Vibrios and Human Health

Because depurated oysters are still alive, the taste differs only because they are slightly saltier. As noted, FDA-approved post-harvest techniques—such as freezing, heat-cool pasteurization, exposure to high hydrostatic pressure, and irradiation—kill the oyster and change the taste and texture, except for rarely used irradiation.

Cold Shock

The Gulf Oyster Industry Council estimates that only 10 percent of oysters currently undergo PHP, and oystermen usually use refrigeration to preserve live oysters. That is why a major focus of my lab also has been “cold shock,” the response of bacteria to cold. What mechanisms do V. vulnificus possess allowing adaptation to cold, and what risk does this pose to consumers?

The optimal temperature for V. vulnificus is 35 degrees Celsius (95 degrees Fahrenheit), not uncommon during summer on the Gulf coast. Most of these oystermen have small boats without refrigeration on board, so they go out for a short time and return to refrigerate their catch.

To study cold shock in V. vulnificus, we created a microarray to evaluate the expression of every one of V. vulnificus’ 4,488 genes at three different temperatures. We confirmed that when taking oysters down to 7 degrees Celsius (44.6 degrees Fahrenheit), the microbes’ proliferation is stopped. In fact, something major happens around 10 degrees Celsius (50 degrees Fahrenheit). That seems to be the threshold temperature where genes start turning on and the microbe gets into gear to handle cold.

When we lowered the temperature to 4 degrees Celsius (39.2 degrees Fahrenheit) instead of 7 degrees Celsius, the bacteria were no longer metabolically active, so our recommendation would be to keep oysters at 4 degrees Celsius.

We found, however, that if you take oysters down to 15 degrees Celsius (59 degrees Fahrenheit), leave them for several hours and then go down to 7 degrees Celsius, you can have problems. In that gap between 7 degrees Celsius and 15 degrees Celsius, the bacteria not only continue to grow, they adapt to the cold and can even proliferate.

HHP

Although oystermen who opt for a PHP method are in a decided minority, one of the most popular PHP methods is hydrostatic high-pressure (HHP) treatment. At the behest of a commercial seafood processor and distributor, we compared HHP-treated oysters to flash-frozen oysters and oysters kept raw at 4 degrees Celsius. We repeated the testing in the winter, the summer, and the fall.

HHP-treated oysters are supposed to have a shelf life of 21 days, and we found HHP treatment indeed eliminated the majority of human pathogens in oysters, at first reducing them to non-detectable levels. The bacteria that remained adapted rapidly and thrived under refrigeration, however, and after one week the HHP-treated oysters had more bacteria than the week-old raw oysters. In fact, I have rarely seen bacteria levels so high—but the good news is that oysters aren’t kept that long. HHP-treated oysters are certainly very safe, but they don’t seem to have a very long shelf life.

Another species of Vibrio, V. parahaemolyticus, infects oysters in the Gulf as well as in the cooler northern waters of the Pacific, from California to Alaska. Depending on the year, and on the salinity and temperature of the water, there may be more V. parahaemolyticus than V. vulnificus in oysters from Alabama’s Dauphin Island.

The CDC estimates vibriosis caused by V. parahaemolyticus causes some 4,500 illnesses annually. We have run a few depuration trials to see if increased salinity affects V. parahaemolyticus. As expected, high salinity depuration was not as effective in removing V. parahaemolyticus as it was in reducing V. vulnificus but an average of 2-log reduction was observed, which makes us optimistic about using depuration to reduce both pathogens.