USDA develops antibiotic alternative for strep in hybrid striped bass

The United States Department of Agriculture’s Agricultural Research Service (USDA-ARS) has developed an alternative treatment for Streptococcus iniae in hybrid striped bass. That pathogen is recognized as a significant cause of mortalities in various farmed fish species. Dr. Michael Deshotel, a research microbiologist with USDA-ARS, says the treatment was developed with an eye on reducing the overuse of antibiotics.

“Anytime antibiotics are used, whether in humans or livestock, it’s always associated with the risk of development of resistance,” says Deshotel. “In the United States, antibiotic use has been curbed almost 40 percent in livestock since 2015. We are intentionally trying to reduce the use of antibiotics to keep the ones that are available more effective for use.”

The treatment uses phage therapy – meaning bacteriophages, which are viruses that only infect bacteria. Deshotel says the origins of the research came out of the war on terror when researchers at Rockefeller University were looking at methods of treating anthrax bacteria.

“They realized that bacteriophage could rupture these cells,” says Deshotel. “The bacteriophage are producing an enzyme called endolysin which sticks to the membrane of the bacteria and enzymatically make a bunch of cuts in the cell wall and the cell then ruptures. The idea was that maybe we can use these endolysin like an antibiotic, and sure enough, it was very effective.”

Phage therapy generally uses the entire virus – though the USDA-ARS are also working on that, this treatment is a subcategory that only uses the virus’ enzymes. The treatment certainly appears effective – Deshotel notes that using the endolysin treatment they had a 95 percent survival rate, compared to 85 percent with the antibiotic, carbenicillin (and five percent with no treatment at all).

The treatment also leaves behind no chemical residue after breaking down.