New Method to Keep Oysters Bacteria Free

GLOUCESTER POINT, Va.Moving farmed oysters into saltier waters just before harvest virtually eliminates the presence of Vibrio vulnificus bacteria, according to a new study conducted by researchers at the Virginia Institute of Marine Science and local oyster growers.

A new FDA regulation takes effect in Summer 2011 and requires Gulf of Mexico shellfish growers to eliminate Vibrio vulnificus from shellfish through the use post-harvest processing (PHP) methods that include low-temperature pasteurization, flash freezing, high pressure and low-dose irradiation.

The findings may offer a low-cost solution to the new regulations that many growers believe will eventually affect the oyster industry in Chesapeake Bay.

The researchers moved farmed oysters from one relatively low- and two moderate-salinity sites in Chesapeake Bay (the Coan River, the York River, and Nassawadox Creek) to Little Machipongo Inlet on the seaside 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. Both dates lie within the FDAs 2010 "risk season." The team 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, measured by the "most probable number" (MPN) of Vibrio vulnificus bacteria in each sample, shows that exposure to salty water decreased Vibrio vulnificus levels from a high of 750 MPN per gram of meat in pre-transplant oysters (with an average of 160 MPN per gram) to less than 1 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.

The findings show that high-salinity relay is a potentially viable method to reduce Vibrio vulnificus in oysters grown and harvested in Virginia. The research team said the "oyster relay" may be just as effective and much cheaper than other PHP methods, which are "expensive, capital intensive, difficult to use with large numbers of oysters, or not readily available."