Oh Shuck!

Raw oysters, delicious with hot sauce, can carry a bacteria that makes people very sick. Researchers are working to better understand Vibrio bacteria and create a warning system for oystermen, which would alert them to the best and worst times to harvest oysters.

BEAUFORT — You might think an oyster is an oyster is an oyster. After all, all oysters look pretty much the same.

But it turns out, where an oyster is raised, in the wild or on a farm, and where it is from, the waters of New England, the Gulf Coast, or North Carolina, makes a big difference on the inside. That’s especially true when talking about Vibrio, the bacteria that is found in all oysters.

Just ask Dr. Brett Froelich, a postdoctoral fellow at the University of North Carolina Institute of Marine Sciences, who has studied oysters for years.

“We found that yes, the Vibrio bacteria are in our oysters, as you would expect with most places, but the salinity levels in our state are much different than in others,” Dr. Froelich explains as he cracks open an oyster in the UNC-IMS lab where he works. “We have the barrier islands here, which affects the salinity levels. Not only that, our water temperature is much different. We're not as warm as the Gulf Coast or as cold as New England. And it’s that intertwining of the temperature and the salt level that produces a specific bacterial profile that is different from another area. So we are trying to hone down what that particular profile is.”

There are two types of naturally occurring Vibrio bacteria found in raw oysters — Vibrio vulnificus and Vibrio parahaemolyticus. Both types can make humans very sick, with diarrhea, vomiting and abdominal pain. The symptoms can last from a couple days up to a week. Both types of bacteria are found naturally in warm coastal waters.

“Vibrio is in the water from Canada to the U.S. to Mexico,” explains Barry Nash, Seafood Technology and Marketing Specialist with North Carolina Sea Grant. “It is a bacterium that is a natural constituent of the water. It is not something you find as a result of pollution, and the incidence of Vibrio is strongest from May through October because those are warm summer months.”

But because the oyster is a filter feeder, which means it gets its food and oxygen by pumping water over its gills, Vibrio bacteria become concentrated in the gut of the shellfish.

And that’s where science and seafood come together. Researchers want to design an early warning system for high Vibrio levels in oysters, and are looking for a way to predict Vibrio levels.

“If we can figure out the conditions that cause these bacteria to multiply, then maybe we can have an early warning of when we need to be careful harvesting our oysters,” says Dr. Froelich, who admits he is a fan of eating oysters.

But to be able to predict Vibrio levels, researchers must first determine what kind of Vibrio bacterium is inside the oyster as well as the concentration of the bacterium. To gain that insight into the inside of an oyster, scientists are applying the science of DNA sequencing. A tissue sample is taken from the oyster to extract and identify the bacterium’s DNA. Researchers are looking for specific types of DNA that are linking to the bacteria that can make people sick.

“We use the DNA sequencing to get an idea of not only the number of bacteria but also the different types,” says Dr. Froelich. “That’s because the types of bacteria really determine whether they will make you sick or not. It’s important to know whether the oyster contains a large number of those dangerous types.”

Then comes the real number crunching. Scientists will have to correlate the records for temperature and salinity in the estuary with weather patterns. Warm weather increases the water temperature while rainfall will decrease the salinity. Wind speed and direction can also impact water conditions. The challenge for researchers is to sort out the relationship between all of the weather variables and the concentration of Vibrio bacteria.

The new testing procedures couldn’t come at a better time because the federal Centers for Disease Control and Prevention is tracking a dramatic increase in Vibrio infections. The CDC’s data shows Vibrio infections are at the highest level since 1996, when tracking of Vibrio first began.

North Carolina bays and sounds once produced hundreds of thousands of oysters a year, but then pollution, overfishing and disease dramatically reduced the catch. The state’s yearly catch records show the industry is coming back. Whether the shellfish is harvested in the wild or raised in a farm, the oyster industry is worth about $3 million dollars per year.

If you close down a shellfish or an oyster growing area, people are losing money, so the sooner you can show it is safe and you can get it back open, the better for the industry and the consumer,” says Dr. Froelich.

Dr. Froelich says the ultimate goal of the project is to create some type of tool that will let oystermen and consumers know that based on the conditions of a particular day, oysters may be a little more dangerous for some people to consume than on other days.

However, even if a warning system can be developed, the risk of eating raw seafood doesn’t go away.

“Anytime you eat raw seafood you are at risk for an infection or a food born illness,” warns Nash. “So first off we want to do everything we can to reduce the risk of that in the raw shellfish and then we want to encourage people to cook their shellfish to make sure they have eliminated the Vibrio threat."


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