A Pursude University study finds that standard cleaning procedures in retail delis may not eradicate Listeria bacteria, posing a serious risk to people with vulnerable immune systems.
Consumers with vulnerable immune systems should buy prepackaged deli meats or heat ready-to-eat meats to 165 degrees, the researchers said. Meat contaminated with L. monocytogenes will not show any signs of spoilage, such as sliminess or odor, so there is no way for consumers to know if it is safe.
The study led by Haley Oliver, assistant professor of food science, found that 6.8% of samples taken in 15 delis before daily operation had begun tested positive for L. monocytogenes. In a second sampling phase, 9.5% 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. monocytogenescan 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.
Risks higher in retail delis
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% 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% of the delis never tested positive for the pathogen, while some delis tested positive in 35% 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."