I first learned that Listeria monocytogenes is a foodborne pathogen in 1985 during the Jalisco cheese outbreak in Los Angeles and Orange Counties in California. There were 29 deaths from 86 documented cases. The veterinary world was very familiar with Listeria in cattle, long before food microbiologists. Industry and the federal Food and Drug Administration (FDA) took this emerging pathogen seriously. Since 1985, there has been active research and investigations at universities and food companies. We know that Listeria is ubiquitous, and current lab methods improve our chances of finding Listeria.

FDA Listeria Draft Guidance for Ready-to-Eat Foods: Part 1, published previously at connectfood.com, discussed Listeria testing on food contact surfaces (FCS) and non-food contact surfaces (NFCS) and from finished product. We encourage you to study FDA’s draft guidance, Control of Listeria monocytogenes in Ready-To-Eat Foods: Guidance for Industry. Here, I present three concepts needed to understand the information in the guidance.

A positive Listeria test from a FCS assumes the presence of Listeria in your product.

The guidance for ready-to-eat food recommends FCS be sampled after three hours of production and five FCS samples be collected per week per line. First consider these sample sites. The FCS sites should be after the final kill step. It does no good to sample FCS sites before the final kill step. If the test is positive, the assumption is that the final kill step killed the pathogen. The FCS sites are before the final product is sealed in packaging. Think about the distance where your product travels after the final kill step to where the package is sealed. In that distance, think about where the product is open to the environment. Listeria is an environmental pathogen. What I mean is if the product is in tubing, the product is not exposed to the environment, and these would not be FCS sites for Listeria. I am thinking about an ice cream plant. The mix travels through tubing until carton filling. The distance the product travels from the end of the tube to the carton is small, and the distance the open carton travels until sealed is small. Compare that to a large bakery. After leaving the oven on conveyor belts, the baked goods travel open to the environment for cooling and the addition of toppings. I have seen distance as long as 50 yards with the transfer onto multiple conveyor belts.

Once you have your FCS sites identified and if a FCS test is positive, it is assumed the product is positive, even without direct sampling and testing of the product. This makes sense. If a FCS is positive, the product touched the FCS. At that point it’s a chicken-and-the-egg discussion of whether the product contaminated the surface or the surface contaminated the product. It doesn’t matter until you do your root cause analysis. What you do know is that you have contaminated product.

A positive Listeria test from an environmental sample has an isolate identified as Listeria innocua or another Listeria species; consider Listeria monocytogenes as present.

Listeria monocytogenes is one of 17 species of Listeria. Only the species Listeria monocytogenes is considered a foodborne pathogen. When your contract lab tests for Listeria genus or spp., a positive test does not tell you the species. Further testing can identify the species, if needed. It is very common for the lab to report negative for Listeria monocytogenes and report the presence of Listeria innocua instead. In no way can you rest easy with a positive Listeria test and negative Listeria monocytogenes test. When the lab reports a positive Listeria test, consider Listeria monocytogenes present.

Consider the sampling site and technique. Let’s use the example of a drain swabbed with a sterile sponge. Assume Listeria monocytogenes is there. Perhaps the sponge did not swab the location of the pathogen. Perhaps the sponge did not get through the layers of goo where the pathogen is harbored. There are many reasons for missing the pathogen in the procedure for swabbing. If the test is positive for a different Listeria species, the conditions at that site are conducive for the presence of Listeria monocytogenes. Corrective action must be done in the same way as if the test were positive for Listeria monocytogenes.

Consider the variety of microflora at the site. The site was not sterile and does contain diverse types of bacteria at different numbers and proportions to each other. Listeria monocytogenes is the wimpy kid on the playground. Listeria innocua outcompetes Listeria monocytogenes not just 10-to-1, but as much as 20:1 or 50:1! The mere chance of finding Listeria innocua instead of Listeria monocytogenes is simply greater. Listeria monocytogenes may have been present in the sample, in the testing, and simply not detected.

An environmental sample tests positive for Listeria genus or Listeria species (spp.); react as if the sample were positive for Listeria monocytogenes.

You will direct the lab in what method to use and at what point to stop the testing. Work with the contract lab in making this determination. After a positive test for Listeria genus or spp., you have the option to continue testing to identify the isolate of Listeria. This costs more money and time. Will you do something different with that information? No! It is not necessary to confirm the presence of Listeria monocytogenes from a positive Listeria genus or spp. test, when your corrective action is the same. Think this through before testing. Know the action the plant will take when a Listeria genus or spp. test is positive. Expect to get positives from zones 3 and 4. FDA inspectors should not have a problem with seeing occasional positive tests. Since Listeria is ubiquitous, you are expected to find it. The important part is what you do as corrective action. Follow-up testing should be negative to prove Listeria is eliminated from the site and the area adjacent to the site.

As you work through the FDA Listeria guidance, the ConnectFood website has free resources, and the folks at ConnectFood are here to help! Contact us.

About the Author
Kathy Knutson, Ph.D.
Kathy Knutson Food Safety Consulting
Dr. Kathy Knutson works nationwide with food manufacturers on recall investigations, problem-solving, training, and Food and Drug Administration (FDA) compliance. After being trained in 2016 as a Lead Instructor with the FDA-recognized curriculum for Preventive Controls Qualified Individuals, she delivered over 20 workshops to industry. With over 35 years in microbiology and 15 years of full-time teaching, Dr. Knutson is passionate about training and is an effective communicator at all levels in an organization. She has taught and consulted with companies on laboratory methods, interpretation of lab results, quality assurance, sanitation, environmental monitoring, Standard Operating Procedures (SOPs), Good Manufacturing Practices (GMPs), Hazard Analysis and Critical Control Points (HACCP) and the FDA’s Food Safety Modernization Act (FSMA). As a life-long learner, Dr. Knutson is trained in prevention of intentional adulteration, a topic on the horizon for the food industry. Dr. Knutson is a contributing author at CannabisIndustryJournal.com. Dr. Knutson writes a food safety blog and contributes expert services to manufacturers through connectfood.com, an online site for writing HACCP and food safety plans. When Dr. Knutson is not traveling, she works from home in Green Bay, Wisconsin, where she lives with her husband, two sons, and an adorable Bernedoodle. Learn more about her at https://www.linkedin.com/in/kathyknutsonphd