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  • Writer's pictureP.K. Peterson

Some Like It Cold: Listeria Outbreak from Contaminated Ice Cream

“Bacteria won’t reproduce in the freezer, but freezing won’t kill them. So whatever was in your food when you froze it will be there when you thaw it.”

- Doug Goff, Professor of Food Sciences at the University of Guelph in Ontario

“My advice to you is not to inquire why or whither, but just enjoy your ice cream while it’s on your plate.”

- Thornton Wilder, American author and playwright

On July 2, 2022, the Centers for Disease Control and Prevention (CDC) issued this Food Safety Alert: “Listeria Outbreak Linked to Ice Cream.” In this week’s post, I provide information on the recent listeria outbreak and, for readers who thought refrigeration is a foolproof way of protecting food, some background on cold-tolerant bacteria.

Listeria outbreak linked to Big Olaf Creamery ice cream. Outbreaks occur when a food is contaminated by a pathogen. The CDC recently identified the multistate outbreak of listeriosis and quickly linked it to ice cream produced by the Big Olaf Creamery in Sarasota, Florida. Big Olaf ice cream is sold only in Florida, but listeriosis cases were reported in ten patients from other states, eight of whom had traveled to Florida (the incubation period for listeria varies from three to 70 days). On July 2, the CDC reported a total of 23 cases due to this outbreak resulting in 22 hospitalizations and one death.

Clinical features of listeriosis. Most people with listeriosis experience fever and muscle aches, and those with the most dreaded form of the disease—meningitis—may complain of headaches, stiff neck, confusion, loss of balance, and convulsions. The diagnosis of meningitis is usually established by blood or cerebrospinal culture, and delay in diagnosis or administration of antibiotic therapy often plays a role in fatal cases (case fatality may approach 25%).

Who’s at risk? L. monocytogenes is an opportunistic pathogen, meaning that it takes advantage of people with compromised cell-mediated immunity (CMI). In addition to organ transplant recipients, this includes pregnant women (diminished CMI protects the fetus against rejection), and those at both ends of the age spectrum. In the current outbreak, half the patients were at least 72 years old and five of the patients were pregnant.

How is Listeria picked up? Listeria can be found in soil, which can lead to vegetable contamination. In the 1990s, most listeriosis cases were linked to contaminated deli meats and hot dogs. In recent years, outbreaks have been tied to contaminated soft cheeses, celery, sprouts, cantaloupe, caramel apples, and, as in the current outbreak, ice cream.

Cold-tolerant bacteria. Bacteria, such as Listeria, Escherichia coli, and salmonella can survive in freezing temperatures. This means that if a food is contaminated with one of these pathogens, freezing the food will not destroy the pathogen.

Bacteria that grow at temperatures in the range of -5C (23F) to 30C (86F), with optimum temperatures between 10C (50F) and 20C (68F) are called psychrophiles. They have enzymes that work best when the conditions are cold, and they have cell membranes that remain fluid at these lower temperatures. These cold-loving bacteria provide scientists with clues for life that may exist on Mars where the average temperature is -60C (-80F).

Polar ecologist, Professor Andrew Clarke, led a study published in 2013 in PLOS One that revealed that below twenty degrees Celsius (-20C [-4F)], single-celled organisms dehydrate, sending them into a vitrified—glass-like—state during which they are unable to complete their life cycle. But these researchers also found that “once a cell is vitrified it can survive right down to incredibly low temperatures. It just can’t do much until it warms up.”

What should you do? Concerned that its original Food Safety Alert about Listeria-contaminated Big Olaf Creamery ice cream had not received sufficient attention and that customers and stores could still have contaminated product in their freezers, the CDC issued a second Food Safety Alert on July 3. It advised, “Consumers who have Big Olaf Creamery brand ice cream at home should throw away any remaining product. People should also clean and disinfect any surfaces, containers, ice cream scoops, or other serving utensils that may have touched Big Olaf ice cream products.”

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Main Page images courtesy of Shuxian Hu, MD. Dr. Hu is a scientist in the Neuroimmunology Research Laboratory at the University of Minnesota.

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