John David 3MBy John M. David, 3M

Listeriosis is a very serious foodborne illness caused by Listeria monocytogenes. While rare, primarily affecting high-risk groups including pregnant women, newborns and immune-compromised individuals, the mortality rate is very high as compared to other foodborne diseases. Just last year in the United States, a Listeria outbreak associated with Italian-imported ricotta salata cheese sickened 22 people, including four deaths.

Dairy foods processors must be especially diligent in their efforts to combat Listeria, as it has been found in pasteurized or unpasteurized milk products, including soft cheeses made such as queso fresco, Brie, Feta and Camembert.

Listeria monocytogenes is a resilient organism

L. monocytogenes is found virtually everywhere in the environment, including soil, water and certain animals. It is a resilient organism, surviving in a broad range of environmental conditions. Unlike other foodborne bacteria, L. monocytogenes can grow at refrigeration temperatures and even at less than 1°C (34 °F), contributing to its ability to cause disease and high risk for certain refrigerated and ready-to-eat foods. Although pasteurization does kill the bacterium, contamination may still occur during the packaging process and processors must be vigilant in their prevention and testing procedures.

Fortunately, thorough and regular plant cleanings that follow HACCP and good manufacturing practices are highly effective in reducing the presence of Listeria. The pathogen can be transferred between sites inside a plant via human activity, so floors, drains, pallets, equipment and even cleaning supplies must be cleaned properly to prevent cross-contamination. And of course, enforcing proper personal protective equipment and employee hygiene, such as regular hand washing, should not be overlooked.

Clean, sample and test for Listeria in a dairy processing plant

Listeria monocytogenes contamination has been found in many types of food processing plants, and in some cases has been show to persist in the environment for years. While proper cleaning and sanitation is crucial, no food safety program is complete without an effective sampling and testing plan to ensure the absence of harmful pathogens. This can be a combination of environmental monitoring and product testing. Regular sampling should occur over a ranged of sites and/or product types, and location and frequency will depend on the layout of the premises, product flow, amount of product produced, and other factors.

As food processors require faster turn-arounds to meet the pace of today’s needs, testing strategies have shifted from slower, traditional culture-based methods to more rapid detection methods. Technologies using nucleic acid amplification have revolutionized the detection of foodborne pathogens from food or environmental samples by providing highly rapid and specific detection. While many molecular methods have utilized polymerase chain reaction (PCR) technology, advances in molecular biology have resulted in newer, robust technologies including loop mediated isothermal amplification (LAMP). A widely recognized method for rapid molecular amplification, LAMP has now been cited in nearly 1,000 scientific articles and has been recognized for its ability to reliably and efficiently target DNA compared to conventional PCR methods.

While PCR uses thermal cycling, i.e., alternating heating and cooling steps to enzymatically amplify DNA, LAMP uses isothermal amplification – amplification at a constant temperature. Using multiple primers to recognize the target gene, unlike the two used in PCR, LAMP’s continuous amplification of DNA results in a process that is both highly specific and rapid.

Isothermal amplification methods for detection of foodborne pathogens 

In recent years, systems using isothermal amplification methods for detection of foodborne pathogens have recently started to become commercially available. In particular, the 3M Molecular Detection System utilizes the innovative combination of LAMP and bioluminescence detection technologies, resulting in a simpler, more robust DNA amplification and detection method and providing significant user benefits over older PCR technology. These two technologies make the system a cost effective, easy to use method with the molecular accuracy requested by customers. Corresponding pathogen-specific assays, or test kits, are also available for the detection of Listeria as well as Salmonella and E.coli O157 (including H7).

Listeria monocytogenes may not be a leading cause of foodborne illness as compared to other pathogens such as Salmonella or E. coli O157:H7, but its resilience and lethality make it a serious concern in the food industry. With robust sanitation practices and food testing, dairy processors can maintain safe environments and products.


John M. David is a global technical services engineer with 3M Food Safety, St. Paul, Minn.