Listeria: A Common and Troublesome Contamination

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Recently, the CDC reported the deadliest outbreak of foodborne illness in the United States in over a decade.¹ The source was cantaloupes that transmitted Listeria monocytogenes—and the terrible tally: 25 deaths and 123 others sickened.² This is the third deadliest outbreak of foodborne illness since the CDC starting keeping such records.¹

What bacteria were responsible for the previous 2 serious outbreaks? It was Listeria and again Listeria both previous times. In 1985, Listeria-contaminated cheese killed 52 people, and in 1998, Listeria growing in hot dogs killed another 21.¹ This recurring theme does not tell the entire story, unfortunately. Just this year there have been less publicized outbreaks, each involving Listeria. In Hawaii, Listeria contaminated 7-Eleven convenience meals in July; Kellogg was warned about this same bacterium at a cookie plant in Georgia in June; and 18,312 pounds of chicken nuggets (Pilgrim’s Pride) were recalled for the same reason—yes, Listeria contamination—again this last July.^1,3

A HARDY “BUG”
There is a definite and disturbing pattern here. The danger is a potentially lethal bacterium with an uncanny knack for surviving with a vengeance in processed foods, and recently in fruits. It defies routine refrigeration practices. To demonstrate its versatility, from 1999 to 2004, there were 4 outbreaks of Listeria infection from sandwiches purchased at hospitals in the United Kingdom.⁴

What makes this “bug” so hardy in conditions that seem to eradicate other bacteria? Agents proven to kill foodborne pathogens fare less successfully against Listeria. For instance, nitrite provides the color of cured meats and is an effective antimicrobial, killing diverse bacteria including Clostridium botulinum. However, nitrite slows the growth of Listeria but does not fully inhibit it.⁵

UNPALATABLE DEFENSE MEASURES
Other studies have succeeded in lowering Listeria counts in food, but at a price. When a cocktail of L monocytogenes was inoculated into cooked chicken, the meat required 2% sodium lactate in combination with pressure treatment (600 Mpa/2 min/20 degrees C) to effectively inhibit the bacterium’s growth.⁶ Other investigators have tried powders from cherry to cranberry with variable success.⁵ When the powders worked, they changed the taste of foods, making them less appetizing.

WHICH FOOD WILL BE NEXT?
Caution must be urged. Although it was impossible to predict the recent cantaloupe contamination, other foods have become predictably worrisome. Refrigerated foods containing chicken—especially sandwiches—that have been stored even at low temperatures for some time can be potentially lethal. For persons who are older or debilitated in any way, there is hidden danger lurking that may not be identified until it is too late. Listeria may be the foodborne pathogen of our generation.