Listeria contamination more likely at retail level than in manufacturing plant

While considerable effort has been expended to reduce the presence of Listeria monocytogenes in the food processing industry, new research suggests that contamination may be occurring at the retail level. All manufacturers’ hard work could be undone by poor sanitation protocols in delis, Purdue University researchers suggest.

Haley Oliver, assistant professor of food science at the university, found that 6.8% of samples taken in 15 delis tested positive for L. monocytogenes. A second sampling phase revealed even greater contamination - 9.5% of samples in 30 delis tested positive. In 12 of the delis sampled, the same subtypes of the bacteria cropped up in several samplings, which could mean that L. monocytogenes can persist in growth niches over time.

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. Manufacturing has a zero-tolerance policy for Listeria, but that dissipates at the retail level,” said Oliver.

“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.”

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

While fewer food contact surfaces tested positive for L. monocytogenes, “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 trace to a source. There are only about 1600 listeriosis cases a year [in the US]. But the likelihood of death is huge [for people with compromised immune systems].

“The vast majority of the [L. monocytogenes] isolates were ‘hot’ - comparable to wild-type L. monocytogenes. These are particularly cause for concern.”

Oliver’s research was published in the Journal of Food Protection.