FDA studies bacteria that could fight salmonella in tomatoes

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By Lyndsey Layton
Washington Post Staff Writer
Saturday, October 31, 2009

The 10 tomatoes sitting in a Tupperware tub at the Food and Drug Administration seem to be doing nothing more than rotting, slowly. But an invisible battle is raging on the surface of the fruit, with provocative implications for food safety and the war that humans have been waging against bacteria for a century.

"This is the wrestling ring," said Eric Brown, a microbiologist at the FDA's Center for Food Safety and Applied Nutrition, as he clicked open the lid to the tub. "This is the smack-down."

Brown and a team of FDA scientists trying to prevent salmonella contamination in tomatoes have stumbled upon what they believe are powerful, naturally occurring "good" bacteria that can slaughter the "bad" bacteria that have become a persistent problem in fresh fruits and vegetables because they harm humans.

"This is highly efficient weaponry, right here," said Brown, pointing to pipettes filled with the "good" bacteria suspended in a saline solution that will be dripped onto the contaminated tomatoes. He presented the initial findings of his research at an international salmonella conference this month in France. "The beauty is that we take something alive and organic and put it back into the field, and by itself, it will kill other bacteria. We're right on the edge of this."

It's a variation on the "enemy of my enemy" philosophy, with scientists like Brown cultivating hostile relatives of harmful bacteria to perform a sort of microscopic fratricide before the bugs can harm humans.

While Brown's findings haven't been applied outside the laboratory yet, in his experiments the microorganisms obliterate not only salmonella on tomatoes but also several other pathogens blamed for food-borne illnesses, including listeria and E. coli O15:H7. So far, only vibrio, the bacterium found in warm seawater that can contaminate oysters and other seafood, has stood its ground against Brown's bacteria.

"It's a phenomenal finding he's got," said Steve Rideout, an assistant professor of plant pathology at Virginia Tech, who has allowed the FDA team to take samples from the university's 200-acre research farm tucked among industrial tomato growers on Virginia's Eastern Shore.

Salmonella has become a leading cause of food-borne illness in the United States. Once largely associated with poultry and eggs, the bacteria live in the intestines of animals. But recently, the bacteria are increasingly in fresh fruit and vegetables, for unknown reasons. Of nine nationwide salmonella illness outbreaks since 2007, just one was linked to a meat product, according to the Centers for Disease Control and Prevention. The rest were associated with vegetables, fruit, nuts or cereal.

Salmonella causes about 1.4 million cases of food-borne illnesses and more than 500 deaths a year in the United States, according to the CDC. While most people recover without treatment, young children, the elderly and those with weakened immune systems can become severely ill.

Salmonella, which describes a group of 2,600 strains of related bacteria, can survive outside living organisms and has lived as long as 18 months in soil.

"Salmonella is turning out to be a far more challenging environmental bug than we ever thought it would be or could be," Brown said. "It's very difficult to get rid of, and we don't even know where it comes from."

Fresh produce

Fruit and vegetable farmers face a particular challenge because produce is often eaten fresh; there is no "kill step," such as cooking. Most produce is not packaged, which increases the chances for contamination. And some pathogens such as salmonella and E. coli burrow inside the fruits and vegetables, making it impossible to wash them away. In fact, Brown has found that his microorganisms are not effective against salmonella when the battle takes place inside the tomato, only on the surface.


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