Traces of Listeria monocytogenes bacteria may not be completely destroyed with current cleaning procedures in place at retail delis, according to new research conducted by Purdue University.

Haley Oliver, a food science assistant professor at Purdue, took samples at 15 delis before daily operation in each had begun. Precisely 6.8 percent of the samples tested positive for L. monocytogenes. A secondary research phase revealed that of 30 delis where samples were collected over a 6 month period, 9.5 percent of them contained the bacteria. The growth of Listeria subtypes––found in 12 of the deli’s monthly samplings––also provides new insight into how the bacteria grows over time. Tests were conducted in national supermarket retailers in across three states. Samples were taken by swabbing surfaces such as meat slicers, countertops as well as surfaces that do not tend to have contact with food such as floors, drains and cleaning equipment.

"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."

Ready-to-eat deli meats are the most common to be contaminated with L. monocytogenes due to its ability to thrive even during refrigerated storage. Other foodborne pathogens cannot continue to cultivate cold environments.

"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."