How Listeria beats sanitation programs

Researchers at the US University of Arkansas have discovered that some dangerous bacteria thrive as strongly without food as they do with it " a finding that could have great implications with current food preservation and hygiene practices.

The bug is Listeria monocytogenes, a pathogenic foodborne bacterium the US Centers for Disease Control estimated to be causing around 2500 illnesses and 500 deaths nationwide each year. When these bacteria get a foothold, it's difficult to eliminate all of them. It has led Prof 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 these cells survive when there's nothing for them to live on," said Johnson.

"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 those that have their buffer changed every four days."

Bwalya Lungu, one of Johnson's doctoral students, 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 made efficient use of the dead cells' waste. It was also noted that the cells go into a suspended state of animation and are not actively metabolising.

"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 endure longer. If more waste nutrients can be eliminated up front, the remaining cells have less opportunity to find something on which to live.

For the food industry, the findings point to the need for diligent efforts at sanitation. If, for example, a space has about 10 million L. monocytogenes cells, then eliminating 99.999 % would seem like a great accomplishment. However, that would still leave behind 100 cells, which is enough to endanger 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 said. "About 25% 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 US Food and Drug Administration has now 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. This means there should be no detectable L. monocytogenes cells in a 25 gram portion of those products.

However, it could prove quite difficult for food manufacturers to comply with this, in lieu of the recent findings.

"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," said Johnson.

"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."

Johnson said cleaning crews use chemicals to cut through the food waste film on surfaces and then apply sanitiser chemicals ideally to destroy any bacteria left. But if any bacteria persist and water is left standing anywhere, then L. monocytogenes have a helping hand towards their survival.