Experimental antibiotic treatment identifies potential pathogens of white band disease in the endangered Caribbean coral Acropora cervicornis

Coral diseases have been increasingly reported over the past few decades and are a major contributor to coral decline worldwide. The Caribbean, in particular, has been noted as a hotspot for coral disease, and the aptly named white syndromes have caused the decline of the dominant reef building corals throughout their range. White band disease (WBD) has been implicated in the dramatic loss of Acropora cervicornis and Acropora palmata since the 1970s, resulting in both species being listed as critically endangered on the International Union for Conservation of Nature Red list. The causal agent of WBD remains unknown, although recent studies based on challenge experiments with filtrate from infected hosts concluded that the disease is probably caused by bacteria. Here, we report an experiment using four different antibiotic treatments, targeting different members of the disease-associated microbial community. Two antibiotics, ampicillin and paromomycin, arrested the disease completely, and by comparing with community shifts brought about by treatments that did not arrest the disease, we have identified the likely candidate causal agent or agents of WBD. Our interpretation of the experimental treatments is that one or a combination of up to three specific bacterial types, detected consistently in diseased corals but not detectable in healthy corals, are likely causal agents of WBD. In addition, a histophagous ciliate (Philaster lucinda) identical to that found consistently in association with white syndrome in Indo-Pacific acroporas was also consistently detected in allWBDsamples and absent in healthy coral. Treatment with metronidazole reduced it to below detection limits, but did not arrest the disease. However, the microscopic disease signs changed, suggesting a secondary role in disease causation for this ciliate. In future studies to identify a causal agent ofWBDvia tests of Henle–Koch’s postulates, it will be vital to experimentally control for populations of the other potential pathogens identified in this study.

Coral diseases have been increasingly reported over the past few decades and
are a major contributor to coral decline worldwide. The Caribbean, in particular,
has been noted as a hotspot for coral disease, and the aptly named white
syndromes have caused the decline of the dominant reef building corals
throughout their range. White band disease (WBD) has been implicated in
the dramatic loss of Acropora cervicornis and Acropora palmata since the 1970s,
resulting in both species being listed as critically endangered on the International
Union for Conservation of Nature Red list. The causal agent of
WBD remains unknown, although recent studies based on challenge experiments
with filtrate from infected hosts concluded that the disease is
probably caused by bacteria. Here, we report an experiment using four different
antibiotic treatments, targeting different members of the disease-associated
microbial community. Two antibiotics, ampicillin and paromomycin, arrested
the disease completely, and by comparing with community shifts brought
about by treatments that did not arrest the disease, we have identified the
likely candidate causal agent or agents of WBD. Our interpretation of the
experimental treatments is that one or a combination of up to three specific bacterial
types, detected consistently in diseased corals but not detectable in
healthy corals, are likely causal agents of WBD. In addition, a histophagous
ciliate (Philaster lucinda) identical to that found consistently in association
with white syndrome in Indo-Pacific acroporas was also consistently detected
in allWBDsamples and absent in healthy coral. Treatment with metronidazole
reduced it to below detection limits, but did not arrest the disease. However,
the microscopic disease signs changed, suggesting a secondary role in disease
causation for this ciliate. In future studies to identify a causal agent ofWBDvia
tests of Henle–Koch’s postulates, it will be vital to experimentally control for
populations of the other potential pathogens identified in this study.

Archived with thanks to Proceedings of the Royal Society B: Biological Sciences