Fighting Listeria

By Dave Edmark, Arkansas Agricultural Experiment Station
Aug. 27, 2007

FAYETTEVILLE, Ark. - Try going for a couple of weeks without food and see what shape you're in, or if you're around at all by then. Let some dangerous bacteria do the same thing and there's a chance they may do more than survive; they can grow stronger and remain as virulent as ever.

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The bug is Listeria monocytogenes, a pathogenic foodborne bacterium the Centers for Disease Control estimates causes about 2,500 illnesses and 500 deaths nationwide each year. When these bacteria get a foothold, it's difficult to eliminate all of them. It has led Michael Johnson's Food Safety Consortium research team at the University of Arkansas Division of Agriculture to examine what keeps these bacteria going and what it takes to bring them down.

"The question is how do these cells survive when there's nothing for them to live on," said Johnson, a food science professor. "One of the presumptions is that they're living on some of the breakdown products of the cells in the population that die. That's supported by the results that the cells sitting in the original waste material survive better than do those which have their buffer changed every four days."

Bwalya Lungu, one of Johnson's doctoral students, explored the issue in a paper she presented in May at the American Society for Microbiology conference in Toronto. She examined the survival of L. monocytogenes bacteria after 28 days and found that even after significant numbers of the pathogen cells are eliminated, those that survived appeared to do so because they make efficient use of the dead cells' waste. Also, Johnson noted, the cells go into a suspended state of animation and are not actively metabolizing.

"They're in a resting stage - not growing, not dying," Johnson said. "You and I can't stop our metabolic train, but this organism apparently has figured out ways to shut down its metabolism."

The cells that die first leave behind the waste products that somehow enable survivors to hang on longer. If more waste nutrients can be eliminated up front, the remaining cells have less opportunity to find something on which to live.

For processors, the situation points to the need for diligent efforts at sanitation. If, for example in a worst-case scenario, a space has about 10 million L. monocytogenes cells, then eliminating 90 percent of them would seem to be quite an accomplishment. However, that still leaves behind 1 million cells, enough to seek out some nutrients to ensure their own survival before they go on to do some damage.

Eliminating 99.999 percent of the original cells still would leave behind 100 cells. That would appear to be an even greater accomplishment, except for persons who are at risk because of low tolerance of infection.

"We don't know what an infectious dose of Listeria is for the susceptible population," Johnson explained. "About 25 percent of the population is immunocompromised due to being pregnant, being an organ transplant patient, having chemotherapy for cancer or being 65 years or older. Those are all situations that could compromise the ability for such persons to fight invading bacteria."

The result is that the federal Food and Drug Administration has declared a zero-tolerance policy on the presence of L. monocytogenes in ready-to-eat foods such as ice cream or deli meat and poultry items. That means there should be no detectable L. monocytogenes cells in a 25-gram portion of such foods, Johnson said.

The researchers' findings about the durability of L. monocytogenes are based on results of work in the laboratory, but they are applicable to the situation that food processing plants face daily, Johnson noted.

"We can't say explicitly, but if the bacteria can survive like this in liquid then it's less of a mystery how they also can survive when they get airborne," he said.

"In food processing plants, when people thoroughly clean down the equipment the floor drains are places without a lot of nutrients left. But there's still potential for a little residue there. Bottom line: It's very hard to reduce Listeria in plants when you have sanitation programs that leave surfaces wet. Keep it dry after you clean."

That's when all the instructions about keeping things clean start to hit home. Johnson said cleaning crews use chemicals to cut through the food waste film on surfaces and then apply sanitizer chemicals ideally to destroy any bacteria left. But if any bacteria persist and water is left standing anywhere, then the bacteria have a helping hand toward their survival even if there are no nutrients left.

Last Updated: 11/27/2007