Deadly Bacteria May Be Moving to a Beach Near You
Illnesses from stealthy pathogens known as Vibrio are advancing northward along numerous coasts, potentially ruining your summer vacation
On a small, gently rocking research boat anchored just offshore in Chesapeake Bay, I lowered a sterile plastic bottle into the water to collect a sample for studying aquatic microbes. Workers nearby dredged oysters from the shallows, and families played in the low waves. To them, it was a perfect summer day. But hidden in the seemingly tranquil waters were Vibrio bacteria, members of a group that exists naturally in coastal environments around the world. Some cause diarrhea, cramping and nausea, and some can produce severe flesh-eating infections and even lead to death.
Vibrio live freely in the water, concentrate in sediment and on plastics, and colonize the surfaces and guts of shellfish, fish and zooplankton. For those organisms, the bacteria can often be harmless or even beneficial. The bacteria also recycle nutrients such as carbon and nitrogen by breaking down organic material. They are found in both saltwater and freshwater bodies, and they thrive in warm water. That’s why for many years Vibrio infections—called vibriosis—generally occurred along the hottest U.S. coastlines, particularly the Gulf Coast. But climate change is warming once cool waters, and vibriosis cases have been relentlessly spreading northward. Today they are reported across the Eastern Seaboard, along the Baltic Sea in northern Europe, and even as far north as Alaska and Finland.
Not only is the bacteria’s favorable habitat expanding, but higher water temperatures can allow some Vibrio species to multiply more rapidly. That’s especially true when storms and heavy rainfall increase the nutrients and alter salinity in coastal waters, creating ideal conditions for their growth. These perfect circumstances raise the likelihood that someone who steps into the surf with a scraped knee or who accidentally swallows a bit of the water could succumb to serious illness.
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Over the past decade the research team I’m part of has tracked the northward advance of environmental conditions favorable for pathogenic Vibrio, as well as an associated rise in severe illnesses—most alarming, species that infect open wounds, potentially leading to life-threatening conditions such as necrotizing fasciitis (flesh-eating disease) or blood poisoning. Now we are trying to forecast risk by developing predictive computer models that use environmental data—such as temperature and salinity—gathered from satellites and monitoring stations, along with analyses of microorganisms in water samples when possible. Our goal is to devise a Vibrio alert system, much like the “red flag” system municipalities use to warn swimmers of dangerous surf. As summers grow hotter and storms more intense, we are trying to design and roll out models that can keep up with a shifting environment and to help coastal communities recognize the increasing risks washing up on their shores.
Scientists have described more than 100 Vibrio species. The comma- or bullet-shaped bacteria have evolved to thrive across a wide range of aquatic environments, from shallow coastal bays to deep-sea hydrothermal vents that present some of the most challenging living conditions on Earth. Many speciesform close symbiotic relationships with their host creatures. For instance, Aliivibrio fischeri organisms colonize the light-emitting organ of Hawaiian bobtail squid, helping the animals emit bioluminescence. Others attach to corals, fishes, oysters, and the exoskeletons of shrimp and copepods—tiny marine crustaceans that are fundamental to the food web and are major reservoirs for Vibrio.
A single copepod can carry more than 10,000 Vibrio cells, so swallowing even a small amount of seawater can be enough to cause disease. These bacteria also concentrate in filter-feeding shellfish such as oysters, which continuously draw in and process large volumes of water, capturing suspended particles—including microbes—in their gills and tissues. Vibrio love this environment and can multiply inside oysters after harvest if the shellfish are stored or transported without proper refrigeration, raising the risk of infection for anyone who consumes them raw.
Temperature is the main prerequisite for Vibrio growth. Like many pathogenic bacteria, Vibrio species flourish in temperatures near that of the human body—around 37 degrees Celsius (98.6 degrees Fahrenheit)—making warm waters especially favorable. Higher temperatures accelerate their metabolism and reproduction and can trigger the expression of genes involved in infection. Salinity is another key factor; Vibrio typically need the sodium ions of salty or brackish water to maintain their cellular function. They are remarkably adaptable, however, and can live in freshwater lakes or ponds.
