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Study Finds Persistent Rates of Listeria in Retail Delis

Rates of Listeria monocytogenes in retail delis suggest that standard cleaning procedures don’t sufficiently eliminate the potentially deadly bacteria, according to a new study by researchers at Purdue University.

During one phase of the study, researchers found 9.5 percent of samples taken from retail delis to be contaminated with Listeria. That includes numerous samples taken from grocery delis of major national retailers.

Deli meats have long been associated with Listeria risk, but this is the first time the presence of Listeria in retail delis has been measured to this degree. While Listeria monocytogenes is only estimated to infect about 1,600 people in the U.S. each year, it kills roughly 260 of them, making it one of the most deadly pathogens found in food.

And, while food manufacturers are required to strictly control for Listeria, retail stores have no such requirements.

“Manufacturing has been under scrutiny for quite some time to control for Listeria, but a deli is a much more complicated environment,” Haley Oliver, assistant professor of food science at Purdue University and lead author of the study, told Food Safety News.

As Oliver explained, it’s more difficult to standardize pathogen controls in the open environments of delis, especially where foot traffic and floor plans vary slightly from one location to the next — even within national retail chains. And, although she doesn’t have the data to back it up, she suspects that socio-economic factors may also play a role — for example, deli workers may be low-wage employees with high job turnover and little training in controlling for something like Listeria.

Over the course of six months, the Purdue team took more than 4,500 samples from 30 retail delis. They looked for contamination in three types of surfaces: food-contact surfaces, such as meat slicers; non-food-contact surfaces, such as floors, and potential transfer points, such as handles and knobs.

They found contamination on 4.5 percent of food-contact surfaces and 14.2 percent of non-food-contact areas, for an average of 9.5 percent contamination for all samples. They also found contamination on 3.3 percent of transfer points.

Oliver said she found those rates incredibly alarming and that pregnant women and people with weakened immune systems should be warned about eating cold deli meat unless they heat it themselves to kill potential pathogens.

At least two noteworthy Salmonella outbreaks have been traced to meat sliced at retail delis in recent years, but no major Listeria outbreaks have surfaced. In 2008, 57 people fell ill and 22 died in a Listeria outbreak linked to deli meat produced by Maple Leaf Foods in Toronto, but that meat was pre-packaged and not sliced at a grocery deli.

So, if there’s really so much Listeria contamination around retail delis, why don’t we see more outbreaks linked to delis?

The answer isn’t so simple. For the most part, tracking such an outbreak is no small feat.

Listeria is a ‘perfect storm’ disease,” Oliver said. “Getting enough cases to fall ill at the same time at one deli and then managing to trace them all back to the specific grocery store is extremely difficult.”

Oliver said it’s very likely that infections occur sporadically from retail deli meat, but not frequently enough for health department investigators to triangulate a source. Listeria infections also have an abnormally wide incubation period, stretching anywhere from three to 70 days, making it even more difficult to identify illness clusters.

Of the infections that have been connected to deli meats, as much as 80 percent have come from deli meats sliced at retail groceries, according to Ernest Julian, chief of the Office of Food Protection at the Rhode Island Department of Health, speaking to Food Safety News last June for an article on illnesses caused by faulty, contaminated deli slicers.

In 2010, a Salmonella outbreak in Rhode Island was ultimately traced back to a slicer in a deli. At some point, tomatoes contaminated with Salmonella were prepared in the machine, leaving contaminated food debris behind in impossible-to-clean areas of the slicer, where it ultimately dripped onto meat being sliced.

“It’s a hidden hazard,” Julian said. “You look at it and it looks OK, but there’s this hairline crack with all this food debris getting through it.”

Oliver said that deli slicers are most certainly not getting cleaned as often as recommended. It can take 20 or 30 minutes to properly clean a deli slicer, with some companies recommending they be cleaned as often as every four hours, she said.

Getting the grocery chains to participate in the study was tricky at first, Oliver said. But through the double-blind study design and a promise to be completely transparent with data, the stores obliged. Some have even volunteered for future phases of the study because they want to understand what risks and challenges their delis face.

For their next paper, the Purdue team is examining how well the delis reduced their Listeria rates after following new or revised sanitation procedures and taking on new intervention steps to reduce contamination.

Those data have not been completely gathered, but so far Oliver said that the researchers have seen “some improvements” in stores where the presence of Listeria was originally moderate. However, high rates still persist in stores where there was originally a high amount of Listeria.

After they publish that study, Oliver and her team plan to expand their work to another 70 stores, bringing the total sample size to 100. By doing so, they hope to better identify ways to predict which delis might have the most trouble with contamination based on shared characteristics and then work to reduce the contamination levels there.

In the study’s original group of 30 stores, eight had what the team considered “very high” prevalence of contamination. But other stores had none at all, which Oliver viewed as proof that the others could reach that point.

“We had stores with no positive samples, which tells us that it can be achieved,” she said. “That’s our takeaway message.”

© Food Safety News
  • Tom Edlind

    This eye-opening report links to a new study by Wang et al. (“Persistent and transient Listeria monocytogenes strains from retail deli environments vary in their ability to adhere and form biofilms and rarely have inlA premature stop codons”), but should cite the primary source of this deli data as Simmons et al. (“Listeria monocytogenes and Listeria spp. contamination patterns in retail delicatessen establishments in three U.S. states”). Also, the answer to “why don’t we see more outbreaks linked to delis” must go beyond tracking issues to include insufficent understanding of strain-dependent virulence (inlA studied by Wang et al. is just one of 27 or so internalins) and human immunity.

  • BB

    Good reason to buy deli meat that has already been sliced at a USDA establishment. At least then you know they have a Listeria testing/control program if there’s post-lethality exposure.

  • Jennifer Johns

    “We had stores with no positive samples, which tells us that it can be achieved,” she said. “That’s our takeaway message.” Actually it poses the most important question of all. What were the stores with no positive sample doing that the other stores were not? Were they following the regulations set by the FDA and/or their state food code? Were they following a set of corporate rules that went a step further than the state/federal rules? There is obviously a difference in the way they did things compared to the other stores where Listeria was found and knowing that difference could be the key.

  • ethanspapa

    I have always wondered about the cooking glass ovens, where they use light bulb heating elements that cook dozens of whole chickens on a skewer as the fat and other little gremlins leech continuously onto the other birds and finally to the window walls and floors of the cooker. I mean how do you clean cooking bulbs and get every nook and cranny of that oven clean.
    That is before they’re not rinsed and injected said birds with massive amounts of salt.
    YUCK!!!