Despite the resilience of corals as a taxonomic group through geologic time, warming oceans, shifting seawater chemistry, overfishing, pollution, and disease currently threaten these habitat-building invertebrates with many coral reef ecosystems in a state of decline.  Researchers have identified a bacterium, Serratia marcescens as the cause of a disease called white pox in elkhorn coral (Acropora palmata).  White pox, more formally known as acroporid serratiosis, can lead to tissue loss and potentially the death of the coral colony.  What makes this especially interesting is that S. marcescens normally causes health troubles in humans–this is the first evidence of a human pathogen to a marine invertebrate.  Acropora palmata was once the dominant coral in the Caribbean, especially in the forereef and reef crest, shallow spots with high wave action.  Today, populations of this coral species has been decimated, reduced by up to 95% in abundance since 1980, and is now considered critically endangered by the IUCN.  Much of this decline is attributable to disease, along with other factors that compound this plight–for example, this species is particularly vulnerable to bleaching.

Previous work done in 2003 noted that S. marcescens was found in both untreated human waster and within A. palmata suffering from white pox, suggesting a relationship between the two.  In a new paper published this week in PLoS ONE, Dr Katheryn Sutherland and colleagues used Koch’s postulates, a standard method for showing disease causation, to  investigate the relationship between the two.  In short, fulfilling these postulates requires researchers to be able to isolate the suspected pathogen (S. marcescens) from the host coral and grown up in culture, the disease to manifest itself when a pure culture of the pathogen is introduced to the host, and isolated yet again from the experimentally-infected host (more on Koch’s postulates here).  The results show that S. marcescens is capable of causing white pox in this coral species speedily, with the coral losing tissue in as little as four days (see figure below).

While this disease is specific to this particular coral, the researchers also found that other coral species could possibly be acting as reservoirs for this pathogen while in seawater, given that the pathogen itself is not adapted well for life in the ocean.  Additionally, a coral predator, a snail, may act as a disease vector or reserve.

Improving wastewater containment and treatment in areas such as the Florida Keys can reduce this pathogen’s transmission, and efforts are ongoing in Florida to improve wastewater management, though this issue is occurring in the wider Caribbean as well.  This study shows an exception to the usual animal-to-human transmission model, but also that this pathogen, found in land-based mammals (us), can cause a disease in a marine invertebrate, jumping not only into a profoundly different environment but also into a much different animal, a colonial invertebrate rather than a vertebrate.  Responding to this issue would be obviously beneficial to corals themselves, but also to human health and for the economies that depend on reef habitats for tourism and resources.  The dynamics of this disease are yet another example that illustrate the interconnectivity of society, ecosystems, and economics.

 Figure:  Sutherland et al. 2011 (CC 2.5)

Sutherland, K., Shaban, S., Joyner, J., Porter, J., & Lipp, E. (2011). Human Pathogen Shown to Cause Disease in the Threatened Eklhorn Coral Acropora palmata PLoS ONE, 6 (8) DOI: 10.1371/journal.pone.0023468