WBD: Just a big group of bullies

This study provides a great insight to the possible solutions of the increasingly addressed issue of coral disease. Investigations have been conducted over the last few decades to identify the causation of coral diseases, with bacteria being deemed as a highly possible culprit. White Band Disease (WBD) is examined within the study by Sweet et al., 2014, which is present in the corals Acropora cervicornis and Acropora palmate, there are two described WBD type 1 and type 2. These corals were chosen due to their listing as critically endangered on the International Union for Conservation of Nature Red list. Coral disease has either been responsible for or a strong influencing factor in the demise of numerous corals, reducing reef habitats and affecting the surrounding ecosystem. The causation and effects of WBD, therefore, need to be monitored and reduced wherever possible.       

A variety of antibiotics were applied to treat WBD type 1 in A. cervicornis, including ampicillin, gentamicin, metronidazole and paromomycin sulfate, followed by the assessment of the microbial community including any bacteria, archaea and ciliates. Two controls were used in the experimentation; a healthy coral with no WBD and a coral with WBD. Both were observed in the same conditions as the treated samples, but received no form of antibiotic treatment. The application of ampicillin and paromomycin sulfate on the WBD infected coral resulted in a halt in the progression of the associated lesions, identifying the disease as being caused by microbes, rather than some previous studies conclusions of physiological stress. This was not the case however with treatments applying gentamicin and metronidazole, which did not prevent the increase in lesion. After antibiotic treatment of the coral disease there remained three possible pathogens responsible for causing WBD: V. charchariae, L. suebicus and a Bacillus sp.

To assess for any correlation between bacteria and WBD, bacterial abundance was measured between the samples used in the experimentation. The results in bacterial abundance of the ampicillin and paromomycin sulfate treated samples showed the return to a bacterial level similar to those without WBD infection. With the treatment samples of gentamicin and metronidazole bacterial abundance was similar to those samples containing WBD, although the communities differed.

Analysis led to the conclusion that WBD was a result of several bacterial pathogens rather than an individual, and additionally that WBD was not caused by physiological stress. A specific ciliate identified as a “potential pathogen of WBD” is also present within white syndrome (WS) in the Pacific, it has been observed digesting coral tissue and associated endosymbionts. Through the removal of the ciliate using antibiotics the tissue lesion continued, enforcing the conclusion that several bacteria were responsible for the WBD. Additional known pathogenic ciliates were also identified through analysis, leading to the belief that ciliates were a defining feature of WBD. 15 prokaryotes were identified as possible secondary pathogens as they were not found within healthy samples of coral.

The primary pathogens of WBD were deemed to be the ciliate and three antibiotic resistant bacteria: V. charchariae, L. suebicus and a Bacillus sp. Previous studies have considered the pathogens of WBD to be a normal part of the hosts microbial community, however, tests on non-diseased coral did not show the three antibiotic resistant species to be present. Whether these pathogens cause WBD individually or together requires further research. The conclusion of this study requires further experimentation to fulfil Koch’s postulates and define the three pathogens and the ciliate as primary causations of WBD. Furthermore, the use of an antibiotic within an open ocean system could be detrimental to other microbes in the ecosystem. However, as stated in the study the possibility of use does exist within enclosed aquaria. It may be possible, that even with antibiotic treatment the addition of physical stress could cause a decrease in coral tolerance to the bacterial pathogens, allowing greater impact from diseases such as WBD before treatment takes effect.

Sweet, M.J., Croquer, A. and Bythell, .JC. (2014) Experimental antibiotic treatment identifies potential pathogens of white band disease in the endangered Caribbean coral Acropora cervicornis. Proc. R. Soc. B 281: 20140094.

http://dx.doi.org/10.1098/rspb.2014.0094