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Public-Private Research Helps Oyster Growers Meet FDA Regs

Oysters (


Researchers at the Virginia Institute of Marine Science (VIMS) in Gloucester Point joined with local oyster growers to sharply reduce a bacterium harmful to humans found in farmed oysters. Their findings may offer a lower-cost solution to new Food and Drug Administration (FDA) regulations that many growers believe can affect the oyster industry in Chesapeake Bay.

The FDA will require Gulf of Mexico shellfish growers to eliminate the bacterium Vibrio vulnificus that causes 25 deaths and 90 cases of illness in the U.S. each year. The agency calls for growers and packers to use post-harvest processing (PHP) methods such as low-temperature pasteurization, flash freezing, high pressure, and low-dose irradiation.

The regulations take affect during this year’s harvest season in the Gulf of Mexico. While the new rules do not apply oyster growers in Chesapeake Bay, local growers and scientists believe the rules will eventually apply throughout the U.S. Many of the Chesapeake oyster growers are small operations for whom PHP methods can be prohibitively expensive.

The research by VIMS and local growers shows that moving farmed oysters into saltier waters just prior to harvest nearly eliminates the presence of Vibrio vulnificus. Using a process nicknamed “oyster relay,” the team moved farmed oysters from low- and two moderate-salinity sites in Chesapeake Bay, to Little Machipongo Inlet on the Atlantic Ocean side of Virginia’s Eastern Shore, where waters are close to full ocean salinity. They moved about 200 oysters from each site, carrying them by truck in insulated coolers.

The team ran two experiments, one beginning in mid-August 2010 and the other in mid-September, considered to be the high-risk season for the bacterium. They sampled the transplanted oysters upon collection, after one week, and again after two weeks, using molecular diagnostics to measure levels of Vibrio vulnificus in oyster tissues.

Their molecular studies show that exposure to salty water decreased the average “most probable number” (MPN) of Vibrio vulnificus bacteria from 160 MPN per gram of meat in pre-transplant oysters to less than 1 MPN per gram. Some samples recorded as many as 750 MPN per gram. The team also found that the shift from fresher to saltier water has little effect on oyster health, with less than 5 percent mortality even among the oysters experiencing the largest salinity change.

VIMS researchers and oyster growers note that further study with a larger number of more highly infected oysters is needed to confirm that an oyster relay can lower Vibrio levels to less than 30 MPN per gram required by the Interstate Shellfish Sanitation Conference (ISSC), a partnership of state and federal control agencies, the shellfish industry, and the academic community. The ISSC sets the sanitation guidelines that regulate the harvesting, processing, and shipping of U.S. shellfish.

Read more: New Vaccines Developed for Farm-Raised Catfish

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