The first efficient bacterial probiotic for treating and preventing stony coral tissue loss disease (SCTLD), a puzzling ailment that has decimated Florida’s coral reefs since 2014 and is swiftly spreading throughout the Caribbean, has been found by researchers with the Smithsonian’s National Museum of Natural History.
The probiotic therapy, which is detailed in a paper published today in Communications Biology, offers an alternative to the use of the broad-spectrum antibiotic amoxicillin, which has so far been the only proven treatment for the illness but has the potential to encourage the growth of antibiotic-resistant bacteria.
At least 20 species of so-called hard corals are affected by SCTLD, and these corals are vital for protecting coastlines from storm damage and serving as vital habitat for countless fish and other marine life with economic and intrinsic value. SCTLD cases have been confirmed in at least 20 nations since they were first identified in Florida in 2014. The exact cause of the disease is still unknown, but once corals become infected, their polyp colonies can quickly become extinct.
“It simply eats the coral tissue away. The live tissue sloughs away, leaving just a white calcium carbonate skeleton.”
Valerie Paul, head scientist at the Smithsonian Marine Station at Fort Pierce, Florida.
Valerie Paul, chief scientist at the Smithsonian Marine Station in Fort Pierce, Florida, and a senior author of the study, said, “It just eats the coral tissue away.”. “A white calcium carbonate skeleton is all that is left after the living tissue sloughs off.”.
Paul has spent decades researching coral reefs, but in 2017 she said she decided to “all in” on SCTLD because it was so dangerous, so poorly understood, and spreading so quickly.
Stony coral tissue loss disease (SCTLD) is obliterating the live tissue on this great star coral colony (Montastraea cavernosa) in Florida. The coral is bleaching and dying as a result of the illness, where the bright white margin surrounds the dark brown, living coral tissue. Credit: Kelly Pitts
In their investigation into the disease’s transmission, Paul and his team—which also included scientists from the University of Florida—found that while some fragments of great star coral (Montastraea cavernosa) quickly developed the recognizable lesions and died, other fragments never became ill at all.
The effectiveness of antibiotics as a treatment suggested that pathogenic bacteria were somehow involved in the development of the disease, despite the fact that the exact cause of SCTLD is unknown.
For this reason, the scientists took samples of the naturally occurring, non-pathogenic bacteria present on a pair of disease-resistant fragments of great star coral for further examination. The research team used these samples to try to determine whether any naturally occurring microorganisms were preventing some great star corals from succumbing to SCTLD.
The first step was to test the 222 bacteria from the disease-resistant corals for antibacterial properties using three dangerous bacteria previously isolated from corals infected with SCTLD. Paul and Blake Ushijima, the study’s co-lead authors and an assistant professor at the University of North Carolina Wilmington who was previously a George Burch Fellow at the Smithsonian Marine Station, discovered 83 strains with some antimicrobial activity, but one in particular, McH1-7, stood out.
The team then performed chemical and genetic analyses to identify the chemical constituents and the genes responsible for producing McH1-7, which has antibiotic properties. The researchers then put McH1-7 to the test using live pieces of great star coral. The conclusive evidence came from these laboratory tests, which showed that McH1-7 prevented the illness from spreading in all 12 transmission experiments, something antibiotics are unable to do. These tests showed that McH1-7 stopped or slowed the disease’s progression in 68.2 percent of the 22 infected coral fragments.
An examination of a section of diseased great star coral (Montastraea cavernosa), which has been cut and is prepared for examination and treatment in an aquarium, Two coral polyps that had already succumbed to the stony coral tissue loss disease (SCLTD) are visible in the white coral skeleton on the left. Credit: Kelly Pitts
Paul stated that in the future, improved delivery mechanisms must be developed if this probiotic is to be applied widely in the field. Injecting the beneficial bacteria into the coral after it has been essentially wrapped in a plastic bag to create a miniature aquarium is currently the main method of applying this coral probiotic. Paul emphasized that it is still unknown whether the bacterial strain isolated from the great star coral will have the same curative and preventative effects for other species of coral, which is perhaps even more significant.
The likelihood that this recently discovered probiotic will aid Florida’s struggling corals without endangering the development of antibiotic-resistant bacteria is welcome news, according to Paul.
There are numerous ways to kill coral, according to Paul, including ocean acidification, coral bleaching, pollution, and disease. “We must take all possible measures to assist them in order to prevent their disappearance.”.
More information: Chemical and genomic characterization of a potential probiotic treatment for stony coral tissue loss disease, Communications Biology (2023). DOI: 10.1038/s42003-023-04590-y
