An Antibiotic Alternative for Fish Farmers

In the Chesapeake Bay area, striped bass were once so abundant they were named the state fish of Maryland. But overfishing throughout the Atlantic coast in the 1970s decimated their populations, and they’ve never fully returned to historic levels. Which is why half of all the striped bass sold across the country are farm-raised. In fact, they are the fourth most farmed fish in the U.S.

That supply relieves the pressure on wild striped bass, but it introduces concerns that are rarely an issue in wild caught fish, namely infections. On a global scale, diseases cause $6 Billion in aquaculture losses annually, and the most widely used preventative measures like antibiotics and disinfectants raise the potential for antibiotic resistance and harmful environmental effects.

Now, new research at the University of Maryland introduces an option for treating diseases in fish without antibiotics. Daniel Nelson, a professor in the Department of Veterinary Medicine, and his colleagues have developed an antibiotic alternative to combatting Streptococcus iniae, the bacteria that causes streptococcosis. A common disease that can be economically devastating to striped bass farmers, Streptococcosis also affects other fish, marine mammals and humans. 

The treatment Nelson and his colleagues developed uses a protein that they created in the lab after studying viruses that attack the Streptococcus bacteria. Their protein, called ClyX-2 is a combination of two proteins used by viruses to attach to and break down the bacteria’s cell walls.

“The exciting thing about this type of protein, called an endolysin, is that it only kills the Streptococcus bacteria without harming the fish or its natural flora,” Nelson said.

Derived from bacteria-attacking viruses, endolysins don’t linger in the environment, either. They break down into naturally occurring amino acids which can be reused in the normal functioning of the fish’s cells.

Nelson and his team of colleagues tested their ClyX-2 treatments by injecting hybrid striped bass with the bacteria Streptococcus iniae. Then they injected some of the infected fish with their new protein. Only 5% of the untreated fish survived, but 95% of the treated fish did. That’s statistically as good - if not better- than common antibiotics used to treat the same disease.

Nelson said the next steps will be to test ways to treat the water or fish food rather than injecting each fish, and to test treatment as a preventative measure before infection is introduced.

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The research paper, “Bacteriophage endolysin treatment for systemic infection of Streptococcus iniae in hybrid striped bass,” was published in Fish & Shellfish Immunology in February 2024.

PhD student Urmil Dave is an additional author of the paper from AGNR’s Department of Veterinary Medicine. Co-authors also include researchers from UMD’s Institute for Bioscience and Biotechnology Research where Nelson is a fellow, and the United States Department of Agriculture, Agricultural Research Service.

The study was supported in part by National Institutes of Health (AWARD: NIAID R01AI168313), The U.S. Department of Agriculture, and the Basil & Anne Hatziolos Scholarship Fund for Veterinary Medical Research.