The Persistence of Foodborne Listeria

Listeria bacteria can be deadly, especially when young children, pregnant women, and the elderly contract infections. Recent news about the bacteria—an outbreak linked to caramel-covered apples and a study that shows their ability to survive in retail environments—underscores the public health threat posed by Listeria and the need for measures to prevent future illnesses.

Known mostly for contaminating processed meats such as deli turkey and hot dogs, the bacteria can also be found in other foods, including unpasteurized milk, soft cheeses, smoked seafood, and certain fresh produce. While infections from Listeria are rare, they are especially lethal: One in 6 people diagnosed will die. The bacteria can also lead to serious complications during pregnancy, such as miscarriage, stillbirth, premature labor, and life-threatening illness in newborns.

In a Listeria outbreak that occurred from Oct. 2014 to Feb. 2015, 35 people were sickened and seven died from infections traced back to caramel-covered apples. One loss of pregnancy linked to the outbreak was also reported. According to the Centers for Disease Control and Prevention, environmental testing revealed Listeria contamination at a facility in California where the apples were packed. The agency announced that the outbreak ended Feb. 12.

That same week, Purdue University released a study showing the persistence of Listeria in retail food environments, even in freshly cleaned delis. In fact, nearly 7 percent of the samples taken around the deli area before the stores opened and 10 percent of samples taken during hours of operation revealed the presence of Listeria.

Contamination from bacteria is not visible to the naked eye, and affected meats are not visually spoiled or altered. "That's the challenge with pathogens such as Listeria, salmonella, E. coli, and norovirus. They don't cause changes in the characteristics of the products," said Haley Oliver, the study’s lead author. "Can you smell a food and tell if it's safe? Absolutely not."