Listeria can survive the standard cleaning procedure used by retail delis, a new study by Purdue researchers has found. Listeria monocytogenes, an uncommon and deadly bacteria, is often associated with ready-to-eat deli meats, hot dogs, refrigerated pâtés or meat spreads, prepackaged deli salads, soft cheeses, smoked fish, raw cheeses and sprouts.
In 2014, three deadly multistate outbreaks were linked to these foods. Bean sprouts produced by Wholesome Soy of Chicago, killed two people and sickened three others in an outbreak announced by the Centers for Disease Control and Prevention (CDC) in November. Listeria in soft cheese was the source of a three-state October outbreak linked to Oasis cheeses that killed one person and sickened two others; and a March, outbreak, linked to Roos Cheese, that sickened eight people in Maryland and California, killing one of them.
One thing that sets Listeria apart from other bacteria is its ability to grow and thrive in cool temperatures.
The Purdue study, led by assistant professor of food science Haley Oliver, found that 6.8 percent of samples taken from 15 delis before the start of the business day were positive for Listeria. In a second six-month sampling phase, 9.5 percent of samples taken from 30 delis tested positive for the bacteria. At 12 of the delis, the same subtypes showed up in multiple monthly samplings, which could indicate that Listeria 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 Listeria 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.”
After an outbreak linked to deli meat killed several people a decade ago, a zero-tolerance policy for Listeria was enacted for manufacturers. “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.”
Researchers took samples from surfaces that come into frequent contact with food, such as meat slicers and counters, and surfaces that typically do not. Most of the positive samples were collected from surfaces that usually do not come into contact with food, such as floors, drains and squeegees. The concern is that bacteria can be transferred unintentionally from these surfaces to food, Oliver 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,” Oliver said.
Based on their findings, Purdue researchers say that standard sanitation procedures at delis can only keep them free of bacteria if the delis are in good condition, area with structural damage such as missing grout, loose wall coverings or a drain that is not working properly create an environment where bacteria can grow.