When not living in or on a host, many Vibrio species survive in the water column, attached to particles of organic matter such as detritus, algae or plankton, which provide both nutrients and protection. They have flagella that allow them to swim toward beneficial conditions and colonize nutrient-rich surfaces. They can also persist when resources are scarce, then rapidly multiply when nutrients become abundant, such as after heavy rainfall or algal blooms.
Humans can get sick from Vibrio by eating infected seafood such as oysters, unwittingly swallowing a mouthful of ocean water or exposing an open wound to the sea. Illnesses fall into two categories: cholera and noncholera vibriosis. Cholera is an acute diarrheal disease caused by consuming food or water contaminated with Vibrio cholerae. In severe cases, cholera can set in and be fatal within hours as a result of rapid fluid and electrolyte loss, leading to hypovolemic shock and multiple-organ failure if not promptly treated. Access to safe drinking water and medical care has essentially eliminated the disease in developed countries—the U.S. sees fewer than 20 cases a year—but it remains endemic in many parts of the world.
Noncholera Vibrio infections lead to an estimated 80,000 illnesses and about 100 deaths annually in the U.S. Vibrio parahaemolyticus is a leading culprit in illnesses contracted from eating contaminated seafood, although cases of foodborne illness caused by Vibrio vulnificus are on the rise. V. vulnificus is one of the deadliest waterborne pathogens—a well-known cause of necrotizing fasciitis and bloodstream infections—with fatality rates exceeding 50 percent in severe cases. V. vulnificus is now responsible for about 95 percent of all seafood-derived deaths related to Vibrio in the U.S. Other species, including Vibrio alginolyticus and Vibrio fluvialis, can cause infections of the skin, eyes, ears and gastrointestinal tract and are increasingly reported as emerging pathogens in coastal areas.
Vibriosis has long followed a distinct seasonal rhythm, with infections peaking along the U.S. Gulf Coast during the warmer months. The same elevated water temperatures and nutrient levels responsible for this trend strongly influence how readily the bacteria are transmitted to people. Climate change is extending summer seasons, encouraging more recreational water use and thus raising the risk of exposure. Increased global travel, aquaculture trade, maritime shipping and populations along coastlines can help spread these bacteria, too.
Between 1990 and 2019 the range of several vibriosis illnesses expanded northward by up to 70 kilometers a year. For instance, V. vulnificus infections rarely occurred north of Georgia in the late 1980s, but by 2018 they were reported as far north as Philadelphia. This movement accelerated in 2023 and 2024, when deaths linked to V. vulnificus occurred in major cities in New York State, Rhode Island and Connecticut—a striking advance. V. alginolyticus illnesses first appeared outside the Gulf Coast region in 1999, in North Carolina, and were reported as far north as Maine by the end of 2004.
Similarly, V. fluvialis and V. parahaemolyticus infections emerged along the mid-Atlantic coast in the late 1990s and reached Maine by the early 2000s. In Europe, Vibrio infections were reported in Finland, Sweden, Denmark and Norway by 2014. Although data from the Southern Hemisphere remain limited, warming in coastal waters suggests comparable expansions may be underway. Scientists project that the economic burden in the U.S. from Vibrio outbreaks will climb from approximately $2.6 billion annually to as high as $8.6 billion by the end of this century.
Among the most troubling climatic phenomena are powerful hurricanes, whose storm surges and flooding generate ideal conditions for Vibrio growth. In early October 2016 Hurricane Matthew unleashed heavy rains across Haiti’s southwestern coast, overwhelming sewers, latrines, drinking water systems, and other sanitation infrastructure just as temperatures soared. The conditions triggered one of the largest cholera outbreaks in modern history, ultimately resulting in more than 600,000 reported cases in the two years following the hurricane. In September 2022, when Hurricane Ian, a Category 5 storm, devastated Florida’s Gulf Coast, the storm surge stirred up coastal sediments and organic matter in the warm waters, creating optimal conditions for pathogenic Vibrio species to thrive. In the month after the storm, 11 people died from vibriosis, according to state health officials.
