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IAFP 2015: Experts May Have Determined How Caramel Apples Caused That Listeria Outbreak

CDC caramel applesIn November 2014, health officials began investigating two concerning clusters of Listeria infections that seemed to be related. The two strains of bacteria had already killed at least five people and hospitalized a few dozen others by the time they got on the case, and they wanted to find the source and stop it as quickly as possible.

Ultimately, the investigation led back to a very unexpected source. It was caramel apples, sold under various brand names but which all used apples traced to one distributor, Bidart Bros. of Shafter, CA.

Perhaps even more unexpected was the next observation: The Bidart Bros. apples that were coated in caramel sickened people, and yet others had eaten the non-caramel apples from Bidart Bros. and did not become ill.

For most people, the first assumption would be that the caramel, or the coating process, was somehow causing the contamination. But that didn’t make sense to investigators, since so many different companies were responsible for the caramel coating.

Testing at the Bidart Bros. facility showed that the contamination was clearly occurring there. That’s when experts began hypothesizing that the caramel coating process was somehow exacerbating the contamination.

To get to the bottom of the matter, Dr. Kathleen Glass from the University of Wisconsin-Madison led a study that replicated the procedures and conditions that the caramel apples had likely undergone. Glass, associate director of the Food Research Institute at the university, presented her preliminary findings on Tuesday at the 2015 International Association for Food Protection annual meeting in Portland, OR.

On their own, neither apples nor caramel struck Glass as foods that would typically foster Listeria growth. Something about the caramel coating process had allowed for bacterial growth.

Her hypothesis: When the sticks (used for holding the caramel apple) punctured the contaminated apples, they spread small amounts of apple juice over surface of the otherwise dry outer skin. Then, once the caramel coating was applied, it locked in the juice and created a micro-environment in which the Listeria on the surface of the apple could grow undisturbed.

To test the hypothesis, Glass’ team inoculated a number of apples with Listeria. They then punctured the apples with a stick and applied caramel coating to half of them, while coating the other half in caramel but leaving out the sticks, so that there was no puncture to allow for juice to escape from the inside of the apples.

Then, half of stick-punctured caramel apples went into a refrigerator, and the other half were left at room temperature. The same was done with the stick-free apples: half in the fridge, half at room temperature.

Both sets of stick-punctured caramel apples — the room-temperature and the refrigerated — grew Listeria at a significantly faster rate than the stick-free caramel apples.

Within days, the amount of Listeria present on the stick-punctured, room-temperature apples more than doubled, while the stick-free room-temperature caramel apples saw mild, steady growth of the bacteria over a much longer period of time. The stick-punctured apples in the fridge still grew a significant amount of bacteria, while the growth of bacteria on the stick-free refrigerated apples was relatively minimal.

Glass said the data were still under peer review, but she believes this is a plausible explanation for how the first-known caramel apple Listeria outbreak could have happened.

Earlier in the same presentation, Robert Tauxe from the U.S. Centers for Disease Control and Prevention (CDC) shared statistics with the audience on the historic numbers of Listeria outbreaks detected in the U.S. each year, dating back to 1983.

For the 14-year period from 1983-1997, the U.S. detected only five Listeria outbreaks, said Tauxe, who is CDC’s deputy director of the Division of Foodborne, Waterborne and Environmental Diseases.

In 1998, CDC introduced PulseNet, a network and pathogen database shared by health departments around the country. With increased outbreak surveillance, the agency began detecting 2.3 Listeria outbreaks each year between 1998-2003.

In 2004, the agency introduced its “Listeria initiative,” an enhanced surveillance system for Listeria infections, and the average number of outbreaks detected each year rose to 2.9.

Finally, in 2014, many health departments around the country had begun to adopt whole-genome sequencing, a method of identifying pathogens much more accurately than previous methods. That year, the agency counted nine Listeria outbreaks — by far the most it has ever detected in a year.

It’s not that we’re experiencing more Listeria outbreaks today than in the 1980s, Tauxe explained. We’re simply getting much better at finding them.

Correction: This article has been updated to correct an error. The caramel apple study was erroneously said to split the apples into two separate groups: Apples that were punctured with a stick and then coated with caramel, and apples that were punctured with a stick, but left caramel-free. That was incorrect. The two groups were divided between apples that were punctured with a stick and then coated with caramel, and apples that were coated with caramel, but not punctured with a stick.

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  • Jack Guzewich

    One important correction to this story as reported. Three of the ill persons in this outbreak did not recall having eaten caramel apples, but they had consumed apples. It was not possible to say where the apples they ate had come from.

  • Kathleen Glass

    Correction: Note that there was no growth of Listeria on the surface of the uncoated (“plain”) apples that we tested during one month storage. Report is correct that there was rapid growth within three day on caramel-coated apples with sticks when stored out of refrigeration and much slower growth at refrigeration.

  • Tom Edlind

    Is there hard evidence that the increased use of whole genome sequencing in 2014 led to the identification of those 9 Listerina outbreaks, or a specific subset of them? Or was WGS only used retrospectively (as it usually is) to confirm identifications made by conventional methods?

  • Dave Walpuck

    Excellent article. Thank you

  • LP

    Nice work Dr. Glass et al.