Bacteria recruited to fight salmonella

WASHINGTON – 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.”

Working in a College Park, Md., laboratory, 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.”

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 being found 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.”

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.

Salmonella is a particular problem for tomato growers in Florida, Georgia, Virginia and other states along the East Coast. And the tomato industry is eager for a way to reduce salmonella contamination and regain consumer confidence, said Reggie Brown of the Florida Tomato Exchange. An outbreak of salmonella illness last year was incorrectly linked to tomatoes – later traced to jalapeno peppers from Mexico – but not before the tomato industry suffered losses of about $150 million. Consumer demand still has not returned to levels before that outbreak, said Reggie Brown, who is not related to Eric Brown.

“Every time someone reports in a news article on food safety about an outbreak associated with salmonella, the word ‘tomato’ will appear out there,” Reggie Brown said. “There’s concern and distrust.”

Like Brown, other researchers have been trying to find a naturally occurring bacteria that can kill pathogens on human food. Scientists at the U.S. Department of Agriculture reported in 2007 that they isolated microorganisms from baby carrots that suppressed the growth of four harmful pathogens. Last year, the same team applied “good” bacteria to kill salmonella on sprouts.

Brown and his colleagues, known around the FDA as “Team Tomato,” made repeated trips to tomato-growing regions on Virginia’s Eastern Shore this spring and summer, where they stomped through fields and swamps and collected more than 200 samples from plants, water and soil. Each trip, they stayed with their coolers of water and muck and plants in the same hotel. “I had to leave a substantial tip to satisfy my conscience,” Brown said.

Back in their College Park, Md., lab, Brown and his main collaborator, Alex Enurah, identified more than 300 species before isolating five “good” bacteria.

The “good” bacteria don’t appear to be harmful to humans but further investigation is needed, Brown and Enurah said. They intend to test the bacteria on tomatoes grown in research hothouses in the coming months. And they are testing whether the “good” bacteria could prevent salmonella contamination as well as treat fruit that is already tainted. Brown envisions a day when “good” bacteria could be sprayed onto fields of tomatoes just before harvest.

Still, what works in the laboratory often runs into trouble on the farm and never makes it to the grocery store, said Jeri Barak, an assistant professor of plant pathology at the University of Wisconsin at Madison.

“The tripping point for biocontrol has always been commercial application,” she said, noting that outside the lab, the process is complicated by the presence of many other bacteria and fungi, and even sunlight. “A lot of times what looks really, really good in the laboratory doesn’t work in the field.”