In late 2024 back-to-back hurricanes Helene and Milton inundated the same region, mixing warm salt water and fresh water into pools teeming with these pathogens. Florida reported a sharp surge in V. vulnificus infections in the month following the storms, including severe cases of necrotizing fasciitis. Officials urgently warned residents to avoid contact with floodwaters and to protect open wounds. As hurricanes grow stronger and more frequent, public health emergencies involving Vibrio may increase.
In 2017, inspired by this work, I joined Colwell’s research group to help expand predictive capabilities to other pathogenic Vibrio species threatening the Chesapeake Bay, where the team has been collecting samples. Continuous surveillance showed that when water temperatures reached approximately 15 degrees C (59 degrees F), the numbers of V. parahaemolyticus and V. vulnificus began to climb steadily. Once the temperatures warmed beyond 25 degrees C (77 degrees F), growth soared. That’s a serious concern given that coastal waters from Florida to the Chesapeake Bay now routinely exceed that level in summer. We also identified specific ranges of salinity that were strongly associated with higher Vibrio abundance. These ranges are far less salty than open ocean water and are typical of brackish conditions found where rivers meet bays. In addition, we found that chlorophyll concentrations typical during modest to large phytoplankton blooms were linked to greater Vibrio numbers, most likely reflecting the presence of abundant nutrients and zooplankton.
What alarmed us most was clear evidence of a long-term increase in Vibrio abundance throughout the Chesapeake Bay between 2009 and 2022. The pathogens are not only growing more abundant; they are active for much longer periods each year. Historically, the number of Vibrio rose in late spring, remained elevated through the summer, then receded in early autumn. Now the bacteria population stays high well into the winter months.
Our team and colleagues at the University of Maryland School of Public Health have found that in Maryland, the annual rate of vibriosis cases between 2013 and 2019 was roughly 40 percent higher than that in the period from 2006 to 2012. Hospitalizations increased by approximately 60 percent. The highest hospitalization rates were in coastal and rural counties of southern and eastern Maryland—particularly near the lower Chesapeake Bay. As climate change transforms coastal ecosystems, Vibrio health risks will last longer, affect a broader geographic range and impact more people every year.
When detected early and treated promptly, most Vibrio infections, especially those causing gastrointestinal illness, can be managed with oral or intravenous rehydration. More severe cases, particularly wound infections such as necrotizing fasciitis or sepsis, require antibiotics and sometimes emergency surgery. Yet these treatments are becoming less reliable as antibiotic-resistant strains of Vibrio become increasingly common. Approved vaccines exist only for V. cholerae, but they typically provide protection for only a few years. No approved human vaccines are yet available for noncholera species, although a few are being developed for the fish and shrimp aquaculture industries. Given these factors, public health officials are emphasizing awareness to lessen exposure.
Building on nearly six decades of cholera research in the Chesapeake Bay, our team has demonstrated that predictive-risk models can help forecast and reduce outbreaks of waterborne disease. The National Oceanic and Atmospheric Administration’s National Centers for Coastal Ocean Science also developed a probability model to estimate the likelihood of finding V. vulnificus in the Chesapeake Bay. Using temperature and salinity data, NOAA provides a daily average prediction for the previous six days, the current day and the next day. The European Center for Disease Prevention and Control created the Vibrio map viewer to predict hotspots across the Baltic Sea. These models, however, are highly location-specific. Environmental factors that heighten Vibrio risk in the Chesapeake Bay will not necessarily raise the same concerns along Florida’s Gulf Coast, where salinity is much higher and seasonal patterns differ. Because each Vibrio species can respond differently to various conditions, the models must be tailored to specific ecosystems. Accurate prediction also requires long-term, site-specific environmental and microbiological data—datasets that are limited in many regions.
With Colwell, Anwar Huq of the University of Maryland, and Antar Jutla and Bailey Magers, both at the University of Florida, we are using machine learning to refine risk models. They include not only local environmental conditions but also human behavior patterns such as recreational water use and seafood consumption. Demographics are key because certain populations—such as older adults and people with liver disease or weakened immune systems—are more susceptible to severe Vibrio infections.
