A research team from Rollins College in Florida and the University of Georgia has identified human sewage as the source of the coral-killing pathogen that causes white pox disease of Caribbean elkhorn coral. Once the most common coral in the Caribbean, elkhorn coral was listed for protection in 2006 under the United States Endangered Species Act, largely because of white pox disease.
Kathryn P. Sutherland, Associate Professor of Biology at Rollins College, and her research collaborators, Erin K. Lipp, Associate Professor of Environmental Health Science, and James W. Porter, Professor of Ecology, both from the University of Georgia, have known since 2002 that the bacterium responsible for killing the coral was the same species as found in humans, but they could not be sure of the source.
Tracking the source
“When we identified Serratia marcescens as the cause of white pox, we could only speculate that human sewage was the source of the pathogen because the bacterium is also found in the waste of other animals,” Sutherland said.
In order to trace the origins of the pathogen, the research team collected and analyzed human samples from the wastewater treatment facility in Key West and samples from various animals, such as Key deer and seagulls. While Serratia marcescens was found in these other species, genetic analyzes showed that only the strain from human sewage matched the strain found in corals afflicted by white pox on the reef.
The final piece of the investigative puzzle was to show that this unique strain was pathogenic to corals. With funding from Florida’s Mote Marine Laboratory “Protect Our Reefs” grant program, Sutherland, Lipp and Porter conducted challenge experiments by inoculating fragments of coral with the strain found in both humans and corals, to see if it would cause disease. The experiments were carried out in a laboratory in closed seawater tanks to eliminate any risk of infection to wild populations of corals.
“The human strain caused disease in elkhorn coral in five days, so we now have definitive evidence that humans are a source of the pathogen that causes this devastating disease of corals,” Sutherland explained.
Addressing the problem
“These bacteria do not come from the ocean, they come from us,” said Porter. “Water-related activities in the Florida Keys generate more than $3 billion a year for Florida and the local economy. We are killing the goose that lays the golden egg, and we’ve got the smoking gun to prove it.”
Serratia marcescens is also a pathogen of humans, causing respiratory, wound and urinary tract infections, meningitis, and pneumonia. Human diseases caused by this bacterium are most often associated with hospital-acquired infections of newborn infants and immune-compromised adults.
The movement of pathogens from wildlife to humans is well-documented, as in the case of bird flu, but the transmission of disease-causing microbes from humans to marine invertebrates has never been shown before. This is the first time that a human disease has been demonstrated to be responsible for population declines of a marine invertebrate.
“Bacteria from humans kill corals – that’s the bad news,” said Porter. “But the good news is that we can solve this problem with advanced wastewater treatment facilities, like the one recently completed in Key West.”
The entire Florida Keys is in the process of upgrading local wastewater treatment plants, and these measures will eliminate this source of the bacterium, hopefully before it is too late for the elkhorn coral to recover.
Reference: Kathryn Patterson Sutherland, Sameera Shaban, Jessica L. Joyner, James W. Porter, Erin K. Lipp. Human Pathogen Shown to Cause Disease in the Threatened Eklhorn Coral Acropora palmata. PLoS ONE, 2011; 6 (8): e23468 DOI: 10.1371/journal.pone.0023468