The surface drifters tracked the movements of surface waters in the estuary over 24-hour periods to test the effect of tidal changes on microbial populations. (Photo by Ben Kirkup, Massachusetts Institute of Technology)
Sarah Chang, a Wellesley College student, collects a water sample in the estuary of Plum Island Sound. Samples were collected each hour over a 24-hour period to explore vibrio population dynamics over tidal cycles. (Photo by Jim Lerczak, WHOI)
Every time you swim at the beach, you are surrounded by zillions of bacteria. There is no escaping them: One drop of seawater contains about 1 million bacteria. Of these, upwards of 10,000 are called vibrios, a diverse group of naturally occurring microbes responsible for 95 percent of all seafood-related deaths.
Before you cancel your summer vacation, however, here’s the punch line: It is highly unlikely that you will ever swim among vibrios’ pathogenic strains—the kind that can make you sick to your stomach, or worse. Your chances of coming across nasty varieties of vibrios in New England waters are not much better than the guy with the metal detector finding a pot of gold beneath a sand castle.
But vibrios are out there, and a cause for concern. The bacteria have caused illnesses in other regions. Handlers of aquacultured fish and shellfish continually take steps to ensure that vibrios don’t show up in their stocks. And recently, scientists found vibrio infections in other animals, such as corals.
Particularly intriguing is vibrios' seemingly split personality. Residing among harmless vibrio populations are small amounts of pathogenic varieties with an “Incredible Hulk” alter ego that can bring people to their knees. Under the right conditions, these nasty strains can grow rampant in what scientists call a bloom event.
“It may be a completely random event, but more likely it’s triggered by environmental factors,” said Martin Polz, a microbiologist and ecologist at the Massachusetts Institute of Technology. Funded by the Woods Hole Center for Oceans and Human Health, Polz and Jim Lerczak, a physical oceanographer at Woods Hole Oceanographic Institution, are hot on vibrios’ rod-shaped bacterial tails. Their goal is to figure out the conditions that favor the transformation of benign vibrios into virulent ones.
Virulent varieties of vibrio
Of the 60 or 70 vibrio species that have been described, Polz and Lerczak are focusing on two: Vibrio vulnificus and Vibrio parahaemolyticus.
“We’re always exposed to Vibrio vulnificus and Vibrio parahaemolyticus—Vibrio vulnificus is always in oysters—but the pathogenic strains are very rare,” Polz said.
In extreme, rare cases, the two strains can be really dangerous. V. vulnificus causes systemic stomach and bloodstream infections—often fatal—and blistering skin lesions that can require amputation of infected arms or legs. Eating raw or undercooked shellfish is the most common route of infection. People whose immune systems are compromised or who suffer from liver disease or hepatitis are most at risk.
V. parahaemolyticus causes acute diarrhea and stomach cramping if consumed in infected shellfish. It causes wound infections if it enters the body through a skin lesion. Over the past decade, health providers have documented more and more V. parahaemolyticus infections; in one case in 1997, more than 200 people became sick, and one died.
Polz suspects that exposure to vibrio bacteria may turn out to play a role in routine eye, ear, and skin infections. But the link between vibrios and human health remains a big question mark. Though vibrios have been integral parts of marine ecosystems for millions of years, researchers lack fundamental knowledge about what kinds of vibrio populations are out there, what factors affect their growth and distribution, and—the biggest question of all—how and why pathogenic strains arise.
Marrying microbiologists with oceanographers
Polz and Lerczak make a strong interdisciplinary team: Polz knows how microbes work, and Lerczak knows how oceans work. In 2005, they launched fieldwork in Plum Island Sound off Massachusetts—an estuary typical of many in the Northeast, Lerczak said, and one whose physics, chemistry, and ecology have been well-documented since the late 1980s. Plum Island Sound has big tides and fast currents that act as a blender that mixes ocean and river waters relatively quickly.
