Header graphic for print

Food Safety News

Breaking news for everyone's consumption

Oyster research could lead to food safety strategies

Microbes battling microbes. This type of warfare is happening all around us — and even inside of us — every day. And as in every battle, there are good microbes and bad microbes, depending on what you want the outcome to be.

Tiny bacteriophages attack larger microscopic organisms such as viruses.

Tiny bacteriophages attack larger microscopic organisms such as viruses and pathogens that are harmful to humans and other animals.

In this case, the bad guys are pathogens that are infecting large numbers of oyster larvae before they have a chance to grow into the oysters that so many people love to eat. They go by the names of Vibrio coralliilyticus and Vibrio tubiashii, with the first one especially harmful to oyster larvae. It’s the same vibrio that infects warm-water corals and contributes to coral bleaching around the world.

The good guys are what scientists call bacteriophages, or phages for short. They’re everywhere — inside of us, on our skin, in the soil, inside and on the outside of plants and animals, and in the ocean. According to researchers in the United States and Australia, humans have 10 trillion or so bacteria in their gastrointestinal tracts — and more than 10 times that number of phages residing and “working” there.

The phages do their work by going after specific, targeted bacteria, infecting and then killing them. In short, they help keep the balance between the good and the bad.

No, this isn’t some obscure science known only to researchers muttering into their microscopes in scientific terms no one can pronounce. Quite the opposite. The U.S. Department of Agriculture recently awarded a $500,000 grant to Intralytix Inc. to develop a phage cocktail that will be effective against the pathogens that kill larval shellfish.

The Baltimore biotechnology company’s scientists are working with USDA researchers in Delaware and at Oregon State University.

And while their research involves vibrios that infect oyster larvae but don’t make humans sick, two of the researchers said it offers the possibility of further research against two vibrios in raw oysters that do make people sick, often with severe digestive symptoms.

One of those, Vibrio parahaemolyticus can infect oysters, often during the summer months. Gary Richards, the lead scientist at the USDA’s Agricultural Research Service (ARS) lab in Dover, DE, said this vibrio is the principle cause of shellfish-related bacterial illnesses in the United States. It annually forces the closure of shellfish harvesting areas .

Vibrio vulnificus is scarier. It can cause serious illness and death. Between 1988 and 2006 the Centers for Disease Control and Prevention received reports of more than 900 Vibrio vulnificus infections from the Gulf Coast states, where most cases occur. Some such infections happen when individuals eat raw or lightly cooked oysters, others occur when the vibrio gets into swimmers’ open wounds.

Cooking oysters thoroughly kills vibrio, but that means cooking them several minutes after they open up. Internal temperatures need to be 145 degrees F.

Vibrios live in salty water and are typically found along the coastlines of the U.S. from Maine to Florida, in the Gulf of Mexico, and along the entire West Coast. These bacteria occur naturally and are not caused by pollution.

Intralytix CEO John Woloszyn said that although the current research project is better described as aquaculture/veterinary medicine, “hopefully, success might lead to a human food safety project.”

rawoyster_406x250

Getting back to the beginning

When adult oysters spawn, they release eggs and sperm into the water. Once fertilized, the eggs hatch into free-swimming larvae. During the three-week larval state, the shell begins to form and size increases.

In the case of commercial production, oyster hatcheries grow larvae to the eyed stage, when they lose their motility. Eyed larvae can be planted into harvesting beds by the commercial shell fishermen, where the larvae will mature in two to three years.

Alternatively, the free-swimming larvae can be reared in the presence of oyster shell. In that case, they will attach to the shell, which can be planted in growing areas, where they will continue to grow until they’re ready to be harvested. This also generally takes two to three years.

Unfortunately, the larvae in some of the Pacific Coast hatcheries have been having trouble developing their shells fast enough. In 2005, the wild and hatchery oyster larvae along the West Coast began to die off in the millions. Things got worse. Between 2006 and 2008, billions of larvae — as many as 80 percent of them — died.

Because commercial growers rely on the larvae and seed they buy from hatcheries, this added up to devastating losses for the hatcheries and the growers who relied on them for sustained production.

All sorts of reasons, among them viral and bacterial infections, were pointed to as possible culprits. But despite a variety of strategies to tackle the problem with infections in mind, intermittent problems persisted.

Then in 2008, researchers from the National Oceanic and Atmospheric Administration and Oregon State University came up with this possibility: Could it be climate change? Perhaps greenhouse gas emissions, specifically carbon dioxide, were changing the ocean’s chemistry and causing a condition known as ocean acidification.

The researchers got to work and found that in places where they conducted studies, acidified water could stop the larvae from developing their shells.

Oregon State University researcher ocean ecologist George Waldbusser said it’s not so much that the acidity of the water was dissolving the shells. Rather, it was inhibiting how quickly the larvae could form the shells.

Turns out that in acidic water conditions, the oyster larvae have to use too much energy to build their shells as quickly as possible, which they need to do so they can continue growing.

“The effort literally kills them,”  Waldbusser said.

And although hatcheries have taken steps to circumvent acidification of their seawater, intermittent problems with high mortalities still occur.

The USDA’s Richards said both vibrios are believed to be “opportunistic pathogens” and that they may infect larval shellfish when the shellfish are stressed by conditions such as water acidification, improper water temperature, salinity or dissolved oxygen levels, poor nutrition, or overcrowding, among others.

