Red apples on sticks, bathed in gooey caramel sounds tasty, but it could send you to the hospital with a bad case of food poisoning, according to a recent study.

Researchers from the University of Wisconsin-Madison's Food Research Institute found that caramel apples punctured with dipping sticks and left unrefrigerated over the course of a couple of weeks may harbor a bacterium called Listeria monocytogenes, The Huffington Post reported. The average population of Listeria increased 1,000-fold on caramel apples with sticks stored at room temperature for three days. By contrast, listerial growth was delayed on caramel apples without sticks stored at room temperature.

The study was prompted by an outbreak of listeriosis in late 2014, in which 35 people from 12 states were infected and seven people died, according to ABC News.

For the study, researchers prepared a cocktail of four L. monocytogenes strains associated with the outbreak and swabbed it on the skin, stem and calyx regions of a group of Granny Smith apples. They inserted wooden sticks through the stems of half of the apples. They dipped all apples into hot caramel using either the sticks or tongs, then allowed them to cool. The apples were then stored either at 25 degrees Celsius (77° F) or 7 degrees Celsius (44.6° F) for up to four weeks.

Researcher Kathleen Glass said dipping apples in hot caramel killed off a lot of the surface bacteria, but "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."

She said caramel apple manufacturers may wish to thoroughly disinfect apples before dipping them in caramel, add growth inhibitors to the caramel coating or apple wax, or use better temperature-time controls to inhibit the growth of L. monocytogenes.

To be safe, she said, consumers should look for refrigerated caramel apples or eat them fresh.

The findings are detailed in the journal mBio.