One of America’s favorite Halloween treats - caramel apples - just might be a breeding ground for deadly bacteria. However, researchers say neither the apple nor the caramel is the culprit - what then is making caramel apples develop Listeria?

A new study shed light on the Listeria outbreak connected to caramel apples the previous winter – an incident that caused seven deaths and made more than 35 individuals sick.

Published Oct. 13 in the journal mBio, the researchers from the Food Research Institute of University of Wisconsin argued that apples are too acidic for the bacteria to live. The caramel, too, does not contain enough water for bacteria to typically grow.

The researchers then focused on the stick used for holding the caramel-dipped apples in their experiment. They put sticks into half the apple samples and left them off the remaining half.

"No growth of L. monocytogenes occurred on refrigerated caramel apples without sticks, whereas slow growth was observed on refrigerated caramel apples with sticks," they wrote, adding that “significant pathogen growth,” on the other hand, was seen within three days at room temperature on apples with sticks.

The researchers pinpointed the release of the apples' juices once the sticks were inserted, offering a moist haven for Listeria growth.

To see how L. monocytogenes could grow in the Halloween dessert, the team swabbed the fruits with the bacteria, dipped them in caramel and allowed to cool, and refrigerated them for four to six weeks.

A surprise for the researchers: even dipping the apple in hot caramel did not kill all surface bacteria, and the caramel coating even provided an ideal venue for bacterial growth.

According to lead author and institute associate director Kathleen Glass, setting out caramel apples at room temperature for a number of days – “maybe up to two weeks” – will give plenty of time for the bacteria to grow.

The team encouraged food producers to consider food components as likely niches for pathogen growth. They added that since it is nearly impossible to eat caramel apples without the stick, makers of this food should try extra disinfection and refrigeration of the fruits and materials used in their preparation.

Infectious disease expert Dr. William Schaffner of the Vanderbilt University in Nashville, Tennessee said that Listeria infection can be tough to identify due to the incubation period that can take weeks.

“But Listeria can smolder after you ingest it, you can be sick up to a month later,” he explained, citing diarrhea, abdominal pains, chills and fever as common symptoms.

Schaffner added that Listeria-linked caramel apples are low in numbers, but those with concerns can simply stick to a fresh, properly washed apple.

Listeria can affect both young and old - particularly the immunocompromised - and can result in a bad stomachache, even meningitis and miscarriages in rare cases.

Listeria is estimated by the U.S. Centers for Disease Control and Prevention (CDC) to be the cause of sickness of 1,600 people in the United States every year, killing as many as 260 people annually.

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