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The Oceans Are Filled with Microbes. Some Are Nasty

What environmental conditions foster outbreaks of pathogenic bacteria?

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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 parahaemolyticusVibrio 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.”

And vice versa.

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