Study looks at Listeria contamination patterns in processors

April 21, 2023 - While several studies have examined Listeria monocytogenes (Lm) prevalence in dairy and meat processing facilities, few have looked at Lm contamination patterns and related sanitation programs in produce processing facilities.

But Ana Allende, Ph.D., and her team from the CEBAS-CSIC research institute in Spain, hope their two-year CPS-funded research will change that. The project is designed to yield practical data about produce facilities’ environmental monitoring plans as well as the efficacy of sanitation programs.

“We started to become interested in the role of environmental contamination following years of attending the CPS Symposium where some researchers, such as Dr. Martin Wiedmann and Dr. Laura Strawn, focused on Listeria control in produce packinghouses and processing facilities,” she said. “We’re trying to bring our experiences from another point of view. The facilities we’re able to sample here could also help us understand the significance of this problem.”

Joining her as co-principal investigators were Mabel Gil, Ph.D., and Pilar Truchado, Ph.D., both from the CEBAS-CSIC.

As part of the project, the researchers enlisted the cooperation of three processing plants: one with a cut iceberg lettuce line, one with a cut fruit line and one with a salad bowl line. 

While the U.S. Food and Drug Administration has zero tolerance for Lm in processed produce samples, the European Commission has set a threshold of up to 100 colony-forming units per gram.
Project looked at Listeria contamination patterns in three produce processing facilities.
Whole genome sequencing, a type of genetic fingerprinting, helped link positive Lm samples to possible sources.
Researchers tested biocides against resident Lm populations to gauge efficacy and potential loss of sensitivity.
Their first objective was to understand how different factors such as zoning, sanitary design and connectivity affected the probability of contamination in different fresh produce processing facilities. In the case of salad bowls, the ingredients included not only leafy greens and other vegetables but also proteins from meat, fish and cheese, or pastas from different sources.

The researchers divided the processing areas into three zones based on their proximity to contact with the produce. Zone 1 involved areas with direct contact, such as knives and conveyor belts. Zone 2 included surfaces that did not contact food but were in close proximity. And zone 3 included more remote non-contact surfaces, such as drains, floors and ceilings, that could potentially lead to contaminating zones 1 and 2. 

They conducted systematic sampling of the facilities at the end of the day before cleaning and sanitizing. They also resampled the three processing lines after the cleaning and disinfection activities.

In addition to the more than 600 total samples from the three zones, the researchers collected 45 samples from raw ingredients and end products.

“By sampling the processing plants before and after cleaning and disinfection, we could understand which might be the entry points of the contamination,” Allende said.

Regardless of the facility, she said they had the highest number of positive Lm samples from zone 3.

The researchers also conducted whole genome sequencing — a genetic fingerprinting — on 100 samples to better understand whether the Lm was transient or persistent. What surprised them was that the same two serotypes of L. monocytogenes were found on the three processing lines after the two samplings, before and after cleaning. 

“This makes us understand that these serotypes are inherent and are moving from zone 3 to zone 1,” Allende said. 

As part of the project, the researchers also evaluated the efficacy of biocides against resident Lm isolates.

“We found, indeed, all of the isolates obtained from the environment after cleaning were sensitive to the biocides,” she said. This allayed concerns that the pathogens were becoming resistant to the sanitizers. 

Although the sanitizers are effective, in many cases, the cleaning activities are not well performed, allowing the microorganisms to persist in the environment, Allende said. 

The research aimed to provide relevant results for the three cooperating produce processors, Allende said. But it also has broader implications for the produce industry about how they should conduct environmental monitoring including sampling after processing just before cleaning. 

In addition, she said, it should help processors better understand the main contamination points in zone 1 and how they relate to identical or similar Lm sequence types in zones 2 and 3.

“One of the hypotheses we had was the raw material was introducing much of the Listeria,” she said. “This was before we did sampling and the whole genome sequencing to understand the isolates and that they were not all coming from the raw material. Some of the contamination was probably coming from zone 3 in the different processing facilities.”
Ana Allende, Ph.D.
Dr. Allende will present her final report at the 2023 CPS Research Symposium.
with Dr. Allende
Learn more about CPS-funded projects here.
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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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