What Processors Should Know About New Research into Geographic Distribution of Listeria
By Claire Zoellner, PhD
Food Safety Scientist
November 1, 2021
Food processors are always concerned with keeping Listeria out of their facilities. This is especially important for commodities that are exposed to the production environment and do not undergo a kill step prior to consumption. We know that Listeria occurs naturally in the environment, as opposed to other pathogens that are introduced from an external source, like animal intrusion. However, there is still ongoing research into this microorganism to better understand the risk factors for outbreaks.
Scientists from Cornell University recently published a study funded by the Center for Produce Safety that analyzed the prevalence of Listeria in the United States. The researchers created a “Listeria atlas” showing the distribution of different Listeria species (classified into groups based on genetic similarity, aka “phylogroups”) across the country. Here are some of the key highlights from the study that food processors should keep in mind.
Nationwide Prevalence of Listeria is Approximately 31%
The researchers collected 1,004 soil samples from non-agricultural soil throughout the United States. Overall, 311 tested positive for Listeria, indicating a 31% prevalence nationwide. Looking more closely at geographic distribution, Listeria was concentrated mostly in the Mississippi River Basin (constrained by longitude 75-100°W and elevation 0-500m), which makes sense given that Listeria tends to favor damp environments. Notably, there were very few positive samples from the Western U.S., including California, Arizona, and Washington. In contrast, the Eastern U.S. had many positive samples.
Overall, the researchers identified 12 different Listeria phylogroups, which they labeled L1 through L12, and nine species within these phylogroups:
- L1, L. monocytogenes lineage III
- L2, L. monocytogenes lineage II
- L3, L. monocytogenes lineage I
- L4, L. marthii
- L5, L. cossartiae subsp. cossartiae
- L6, L. innocua
- L7, L. farberi
- L8, L. welshimeri
- L9 and L10, L. seeligeri
- L11, L. immobilis
- L12, L. booriae
Listeria Geographic Distribution Varies by Species
The researchers found distinctive geographic patterns of Listeria species distribution. Notably, the phylogroups that contained L. monocytogenes (the human pathogen that causes foodborne illness) were concentrated in these areas:
- L1 – identified in the South and Midwest, in states ranging from Iowa, Arkansas, and Louisiana, to Pennsylvania, Maryland, and South Carolina.
- L2 – identified mostly in the East and Midwest, with a positive sample from Oregon as well.
- L3 – Found mostly in the East and South, from New York, Connecticut, and Rhode Island down to Alabama and Georgia, plus a positive sample in California.
Out of the 12 different groups identified, L5 had the narrowest geographic range and L10 had the widest.
Soil Composition Is a Better Predictor of Listeria than Climate or Land Use
Another key finding was that soil composition is more important than climate or land use when predicting Listeria presence. Overall, the best soil conditions were lower sodium, higher molybdenum concentration, and higher moisture/organic matter content. Although researchers found the ideal range in soil is 25–50mg/kg sodium, it is known that Listeria can survive in concentrations of up to 10% sodium chloride. Processing facilities for a wide range of food products must be vigilant about testing for Listeria and should consider local environmental conditions when determining different corrective action and control strategies.
Geographic Distance Doesn’t Have a Major Impact on Genetic Variation
When the researchers examined genetic variation between different Listeria strains within a phylogroup across the United States, they found high similarity, even at great geographic distances. Overall, “any two genomes within the same geographic location had a gene content difference of ~10%. If the genomes were sampled from two extremes of the country (for example, Florida and Washington state), the gene content difference would increase only by 4–5%.” In other words, distance does not cause vast differences in gene content. This is important to keep in mind when investigating the source of a Listeria outbreak. This information might help regulatory agencies establish an appropriate cutoff to use in implicating a food or facility.
How Food Processors Can Apply These Findings to Their Food Safety Practices
This research gives us some additional insight into the types of non-agricultural environments Listeria grows in and where it’s prevalent. Since this pathogen is naturally present in soil and not introduced by an external factor (e.g., animal intrusion or fecal matter), processors need to be cognizant of points of potential contamination in their facilities. This includes employees’ shoes, forklifts, loading docks, and other surfaces that may have contact with the soil outside the facility.
Food processors should also keep in mind their geographic location and the soil conditions near their facilities. Those in higher risk areas where L. monocytogenes is more prevalent might want to implement stricter control and testing protocols. Processors should be testing for Listeria spp. instead of only testing for L. monocytogenes. The presence of any Listeria species indicates that you have conditions that will likely allow L. monocytogenes to grow, and you need to take action to eliminate this risk.
Another important takeaway is that this research will expand our Listeria whole genome sequencing (WGS) data. FDA investigators utilize WGS in their outbreak investigations. Knowing the extent of genetic variation within Listeria monocytogenes will help narrow down locations during a traceback.
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What Does Listeria Contamination Risk Look Like in a Facility?
While this research gives us a lot of insight into how Listeria occurs naturally, it’s still difficult to visualize points of potential contamination in a built environment, such as a processing facility or food distribution center. Let’s look at another research study that examined different areas of facilities that were tested as part of an environmental monitoring program (EMP).
Cornell University researchers completed a meta-analysis of research on environmental monitoring sampling plans. They also surveyed experts in the industry who had experience working on EMPs in produce, dairy, meat, seafood, and retail facilities. The experts outlined key sampling sites that should be included in an EMP sampling plan and ranked the importance of each sampling site on a scale from 1 to 5 (5 being the highest priority). Some of the most important sampling sites for facilities included:
- Cutting boards used for finished product
- Brushes and other equipment that touch finished product
- Bowl cutters (interior of castings bowls, blade spinner, and other difficult to clean food contact areas)
- Equipment (e.g., vats, tanks, tables) that comes in contact with product after CCP or kill step
- Slicer, peelers, and choppers
- Bumper guards (e.g., on sorting tables, along conveyances) that contact finished product
- Conveyor systems, belt, and other food contact sites
- Packaging machines
- Interior of pipes and tubes that transfer finished product
- Holding vats used to store finished product
- Buckets/bins that come in contact with finished product
- Scraper blades
- Skinning machines
- Sorting tables (surfaces that contact finished product)
These areas are a good starting place for processors looking to revise their EMP program or validate routine sampling sites.
Your Environmental Monitoring Program Is Your First Line of Defense Against Contamination
While we are still learning more about Listeria and fine-tuning rapid detection methods, your EMP remains a vital line of defense against contamination. The best thing you can do is to make sure your EMP is fully optimized and routinely reviewed so that you have actionable information about points of contamination throughout your facility and can take appropriate action. iFoodDS can help.
Our team of food safety scientists offer a range of services, including a detailed review of your current program, a swab-a-thon to establish baseline contamination or validate procedures, and an advanced predictive model that simulates Listeria movement across your facility. Request a consultation of our Environmental Monitoring Solutions to see how we can help keep your buyers and consumers safe.