Purdue University research shows that standard cleaning procedures in retail delis may not eradicate Listeria monocytogenes, 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.

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.”

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.

The paper was published in the Journal of Food Protection. The abstract is available at http://www.ingentaconnect.com/content/iafp/jfp/2014/00000077/00000011/art00012