Study: Listeria pathogen is prevalent, persistent in retail delis

February 10, 2015  

Oliver listeria

Haley Oliver (right) and Jess Buening, a senior in animal sciences, examine a meat slicer. (Purdue Agricultural Communication photo/Tom Campbell)
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WEST LAFAYETTE, Ind. - Purdue University research shows that standard cleaning procedures in retail delis may not eradicate Listeria monocytogenes bacteria, which can cause a potentially fatal disease in people with vulnerable immune systems.

A study led by Haley Oliver, assistant professor of food science, found that 6.8 percent of samples taken in 15 delis before daily operation had begun tested positive for L. monocytogenes (mah-noh-sy-TAH’-gin-eez).

In a second sampling phase, 9.5 percent of samples taken in 30 delis during operation over six months tested positive for the bacteria. In 12 delis, the same subtypes of the bacteria cropped up in several of the monthly samplings, which could mean that L. monocytogenes can persist in growth niches over time.

"This is a public health challenge," Oliver said. "These data suggest that failure to thoroughly execute cleaning and sanitation protocols is allowing L. monocytogenes to persist in some stores. We can't in good conscience tell people with weak immune systems that it is safe to eat at the deli."

In healthy individuals, eating food contaminated with L. monocytogenes may lead to common food poisoning symptoms such as diarrhea or an upset stomach. But the bacteria can cause listeriosis - a serious systemic infection - in immunocompromised people such as the elderly, infants and children, pregnant women and people with HIV. In severe cases, L. monocytogenes can pass through the intestinal membrane and into the bloodstream or cross the blood-brain barrier. The bacteria can also cross the placental barrier in pregnant women, which can trigger abortion.

Ready-to-eat deli meats are the food most associated with L. monocytogenes, which can grow at refrigerator temperatures, unlike Salmonella and E. coli.

Stringent control measures and inspections have tamped down the presence of L. monocytogenes at meat processing plants, but there are no regulations specific to Listeria for retail delis. Recent risk assessments suggest that up to 83 percent of listeriosis cases linked to deli meats are attributable to products contaminated at retail.

"It's kind of the Wild West," Oliver said. "Manufacturing has a zero-tolerance policy for Listeria, but that dissipates at the retail level. The challenge of developing systematic cleaning procedures for a wide variety of delis - which are less restricted environments than processing plants - can make Listeria harder to control."

Oliver and her team tested for L. monocytogenes and other Listeria species in 30 delis in national supermarket chains in three states. The researchers swabbed surfaces that come into frequent contact with food, such as meat slicers and counters, and surfaces that typically do not.

About 30 percent of the delis never tested positive for the pathogen, while some delis tested positive in 35 percent of the samples collected over six months.

"The prevalence of L. monocytogenes is higher than we expected in a significant percentage of delis, and the bacteria is persisting in these delis over time," Oliver said.

Most of the positive samples were collected from surfaces that usually do not come into contact with food, examples being floors, drains and squeegees. But the bacteria can be transferred unintentionally from these surfaces to food, Oliver said.

While the percentage of L. monocytogenes found on food contact surfaces was low, "these numbers would never be acceptable in manufacturing," she said. "The reason we haven't had a listeriosis outbreak tied to a deli is because it's a disease with a long incubation time, and it's difficult to track to a source. There are only about 1,600 listeriosis cases a year. But the likelihood of death is huge."

The research team tested 442 of the L. monocytogenes isolates collected from delis to determine how virulent the isolates were - that is, how great the likelihood was that they could cause disease. They found that less than 3 percent of the isolates had a lower potential for virulence.

"The vast majority of the isolates were 'hot' - comparable to wild-type L. monocytogenes," Oliver said. "These are particularly cause for concern."

She said that delis' standard sanitation operating procedures can keep the bacteria at bay only if the delis are in good condition, thoroughly cleaned and have sloped floors. But cleaning and sanitation may not effectively manage Listeria in a deli with structural damage such as missing grout, loose wall coverings or a drain that is not working properly. L. monocytogenes can flourish when it finds a moist niche that is infrequently cleaned.

