Mark Dennett of Woods Hole Oceanographic Institution and Linda Amaral-Zettler of Marine Biological Laboratory take samples of water in Mount Hope Bay, R.I., to investigate microbe populations living in the ocean. (Abby Laatsch, Marine Biological Laboratory)
Legionella pneumophila is the leading cause of Legionnaire's Disease, an often fatal disease first discovered in 1976 among a group of elderly men attending an American Legion Convention in Philadelphia. Scientists investigating microbe populations in the oceans unexpectedly turned up 32 strains of Legionella, which previously hadn't been known to exist in saltwater environments. The majority of Legionella bacteria are not pathogenic. (Image Copyright Dennis Kunkel Microscopy, Inc.)
Salty ocean water can be a nuisance. It’s undrinkable and it corrodes
nearly everything it touches. But salt water’s inhospitality has always
had one benefit: The salt kills microbes, making the ocean a fairly
antiseptic environment. Or so we thought.
In the late 1990s, oysters in Chesapeake Bay began turning up infested
with Cryptosporidium, a freshwater microbe familiar to backpackers who
drink unfiltered stream water. It causes diarrhea and is occasionally
fatal in young or weakened people.
Then last year Rebecca Gast, a biologist at Woods Hole Oceanographic
Institution, found another common freshwater microbe in the Great Salt
Lake, Utah: Legionella. Several species of this bacterium cause
Legionnaires’ disease, an illness that causes pneumonia, confusion, and
fatigue.
Suddenly, the possibility arose that freshwater microbes traveling by
stream, gutters, and sewage systems might be surviving the jump into
salt water.
So for the past year, Gast and colleague Linda Amaral-Zettler of the
Marine Biological Laboratory in Woods Hole have been conducting a
sampling project to see what’s actually lurking out there in the oceans.
“People have not paid attention to this,” Amaral-Zettler said. “We
need to examine the extent of the problem—whether human pathogens end
up in the coastal marine environment, how they are distributed, and how
they persist.”
Warmer waters may increase threat
With funding from the Woods Hole Center for Oceans and Human Health,
they have been working in nearby Mount Hope Bay. Wedged between Fall
River, Mass., and Bristol, R.I., the bay receives plenty of runoff,
which can be rich in the nutrients and organic matter that microbes
thrive on.
Mount Hope Bay also has relatively warm waters that might make it
easier for human pathogens to survive, Gast said. The summer sun heats
the shallow bay, and a power plant empties water approaching
human body temperature (98°F or 37°C) into the bay.
Initial findings proved a basic premise in marine microbiology: “The more you look, the more you find,” Amaral-Zettler said.
Samples taken in 2005 turned up 32 strains of Legionella. Subsequent
DNA typing indicated that none were the strain responsible for
Legionnaires’ disease. Still, the saltwater discovery was unexpected.
“We’re seeing a huge diversity of Legionella in marine environments
that mirrors what people see in fresh water,” Gast said. After combing
through a hefty green textbook titled simply Legionella, Gast still
hasn’t found the names of the 32 strains—or whether they’ve been
formally described at all.
New to science or not, it would help to know whether the Legionella in
Mount Hope Bay can make beachgoers sick. To find out, Gast found
published DNA sequences for disease-causing genes in other Legionella.
Now, she is constructing experiments that will look for those harmful
genes in the Legionella from Mount Hope Bay.
Polysyllabic pathogens
Gast and Amaral-Zettler started looking for pathogenic Legionella
because of a quirk: The bacteria can invade amoebae and in doing so
become more virulent. Amoebae fight infections by trying to digest the
invaders. That’s the same way human white blood cells operate, so
Legionella cells that have survived inside amoebae also tend to foil
human immune systems.
The two scientists are also testing the Mount Hope Bay samples for two
kinds of amoebae that cause problems all on their own: Naegleria and
Acanthamoeba. People most often encounter Naegleria by taking a dip in
warm, stagnant fresh water. Though cases are rare, it causes a brain
infection that is fatal. Acanthamoeba is more widespread, found in
fresh to salty water and even contact lens cases. It can cause
persistent eye infections and occasionally attacks the brain.
Next, Gast and Amaral-Zettler will test samples for Cryptosporidium and
Giardia, another common stream-water microbe that can cause desperate
sprints to the bathroom.
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.
In 2004, two
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 sources).
The Woods
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 turbulent,
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.
