Halloween is nearly upon us, and that means spooky costumes, trick-or-treating, and caramel apples! However, it's not just unwrapped candy that vigilant parents should look out for. According to new research, Listeria monocytogenes can grow on candy-coated apples.

According to the US Centers for Disease Control and Prevention (CDC), a Listeria infection (Listeriosis) affects 1600 Americans each year. For most, the bacterial infection passes in a few days, causing fever, muscle aches and intestinal problems. However, for those particularly vulnerable -- namely pregnant women and the elderly -- a persistent infection can prove fatal. In fact, the disease kills a stunning 260 people in the US every year. The World Health Organization (WHO) estimates that it's even worse for undeveloped worlds, with food-borne illness killing nearly a fourth of its victims worldwide.

The good news is that Listeria doesn't stand a chance against a healthy immune system, and its main carriers -- mainly melons and other firm-skinned produce -- are thoroughly cleaned before arriving at your local grocery. However, on the off-chance an infection does fight its way into your body, the bacteria is infamously hard to detect until it has entrenched itself.

That's why investigators from the University of Wisconsin-Madison's Food Research Institute decided to turn their attention to Listeria hosts the average person will encounter this autumn -- a time when our immune systems are struggling to cope with things like temperature shifts, mildew allergies, and flu season. (Scroll to read on...)

Normally, neither caramel nor apples are prime real-estate for Listeria. However, according to Kathleen Glass, the associate director of the institute, that changes when you put them on a stick.

How can that be? Stabbing the apple causes some juice to come to the surface, and that moisture trapped under a layer of caramel, "creates a microenvironment that facilitates growth of any L. monocytogenes cells already present on the apple surface," Glass explained.

In fact, a 2014 outbreak of Listeria which tragically killed seven people was linked to pre-packaged caramel apples.

To properly assess this threat, Glass and her colleagues studied Listeria growth on a group of Granny Smith apples dipped in caramel. Some of these treats were stored in either a refrigerator, or at room temperature. (Scroll to read on...)

They were then left to sit for a whopping four weeks and tested at scheduled times throughout the wait. Disturbingly, in just three days, Listeria colonies on the unrefrigerated apples grew about 1000-fold in size. As for the refrigerated group, no growth was seen at all until the end of the first week. A third group, dipped apples without sticks, showed no bacterial growth at all despite being stored at room temperature.

According to Glass, the risk these seasonal treats pose in the first place should not be great, as dipping apples in hot caramel usually kills most surface bacteria.

However, "those that still survived were the ones that were able to grow. If someone ate those apples fresh, they probably would not get sick. But because caramel-dipped apples are typically set out at room temperature for multiple days, maybe up to two weeks, it is enough time for the bacteria to grow."

To be safe then, it might be wise to eat caramel or candied apples only if they have been refrigerated. Otherwise, one can always eat them fresh -- a strategy that's as safe as it is delicious.

The results were recently published in this month's issue of mBio, a journal of the American Society for Microbiology.

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