Listeria pathogen is prevalent, persistent in retail delis: Study
Oliver and her team tested for L. monocytogenes and other Listeria species in thirty 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. The bacteria, however, 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. Cleaning and sanitation, however, 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 release notes that 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 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. That research was funded by the Purdue University Food Marketing Institute and the USDA-Agricultural Research Service.
— Read more in Courtenay Simmons et al., “Listeria monocytogenes and Listeria spp. Contamination Patterns in Retail Delicatessen Establishments in Three U.S. States,” Journal of Food Protection 11 (November 2014): 1929-39; and J. 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,” Foodborne Pathogens and Disease 12, no. 2 9February 2015):151-58 (doi: 10.1089/fpd.2014.1837)