By incorporating these factors, we can better predict vibriosis risk. Since 2022 we have been sampling regions of the Gulf Coast that were severely affected by hurricanes and have recently reported spikes in Vibrio infections, such as Lee County and Tampa Bay in Florida. And we have been collecting water and oyster samples from numerous Gulf sites. By combining these real-world data with environmental variables sensed by satellites, we are developing real-time early-warning systems that reflect the unique ecological dynamics of each region. To protect coastal communities around the world, we need more environmental monitoring, standardized reporting of human infections, and long-term datasets to help train models—not only for Vibrio but for a broader range of waterborne pathogens.
Ideally, early-warning systems for vibriosis would operate much like air-quality or rip-current alerts: when conditions become favorable for Vibrio growth, automated messages could notify beachgoers, marine workers and aquaculture fisheries through cell-phone alerts and social media or public advisories. This real-time information could prompt simple but potentially lifesaving behavior changes and practices such as covering any cuts or scrapes with waterproof bandages, avoiding water altogether for people who have open wounds, and abstaining from eating raw shellfish unless it is sourced from monitored waters. Public health messaging must also counter outdated beliefs such as the myth that salty seawater “cleans” wounds. It doesn’t; exposing open wounds to seawater can significantly increase the risk of severe infection. These strategies could be extended to freshwater areas as well.
As Vibrio threats evolve, science must, too. An intriguing feature of Vibrio is their ability to withstand harsh conditions by entering a state known as viable but nonculturable (VBNC), first described by Colwell and her students at the University of Maryland, College Park. In the VBNC state, bacteria remain alive but become inactive. As a result, they cannot be grown using standard laboratory techniques that are widely employed to monitor environmental pathogens. Despite their dormancy, VBNC cells can quickly revert to an active, infectious state once conditions improve—exactly the case in the warm, nutrient-rich environment of the human gut.
For years it was hard for researchers to detect cells in the elusive VBNC state. Advances in molecular analysis have significantly improved our ability to find them in the environment, including the polymerase chain reaction technique, which can amplify trace amounts of DNA to detectable levels, and high-throughput sequencing, which provides the order of DNA bases across millions of fragments simultaneously—allowing us to identify which species and even what genes are present. In my research, I use DNA-based techniques known as metagenomics to profile all microorganisms in a sample—including bacteria, viruses, fungi, and protists (organisms that fall between the other taxonomic rankings)—and to identify pathogens and detect antibiotic-resistance genes. My colleagues and I also apply RNA-based methods to assess gene expression in suspect microbes, which gives us a clearer picture of not just “who” is there but also “what” they are doing. These approaches are especially valuable because they can circumvent issues associated with detecting microorganisms in the VBNC state.
Using these tools, we have discovered a much broader diversity of Vibrio in coastal waters than previously recognized. In one sample from Florida’s Gulf Coast, we identified more than 80 distinct Vibrio species, including strains that are known to cause disease in humans and many that carry genes for antibiotic resistance. These high-resolution datasets enable us to predict not just when V. vulnificus or V. parahaemolyticus might be present but when the bacteria are likely to become more active or dangerous. This information also allows us to account for complex ecological interactions—for example, how blooms of algae or shifts in salinity can enhance or suppress certain Vibrio populations.
As we deepen our understanding of Vibrio genetics, new concerns emerge. The bacteria are remarkably adaptable, frequently acquiring new traits through mutation and horizontal gene transfer—the direct exchange of genetic material—which allows them to rapidly evolve in response to changing environmental pressures. Indeed, V. cholerae originally gained its capacity to produce cholera toxin from a bacteriophage, a virus that infects bacteria. Some V. vulnificus strains have recently evolved greater heat tolerance, letting them persist longer in warm waters. Certain V. parahaemolyticus strains have acquired genes that improve their ability to infect hosts.
These adaptations not only make infections potentially more severe and harder to treat, especially when antibiotic-resistance genes are involved, but also complicate the design of early-warning systems. A predictive-risk model may fail to account for newly evolved strains or those in a VBNC state. That’s why long-term environmental surveillance is so essential to providing a powerful public health tool. If we identify when and where the risk of Vibrio exposure is high, we can issue timely alerts to the public, support coordinated responses, guide resource allocation and inform health policy decisions.
Source: www.scientificamerican.com