the use of phage therapy as an in situ means for disease control in environmental corals is not possible at this time. This is due in part to the fact that many of the target hosts simply aren’t amenable to culturing, a fact which means the correct phages may be difficult to identify. When coupling this with varying levels of phage specificity and bacterial host susceptibility it is clear that phage therapy may not be ready for practical applications. This paper does however highlight its potential, as well as helping to gain further insight into the mechanisms behind coral immune response.

Reference:

Soffer, N, Zaneveld, J. and Thurber, R. V. (2015). Phage-bacteria network analysis and its implication for the understanding of coral disease. Environmental Microbiology. 17 (4), 1203-1218.