Vibrio vulnificus in Florida: the flesh-eating bacteria you may have heard about

Pensacola beach after sunset. The foreground is a wide stretch of open sand, with some people in the distance. The sky is bright orange and yellow.
Microbiologist and molecular geneticist Paul Gulig suspects he was exposed to Vibrio vulnificus at Pensacola Beach, pictured above. (Photo courtesy of Paul Gulig)

While at home in Gainesville, Paul Gulig stepped on something in his driveway and got a small cut on the bottom of his foot. The injury was so small he almost immediately forgot about it.

Later that week Gulig, a member of the UF Emerging Pathogens Institute and a professor at the UF College of Medicine, went swimming in the Gulf of Mexico while visiting family in Pensacola. The warm coastal waters are brackish – saltier than freshwater, but less salty than the ocean – and provide the bacteria Vibrio vulnificus with its ideal growing conditions. Though Vibrio vulnificus is very often present in those waters, unfortunately for Gulig, the Florida Department of Health had recently detected higher levels than normal.

Over the next few days, Gulig developed diarrhea. That alone wasn’t too concerning, but the cut on his foot slowly became increasingly sensitive. He went to the hospital, thinking maybe he had a piece of glass lodged in there. The medical team, however, found something entirely different when they took an ultrasound of his foot: an infection deep in his skin tissue.

Luckily, all Gulig needed to recover was a round of antibiotics. But he still doesn’t know for sure what kind of infection he had since the medical team never took a culture from his wound. Having spent many years researching Vibrio vulnificus, though, he suspects he had a mild encounter with the infamous flesh-eating bacteria.

Vibrio vulnificus, a quick killer

Vibrio vulnificus is often transmitted when someone with an open wound enters brackish water. The infection can lead to necrotizing fasciitis, a condition where the flesh around the wound dies. People may also ingest Vibrio vulnificus by eating contaminated seafood like raw oysters – consequently experiencing vomiting, diarrhea and stomach pain – which may also ultimately lead to a systemic infection of the skin.

3D illustration of the bacterium Vibrio vulnificus.
Vibrio vulnificus infections are rare, but serious. In 2023, Florida saw 46 confirmed cases and 11 deaths. (Dr_Microbe – Adobe Stock)

According to the Centers for Disease Control and Prevention, about 1 in 5 people with a Vibrio vulnificus infection will die. Because the bacteria prefer high levels of iron, people with genetic conditions like hemochromatosis face an especially high risk.

Although people without any preexisting health conditions are more likely to develop only a mild disease, the infection can quickly become fatal if Vibrio vulnificus enters the bloodstream. From there, it causes blistering skin lesions, fever, chills and dangerously low blood pressure.

“That’s what will kill you,” Gulig said. “That process, from eating an oyster to dying from sepsis, can happen within two days.”

By the time most people realize they don’t have a run-of-the-mill intestinal issue, it’s often too late. But this doesn’t mean you have to stop eating raw oysters and stay out of the water altogether – unless you have an open cut, in which case it may be wise to consider sitting a beach day out. Instead, Gulig recommends that people be diligent about monitoring themselves for symptoms. Because Vibrio vulnificus replicates so quickly, just a few hours can mean the difference between life and death.

Getting funding to study Vibrio vulnificus has been an uphill battle for Gulig, because the disease is so rare. When he applied for grants, he would receive comments that there was no need to study the pathogen, because people should just stop eating oysters. But he knew this wasn’t a tenable approach.

“What, are you going to tell people in Florida not to go in the water? No, just like you can’t tell people not to eat oysters,” Gulig said.

Vibrio vulnificus makes headlines

For all its rarity, Vibrio vulnificus continues to attract plenty of publicity. Gulig credits Hurricanes Katrina, Irma and Ian with putting the bacteria on the map. When streets flooded afterward, more people were exposed to the brackish waters that Vibrio thrives in, and the number of cases increased.

Out of all the millions of people who go into the water in Florida every year, Gulig said, only 50 will become infected. But there is only a small window for someone to get treatment, and it can be frightening to hear that someone who was healthy on Saturday ends up dead on Monday.

Kayak boat floating on flooded street surrounded by hurricane Ian rainfall flood waters homes in Florida residential area.
In 2022, the number of confirmed Vibrio vulnificus cases in Florida spiked to 74 due to the impacts of Hurricane Ian. (bilanol – Adobe Stock)

“Everyone pays attention,” Gulig said. “Vibrio is so fast and scary, it’s considered newsworthy. A lot of other diseases, you have time to treat them.”

Compared to other bacteria, Vibrio vulnificus replicates unusually fast in skin tissue. In the lab, Gulig has found that Salmonella bacteria and E. coli in a nutritious broth will double every 20 minutes, but both take several hours to double in animal tissue.

Most pathogens don’t find animal hosts to be hospitable environments, since the immune system is actively working to restrict their growth. But Vibrio vulnificus replicates just as quickly in mouse tissues as other bacteria do in broth.

Gulig also showed that Vibrio vulnificus had a higher growth rate than people first realized. Animals in the lab will fight an infection, making it so the number of bacteria that researchers recover after a few hours don’t necessarily reveal the exact replication rate. To get around this, Gulig’s lab developed a genetic tool that let them count how many generations of bacteria there had been, regardless of how much the animal host had killed.

When is your risk of a Vibrio vulnificus infection higher?

The number of Vibrio vulnificus infections tends to rise in the summertime, when water gets warmer and increased thunderstorms create more freshwater runoff to dilute the saltwater in estuaries and other brackish waters. The ocean is too cool and salty for Vibrio vulnificus, so in Florida the bacterium is most likely to be found in brackish waters near the shore, like in the Gulf of Mexico, Tampa Bay, Pensacola Bay and Escambia Bay.

Not all strains are the same though. Some can replicate in the skin, but are unable to enter the bloodstream and cause a systemic infection. Gulig and other scientists have tried to identify specific genes that can help predict the virulence, or severity of infection, of a particular Vibrio vulnificus strain, but have so far been unsuccessful.

In 2021, however, Gulig and a team of researchers discovered a relationship between season and virulence. They noticed that the Vibrio vulnificus strains they collected from the water in colder months were more virulent than those collected from warmer waters.

“There’s something about the ecology between the warm and the cold waters that is selecting for different strains of Vibrios,” Gulig explained.

Environmental conditions during the summertime are so favorable for Vibrio vulnificus that the strains tend to be less virulent, since the bacteria don’t need to work as hard to survive. Then when water temperatures drop in the winter, the strains that stick around must be tougher to survive. Inside a human host, this can translate to a more severe infection. However, scientists don’t understand the details of the relationship, like how the colder water affects the virulence of the bacteria.

Protecting yourself from the flesh-eating bacteria

Gulig doubts anyone will ever develop a vaccine, since Vibrio vulnificus is so rare and can be treated. But the seafood industry hopes to eventually develop a way to remove the bacteria from oysters and make them safe to eat.

In the meantime, Gulig said the best way for people to stay safe is to remain vigilant about monitoring themselves for symptoms after eating raw oysters or entering water.

As he prepares for retirement, Gulig is hopeful that scientists will continue developing better ways to predict when the risk of a Vibrio vulnificus infection is especially high.

“We don’t need better treatments, because antibiotics are so good,” Gulig said. “If we could figure out the ecology and pathogenesis, we could help prevent encounters of people and Vibrio. I think that’s what the future is.”

By: Jiayu Liang