The optimal temperature for V. vulnificus is 35 degrees Celsius (95 degrees Fahrenheit), not uncommon during summer on the Gulf Coast. Most of the Gulf 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 118 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 also adapt to the cold and can even proliferate.
Hydrostatic High-Pressure Treatment Although it seems few oystermen opt for a PHP method, 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 Centers for Disease Control 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 make us optimistic about using depuration to reduce both pathogens. Development of a high-salinity depuration system to reduce pathogenic vibrios in Gulf oysters will promote a struggling industry and reassure millions of oyster-eaters that they can eat the raw delicacy with confidence. There is regional interest in developing intensive oyster aquaculture, so oyster farmers also would benefit tremendously. Most important, a reliable, economical oyster depuration system would save lives.
A version of this article first appeared in the Journal of Applied Microbiology.