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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.
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 species form 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.
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