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Fighting fire with fire

By Sharon Omahen
University of Georgia

University of Georgia scientists have found bacteria that kill listeria in processing plant floor drains, where the pathogen is known to settle and multiply.

"There are just a few thousand cases of listeria in humans each year," said Michael Doyle, a microbiologist and director of the UGA Center for Food Safety in Griffin, Ga. "But, of those, about 500 die. That's a high mortality rate, and that's why listeria infections are a major concern in our country."

Pregnant women, cancer patients and transplant patients are among the most frequent cases seen. "Listeria strikes these immunocompromised populations hardest," Doyle said.

Luncheon meats = high risk

The U.S. Department of Agriculture identifies luncheon meats as high-risk products for listeria infections, Doyle said. Sliced turkey deli meats are high on the list.

"Listeria can grow to billions of cells in some refrigerated luncheon meats," he said. "And there have been major outbreaks traced to sliced turkey luncheon meats."

The deadly pathogen is often found in processing-plant floor drains. To help fight it, representatives of the processed meat industry asked Doyle and his UGA colleagues to help find a solution.

"Listeria can be widely distributed in processing plants," Doyle said. "It grows where there is water in areas like floor drains, where listeria can set up a home."

Unfortunately, floor drains are one of the toughest areas to effectively clean and treat for listeria in processing plants.

Biofilm forms protective coating

"Over time, a biofilm, or slime layer, develops in the drains," Doyle said. "This biofilm protects the listeria when cleaners and sanitizers are poured down the drains."

Knowing this, Doyle took a fight-fire-with-fire approach to killing the drain-dwelling listeria. He uses the same technology he developed for controlling E. coli and Salmonella.

"We took samples of biofilm from floor drains in processing plants where there was a history of low levels to no listeria," he said.

With the help of the plant operators, biofilm samples were taken from dairy, poultry and infant food processing plants. The scientists found nine different bacteria from biofilms that were highly effective in competing with and killing listeria.

"From these bacteria, we chose two strains that could grow with listeria and ultimately outcompete it," Doyle said.

The researchers then tested these two strains in a fresh poultry processing plant. Working with Ecolab Inc., UGA scientists used a foaming agent to apply the bacteria to drains.

"The foam adheres to the drain's surface and gives the good bacteria an opportunity to attach and grow in their new environment," Doyle said.

Foam/biofilm mixture successful

The drains were monitored for more than three months. The foam/biofilm mixture eliminated listeria in most drains to undetectable numbers for several weeks.

"In some drains, where there was a continuous influx of processing wastes, we were able to bring the numbers down dramatically," Doyle said, "but we couldn't totally eliminate the listeria."

Next, the UGA researchers tested the bacteria mixture in a ready-to-eat deli meat processing plant. The listeria numbers are much lower in these plants than in fresh meat plants.

Six drains were treated, and two were used as natural control drains. After eight weeks, the scientists found five of the six treated drains were free of detectable listeria.

Ecolab has licensed this technology from UGA and is developing a formulation that will be further tested. The company intends to make the product commercially available after regulatory review and approval.

(Sharon Omahen is a news editor with the University of Georgia College of Agricultural and Environmental Sciences.)

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