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 also 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 leaving the other half caramel-free.
Then, half of the caramel-coated apples went into a refrigerator, and the other half were left at room temperature. The same was done with the caramel-free apples: half in the fridge, half at room temperature.
Both sets of caramel-coated apples — the room-temperature and the refrigerated — grew Listeria at a significantly faster rate than the caramel-free apples.
Within days, the amount of Listeria present on the caramel-coated room-temperature apples more than doubled, while the caramel-free room-temperature apples saw mild, steady growth of the bacteria over a much longer period of time. The caramel-coated apple in the fridge still grew a significant amount of bacteria, while the growth of bacteria on the caramel-free refrigerated apple 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 Related artic
much better at finding them.