Delis with contamination problems should "minimize the 'stuff' in the deli," Oliver said, to make it possible to clean the area thoroughly and train employees on how to maintain a sterile environment.

Consumers with vulnerable immune systems should buy prepackaged deli meats or heat ready-to-eat meats to 165 degrees, she said. Meat contaminated with L. monocytogenes will not show signs of spoilage, such as sliminess or odor.

"That's the challenge with pathogens such as Listeria, Salmonella, E. coli and norovirus: They don't cause changes in the characteristics of the products," she said. "Can you smell a food and tell if it's safe? Absolutely not."

The study was a collaboration between researchers at Purdue University, Cornell University, the U.S. Department of Agriculture and North Carolina Agricultural and Technical State University.

The paper was published in the Journal of Food Protection. The abstract is available at

The USDA’s Food Safety and Inspection Service provided funding for the research.

A second study that tested the virulence potential of the strains of L. monocytogenes found in retail delis was published in Foodborne Pathogens and Disease. The abstract is available at That research was funded by the Purdue University Food Marketing Institute and the USDA-Agricultural Research Service. 

Writer:  Natalie van Hoose, 765-496-2050,

Source: Haley Oliver, 765-496-3913, 

Related website:

Purdue University Department of Food Science: 


Listeria monocytogenes and Listeria spp. contamination patterns in retail delicatessen establishments in three U.S. states

Courtenay Simmons 1; Matthew J. Stasiewicz 1; Emily Wright 1; Steven Warchocki 1; Sherry Roof 1; Janell R. Kause 3; Nathan Bauer 3; Salam Ibrahim 4; Martin Wiedmann 1; Haley F. Oliver 2

1 Cornell University Food Science Department, 410 Stocking Hall, Ithaca, NY 14850, USA

2 Purdue University Department of Food Science, 745 Agriculture Mall Drive, West Lafayette, IN 47907, USA

3 Food Safety and Inspection Service. U.S. Department of Agriculture, 355 E. Street SW, Suite 9-191, Washington, D.C. 20024, USA

4 North Carolina A&T State University, Department of Human Environment and Family Sciences, 171 Carvar Hall, Greensboro, NC 27411, USA


Post-processing contamination in processing plants has historically been a significant source of Listeria monocytogenes in ready-to-eat delicatessen meats, and therefore a major cause of human listeriosis cases and outbreaks. Recent risk assessments suggest that a majority of human listeriosis cases linked to consumption of contaminated deli meats may be due to L. monocytogenes contamination that occurs at the retail level. To better understand the ecology and transmission of Listeria spp. in retail delicatessens, food and nonfood contact surfaces were tested for L. monocytogenes and other Listeria spp. in a longitudinal study conducted in 30 retail delis in three U.S. states. In phase I of the study, seven sponge samples were collected monthly for 3 months in 15 delis (five delis per state) prior to start of daily operation; in phase II, 28 food contact and nonfood contact sites were sampled in each of the 30 delis during daily operation for 6 months. Among the 314 samples collected during phase I, 6.8% were positive for L. monocytogenes. Among 4,503 samples collected during phase II, 9.5% were positive for L. monocytogenes; 9 of 30 delis showed low L. monocytogenes prevalence (<1%) for all surfaces. A total of 245 Listeria spp. isolates, including 184 Listeria innocua, 48 Listeria seeligeri, and 13 Listeria welshimeri were characterized. Pulsed-field gel electrophoresis (PFGE) was used to characterize 446 L. monocytogenes isolates. PFGE showed that for 12 of 30 delis, one or more PFGE types were isolated on at least three separate occasions, providing evidence for persistence of a given L. monocytogenes subtype in the delis. For some delis, PFGE patterns for isolates from nonfood contact surfaces were distinct from patterns for occasional food contact surface isolates, suggesting limited cross-contamination between these sites in some delis. This study provides longitudinal data on L. monocytogenes contamination patterns in retail delis, which should facilitate further development of control strategies in retail delicatessens. 

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