“Stresses make the larvae more susceptible to infection by opportunistic bacteria,” he said.

He also said the USDA grant project will determine if similar reductions in mortality that he and his fellow researchers have already achieved in the lab with phages can be achieved in a small-scale hatchery.

Another scientist working on the project, Claudia Hase of the College of Veterinary Medicine at Oregon State University, said that Vibrio coralliilyticus is highly infectious to both Pacific and Eastern oyster larvae.

She also said it has a powerful toxin delivery system. And if that’s not enough, vibrios are among the smartest of all bacteria. She said that in an “IQ test” administered to bacteria, vibrios ranked in the top bracket.

“They can smell, sense things, and swim toward a host,” Hase said. “They can even ‘talk’ to each other. They’re incredibly powerful.”

No wonder then that the shellfish industry is so interested in finding ways to build up an arsenal against these pathogens. It’s a matter of economic survival.

“The focus right now is on larval mortality,” said Bill Dewey, spokesman for Taylor Shellfish Farms. “We are relying on research to lead the way.”

Intralytix chief scientist and principal investigator Alexander Sulakvelidze said the goal will be to develop an all-natural product that can eliminate or significantly reduce larval shellfish mortality caused by the vibrio pathogens that are causing mortalities in excess of 59 percent.

Phages are considered “all-natural” because they are part of nature, and not manufactured.

“We view this grant as an important first step in developing a series of phage-based natural products for the aquaculture industry,” said Intralytix CEO Woloszyn.

Woloszyn said the two vibrios (V. coralliilyticus and V. tubiashii)  are major causes of larval shellfish mortality, which results in increased costs to the aquaculture industry and consumer.

“Our phage preparation could have very significant impact on reducing larval mortalities in hatcheries and on reducing the cost of producing and buying oysters,” he said. “We are confident of success that will result in a bi-coastal benefit to both oystermen and oyster lovers.”

Just what phages will be used in the cocktails and how they’ll be administered will be a major focus of the research project.

Intralytix has already introduced three phage-based food safety products approved by U.S. regulatory authorities. ListShield, EcoShield and SalmoFresh are the company’s current commercial offerings effective against Listeria monocytogenes, E. coli O157:H7 and Salmonella spp., respectively. In the seafood industry, the company’s ListShield has become popular among smoked salmon processors to substantially reduce or eliminate Listeria monocytogenes.

So what are phages anyway?
Not discovered until 1915, bacteriophages — so named because they appeared to eat bacteria — are naturally occurring viruses that can infect and kill bacteria, among them typhoid, cholera, tuberculosis, meningitis, as well as foodborne pathogens such as E. coli, Listeria, Salmonella and Campylobacter.

Until the electron microscope was invented in 1940, these bacteriophages couldn’t be seen as anything but a clear spot under a regular microscope.

The way they work is nothing short of amazing. Some bacteriophages, for example, have hollow heads, where their DNA or RNA is stored. At the other end they sport tails that could be compared to tunnels. The tips of these tails can “dock” onto molecules on the surface of the specific bacteria they’re targeting.

 

With that mission accomplished, they begin shooting their viral DNA through their tails into the targeted cell. Once inside, the DNA takes over and starts directing the production of progeny phages — often more than a hundred in a mere 30 minutes. These young phage-warriors then burst out of the host cell, killing it in the process, and eagerly head off in search of more bacteria to infect and kill.

As powerful as phages are against bacteria, they’re nontoxic to mammals and the environment because they require the specialized form of cellular machinery found only in bacteria to multiply. Another plus is that no genetic engineering is required to make phage cocktails. Scientists say because bacteriophages are so plentiful, it’s not difficult to combine them into cocktails that can be highly effective against bacteria.

This is not pie-in-the-sky science fiction. Before antibiotics, phage therapy was used with varying degrees of success against a range of bacterial diseases. But with the advent of antibiotics during World War II, interest in phage therapy plummeted. However, increased concern about antibiotic resistance has once again put them into the spotlight.

Phage cocktails and food safety
Phages don’t affect the taste, smell, consistency or the freshness of food. And in the case of oysters, they would not kill them.

“Making food safer Nature’s way,” is how an Intralytix video about phages and food safety describes it.

USDA lead scientist Richards said he anticipates phage-intervention will be the first of many treatments for use by the U.S. seafood industry “as we continue to strive to improve aquaculture operations and develop novel processing technologies to enhance seafood safety.”

“Clearly the use of naturally occurring phages to combat pathogens in the food supply would represent a ‘green’ approach for tackling some of today’s problems facing agricultural production and food safety” he said.

“Absolutely,” agreed veterinary scientist Hase when asked about the possibility of using phages for food safety in oysters, especially for raw oysters on the half shell.

“Consumers are so conscious about food safety,” she said. “Fear can drive people away from a product like raw oysters. But when they know the oysters are safe, they want them.”

Taylor Shellfish spokesman Dewey agreed.

“The main market and excitement in oysters revolves around live oysters on the half shell,” he said. “We’re looking for a silver bullet that would eliminate vibrio and leave the oysters alive.”

While the company is achieving good success by putting the oysters in ice as soon as they’re harvested and keeping them below 50 degrees, phages might provide a better alternative.

“A live seawater oyster-holding system with the phages in it might be one way to eliminate Vibrio parahaemolyticus,” he said. “It’s certainly a potential down the road.”

(To sign up for a free subscription to Food Safety News, click here.)

 

© Food Safety News