The scientists tracked vibrio populations and measured environmental conditions—including temperature, salinity, nutrients, and tides—throughout each of four seasons. Preliminary results show that temperature is a big factor. Vibrios grow better in the summer, and the threshold number is 61°F (16° C), Polz said; once the water warms, vibrio populations can double in one hour and shift toward potentially pathogenic strains.
According to one theory, viruses could be culprits. Pathogenic and non-pathogenic vibrios differ just by a few genes, Polz explained. Viruses, ubiquitous in the marine world, may insert nasty genes into otherwise benign vibrio cells as they reproduce.
Next, the team will test the effect of tidal changes. The team headed out in July with an instrumented drifting device to literally follow a parcel of water up and down the estuary and measure how its physical and chemical properties change along the way.
By the end of the five-year study, the team will have some basic information about the ecological factors that regulate vibrio populations. Lerczak said the COHH study could help scientists predict or even prevent outbreaks that affect human health.
“Scientists have to study the system as it evolves in order to get the data necessary to someday be able to better predict when a vibrio outbreak may occur,” he said. “We’re looking at little pieces of the puzzle now.”
“The bacteria are incredibly diverse, and (studying them) is a challenge,” Polz said. “But microbes keep the entire ocean going. If you want to understand the oceans, you have to understand the microbes.”
Researchers Mobilize to See What Hurricane Katrina Stirred Up
When the levees broke in New Orleans after Hurricane Katrina, officials
feared that Lake Pontchartrain, the second-largest saltwater lake in
the United States, might be infiltrated with disease-causing microbes.
Rumors circulated that people died from wound infections caused by
Vibrio vulnificus, said MIT microbiologist Martin Polz (at left).
Polz is part of a team of scientists mobilized by the Woods Hole Center
for Oceans and Human Health to assess the situation in New Orleans.
Polz is working with two other Woods Hole biologists, Rebecca Gast of
Woods Hole Oceanographic Institution and Linda Amaral-Zettler of the
Marine Biological Laboratory, to study how the hurricane affected
microbe populations in Lake Pontchartrain and the possible impacts on
human health. Collaborators include teams at the University of Hawaii,
the University of Miami, and Louisiana State University.
The scientists have taken, and are analyzing, water and sediment
samples to assess microbial life in Lake Pontchartrain after Katrina.
In the long term, Polz said, health impacts from waterborne diseases
will probably be minimal because the volume of water entering the lake
after the levees broke was less than 5 percent of the total lake
volume, he said. And salt water is frequently flushed between lake and
Still, Amaral-Zettler said, “We want to be able to say, ‘This is what is there, not what we think is there.’ ”
With a rising tide of harmful algal
blooms and waterborne pathogenic
microbes causing illnesses and closing beaches and shellfish beds,
policymakers recognized a growing need to examine links between the
oceans and human health. But few links existed between oceanogaphers
and scientists specializing in human health.
federal scientific agencies that rarely interacted embarked on a
groundbreaking collaboration, which sparked a cascade of novel
partnerships. The National Science Foundation’s Division of Ocean
Sciences and the National Institute of Environmental Health Sciences
created four Centers for Oceans and Human Health (COHH) around the
country, including one in Woods Hole, Mass. Each center harnessed the
expertise and resources of scientists from several institutions and
disciplines to study “risks and remedies from the sea” (the remedies
being potential pharmaceuticals from marine
Hole Center for Oceans and Human Health, for example, has brought
together physical oceanographers, biological oceanographers,
microbiologists, and genomics experts from Woods Hole Oceanographic
Institution (WHOI) , the Marine Biological Laboratory, and the
Massachusetts Institute of Technology, said John Stegeman, director of
the center and a WHOI biologist.
“The ocean is a
fluid medium that’s changing all the time,” said Dennis McGillicuddy, a
WHOI physical oceanographer and the center’s deputy director. “To make
significant progress in health concerns, we have to grapple with how
physics, biology, and chemistry intersect and interact. It’s really a
fundamentally new direction for this research.”
In its first two years, the Woods Hole COHH launched several
investigations on algae, bacteria, viruses, and other organisms that
threaten to compromise the safety of our seafood supply and the
commercial and recreational use of coastal waters.