Bacterial Strain BA-3 and a filterable factor cause a white plague-like disease in corals from the Eilat coral reef

During the last 2 years a white plague-like disease has spread over the Eilat coral reef, Gulf of Aqaba, Red Sea. Two of the major reef-building coral genera, Favia and Goniastrea, were most affected by this disease. Approximately 10% of these corals showed progressive signs of the disease or were already dead as a result of the disease. Controlled aquarium experiments demonstrated that the disease is infectious and that transmission from diseased to healthy corals does not require direct contact. Infection was not genus-specific, since diseased F. favus infected 3 other coral genera. Although diseased F. favus contained 80 to 10 000 times more culturable bacteria than healthy specimens, none of the 25 isolates initially tested infected healthy corals. Filtration of aquarium water containing diseased F. favus indicated that the infectious agent was larger than 0.2 μm and smaller than 3 μm. Microorganisms retained by the 0.2 μm filter did not infect corals; however, when the retentate was combined with the 0.2 μm filtrate, infection took place. This suggested that the infectious agent requires a filterable factor to cause the disease. Combining the 0.2 μm-filtered water from an aquarium containing a diseased coral with each of the 25 pure cultures previously obtained from diseased F. favus allowed for the recognition of 1 strain that caused rapid lysis of F. favus. The pathogen, referred to as BA-3, is a Gram-negative bacterium whose 16S rDNA sequence indicates it is a new species and possibly a new genus. Attempts to re-isolate Strain BA-3 from diseased corals in the field were unsuccessful. The nature of the filterable factor, which is >5000 molecular weight (MW) and heat sensitive, remains to be determined.