Newly discovered probiotics could protect coral reefs in the Caribbean that are threatened by deadly and devastating diseases

Researchers at the Smithsonian’s National Museum of Natural History have discovered the first bacterial probiotic effective for treating and preventing stony coral tissue loss (SCTLD), a mysterious disease that has ravaged Florida’s coral reefs since 2014 and is spreading rapidly throughout the Caribbean.

Probiotic treatment, described in a paper published today in Communication biologyprovides an alternative to using the broad-spectrum antibiotic amoxicillin, which has so far been the only proven treatment for the disease, but which carries the risk of promoting antibiotic-resistant bacteria.

SCTLD infects at least twenty species of so-called hard corals, which provide essential habitat for countless fish and marine animals of economic and intrinsic value while also helping to defend coastlines from storm damage. Since it was discovered in Florida in 2014, cases of SCTLD have been confirmed in at least 20 countries. The exact cause of this disease is still unknown, but once a coral is infected, a colony of polyps can die within weeks.

“It eats away coral tissue,” said Valerie Paul, senior scientist at the Smithsonian Naval Station in Fort Pierce, Fla., and senior author of the study. “The living tissue sloughs off and what’s left is just a white skeleton of calcium carbonate.”

Paul has been studying coral reefs for decades, but said she decided to go “all in” in the SCTLD in 2017 because they were too deadly, misunderstood and spread too quickly.

While investigating how the disease spread, Paul and a team including researchers from the University of Florida discovered that some parts of the great star coral (Montastraea cavernosa) rapidly developed characteristic SCTLD lesions and died, but other pieces did not get sick at all.

Although the exact cause of SCTLD is not known, the effectiveness of antibiotics as a treatment suggests that pathogenic bacteria were somehow involved in the development of the disease.

For this reason, the researchers collected samples of naturally occurring, non-pathogenic bacteria found on a pair of large, disease-resistant coral fragments for further testing. With these samples, the research team aims to identify the naturally occurring microorganisms, if any, that protect some star corals from SCTLD.

First, the team tested 222 bacterial strains from disease-resistant corals for their antibacterial properties using three strains of harmful bacteria previously isolated from corals infected with SCTLD. Paul and Blake Ushijima, lead author of the study and assistant professor at the University of North Carolina Wilmington who was formerly a George Burch Fellow at the Smithsonian Naval Station, discovered 83 strains with some antimicrobial activity, but one in particular, McH1-7, stood out.

The team then conducted chemical and genetic analyzes to discover the compounds behind the antibiotic properties of McH1-7 and the genes behind the production of these compounds. Finally, the researchers tested McH1-7 with live pieces of great stellate coral. These lab experiments provided the final bit of conclusive evidence: McH1-7 stopped or slowed disease progression in 68.2% of the 22 infected coral fragments, and most importantly prevented disease from spreading in all 12 transmission experiments, something no antibiotics could do.

Going forward, Paul said work is needed on improved delivery mechanisms if this probiotic is to be widely used in this area. Right now, the primary way to apply this coral probiotic is to wrap the coral in a plastic bag to create a small aquarium and then inject the beneficial bacteria. Perhaps most importantly, Paul said it remains to be seen whether the bacterial strain isolated from large stellate corals will have the same therapeutic and preventative effects as other coral species.

The potential of this newly identified probiotic to help beleaguered coral reefs in Florida without the risk of unintentionally breeding antibiotic-resistant bacteria presents some much-needed good news, Paul said.

“Between ocean acidification, coral bleaching, pollution, and disease, there are a lot of ways to kill coral,” Paul said. “We need to do everything we can to help them so they don’t disappear.”