Research examines potential
Listeria risks in distribution centers

November 18, 2020 - Numerous research projects have examined potential risks from foodborne pathogens in field-grown produce as well as in packinghouses, but little work has looked at whether produce distribution centers (DCs) may be potential contamination sources.

“I think the DCs are a little out of sight, out of mind,” she said. “We have been so focused on foodborne outbreaks and what’s happening at the field level or packinghouse wash water and employees and hand hygiene.”

Her two co-principle investigators -- Laura K. Strawn, Ph.D., with Virginia Tech, and Ben Chapman, Ph.D., with North Carolina State University – are veterans at studying potential foodborne outbreak risks at the retail level.

“At least from my perspective, I don’t typically think of DCs,” Dunn said of potential risks. “It’s kind of something that our eyes have been opened up to.” Should they find potential Listeria risks, the researchers plan to develop written risk-reduction recommendations for DCs.

“I think given the trends we’re already seeing, we’re already pretty clear there will need to be some written guidances for DC management,” Dunn said. “These will be based on some high-risk areas, practices and/or equipment in DCs that we’ve found in the study.”

The researchers are focusing efforts on vented produce in breathable containers and stored in coolers. Among the items are berries, tomatoes and other items packed in plastic clamshells and onions in mesh bags.

Dunn said they are concentrating on Listeria because it forms biofilms and, once in a facility, is difficult to eliminate. Listeria also is more of an indoor pathogen and doesn’t compete as well in fields as Salmonella and E. coli, which tend to do better outdoors.

The researchers also are testing just for the Listeria genus and won’t be going further to differentiate between pathogenic Listeria monocytogenes and other more benign Listeria species.
• Produce distribution centers have been largely overlooked as potential foodborne pathogen sources.
• Project focuses on the Listeria genus because they do well indoors, form biofilms and are difficult to eliminate once established.
• Project outcome may yield a set of risk-reduction guidances for DCs.
“They’re family members, and they have similar growth environments, conditions and preferences,” she said. “We assume if we find Listeria that the conditions could possibly be right for Listeria monocytogenes to be there.”

Chapman will analyze the meta data to see if he can correlate positive finds at different facilities to commonalities. As part of the analysis, he’ll look at a myriad of variables, including on-site management practices, employee shifts, facility design, sanitation practices and sanitation materials.

As originally proposed, Dunn and colleagues were to visit at least 25 DCs throughout the nation to collect samples by swabbing numerous surfaces from Zones 2 through 4. Those areas don’t contact produce directly but are farther removed, such as pallets, forklift tires, floors, floor cracks, truck trailers and employee shoes.

Then the COVID-19 pandemic hit in March, and two of the three researchers are still under travel restrictions. As of this writing, they’ve collected samples from 11 DCs, and most of that sampling was before the pandemic.

Although the researchers had hoped to visit cooperating California DCs, Dunn said they may settle with operations east of the Mississippi River. So far, they’ve visited facilities in Texas, Michigan and New York and are trying to recruit one more partner from Florida, Georgia or Alabama, she said. Michelle Danyluk, Ph.D., with the University of Florida, has helped by collecting samples from DCs in her state.

What has made the project possible is the input and cooperation from participating DCs, Dunn said. In fact, distribution center representatives were the ones who brought their concerns to the Center for Produce Safety initially.

“We couldn’t do the work without them,” Dunn said. “This was one of the CPS target areas a couple years ago, and it was because our partners brought it forward. If they weren’t onboard, we couldn’t have brought this forward. Our partners have been a huge help. They conceived the idea, were instrumental in the project design and have been so accommodating in getting us into the facilities.”

She said the DCs wanted to be proactive, identify any potential risks and take care of them before they became issues.

Despite the COVID-19 challenges, Dunn said the researchers have been able to collect a substantial number of samples from each facility, and they should be able to put together a “nice packet of information” that will benefit DCs.
Laurel Dunn, Ph.D.
University of Georgia
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The Center for Produce Safety (CPS) is a 501(c)(3), U.S. tax-exempt, charitable organization focused exclusively on providing the produce industry and government with open access to the actionable information needed to continually enhance the safety of produce.

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