Abstract
Porites Ulcerative White Spot Disease (PUWSD) is manifested as round,
3-5 mm in diameter, multi-focal to coalescing, bleached tissue or erosions. Foci of bleached,
intact tissue either regress, or progress to erosions. Occasionally, discrete erosions coalesce,
and after 3 to 5 months, entire coral heads die, leaving a bare skeleton for opportunistic plants
and invertebrates to colonize. Discovered in the central Philippines in 1996, PUWSD has been
observed on 10 of the commonest Porites coral species. From 1997-1998, an El Nino
year, its severity on coral heads monitored increased dramatically.12
In 1998, 120 bacterial isolates were taken from the surface of colonies of
Porites attenuata with and without characteristic lesions. Sixteen-s ribosomal DNA
extracted from bacterial isolates was amplified and restriction fragment length polymorphism
(RFLP) analysis performed. RFLP patterns were used to assemble similar or identical strains into
12 groups. Two of these were principally composed of isolates taken from diseased coral (18/21 and
12/16 total isolates). There is a discernable genetic distinction between the population of
isolates from diseased versus clinically healthy coral.
One or 2 representative isolates from each group (15 total) were then
subjected to DNA sequence analysis (850 bp). Results of a BLAST search (NCBI) revealed that the 2
representatives from the RFLP groups dominated by isolates from diseased coral (putative pathogens
1 and 2) had 89% identity with Vibrio carchiariae and 84% identity with Vibrio nereis,
respectively. Sequences were aligned, and neighbor-joining trees were assembled, in order to
examine the phyolgenetic relationships between the two bacterial populations.5 Due to
the relatively large distance of the branches containing the putative pathogens, compared to the
other Vibrio species included in the trees, the putative pathogens may represent an
undescribed Vibrio species.
Both coral surface microorganisms and the coral itself were chemically
extracted and assayed for antimicrobial activity against the putative pathogens and two reference
strains.7,8 A chemical extract of healthy P. attenuata had antimicrobial
activity against all 20 of the coral bacteria assayed, and no activity against reference strains
of E. coli and B. subtillis. On average, bacterial strains isolated from diseased
coral were more resistant to the antimicrobial compound than those isolated from healthy coral
(P=0.006). The two putative pathogens were the most resistant of all, and putative pathogen 2 was
found to be significantly more resistant than the average from the disease group (P=0.000).
When viewed with the light microscope, histologic preparations of 5 diseased
Porites species contained multifocal coral tissue necrosis, paucity of symbiotic algae, and
an increase in filamentous algae and bacteria.2,3,4,10,11 These changes are
non-specific, and may represent a primary bacterial infection, or they may be due to effects of
microbial invaders secondary to viral infection or environmental stressors. Coral bleaching, or
loss of symbiotic algae, has a complex etiology, and may be related to elevated sea surface
temperatures, increased UV radiation, pollutants, or primary microbial pathogens.1,6,9
It is likely that PUWSD represents a manifestation, although atypical, of the bleaching process.
Acknowledgements
The authors would like to thank the follow people for their contributions:
Aileen Maypa M.S., Silliman University Marine Lab, Negros Philippines; Tom Schmidt, Ph.D., Alice
Child, M.S., Marine Biological Laboratory in Woods Hole, MA; Esther Peters, Ph.D, National Tumor
Registry, George Washington University; Andrew Lackner, D.V.M., Ph. D., New England Primate
Center, Harvard Medical School; and Mike Waddington, Ph.D., Accugenix, Newark, DE.
This research was supported by the National Institute of Health Research
Training Grant, the Geraldine R. Dodge Foundation, Tufts Institute of the Environment, and
International Programs, Tufts University School of Veterinary Medicine.
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