As the United States is experiencing Listeria outbreaks in December 2021, January and February 2022, let's review what is Listeria, how it can contaminate food and how an effective environmental monitoring program can help control risks linked to this pathogen.
What is Listeria?
Listeria monocytogenes is a -gram-positive, intracellular rod bacterium that causes Listeriosis. Listeriosis is a rare foodborne disease; however, it can result in mortality rates of 20% - 30% in the elderly, pregnant persons, neonates, and immunocompromised individuals. [1] Infection via Listeria includes but is not limited to sepsis, meningitis, encephalitis, spontaneous abortion (Listeriosis having been passed to the unborn fetus) or fever and self-limiting gastroenteritis in a healthy adult (Bomar NIH.Gov). According to the Center for Disease Control (CDC), approximately 1,600 people get Listeriosis each year with approximately 260 people dying from the disease. Of the 1600 individuals who are diagnosed with Listeriosis, 94% will require hospitalization.
Incubation periods range from one to over 90 days, and impact the ability of outbreak traceback, as affected individuals frequently have little concept of the items they consumed that may been contaminated.
Food Safety and Inspection Service, USDA’s use of Genome Sequencing in samples from suspected infected consumers has greatly enhanced these traceback efforts.
Where can Listeria be found?
Listeria is widespread both in soil and water, may have animal and human carriers, can be found - from farm to retail. 
The bacterium is unfortunately able to survive in a diverse range of environmental conditions: it can tolerate both acidic (pH 5 to pH 9) and salty conditions (10% NaCL), both high and low temperatures (0 – 45 ⁰C) and a fairly low moisture content. [2] It have also the ability to form high resistant biofilm and thus very complicated to remove. The ability of Listeria to grow/multiply under refrigerated conditions and/or in Ready-To-Eat (RTE) commercially processed foods is of the greatest concern. When foods have been linked to Listeriosis outbreaks, they have been found to contain ≥1,000 CFU/g of L. monocytogenes. In order to have these levels of the bacterium, it is highly probable that it had the ability to grow/multiply post processing. Consequently, consumer protection is best served by focusing on those foods in which the bacterium can grow/persist to these levels. Examples of items susceptible to this are cheese, deli meats, seafood, produce and smoked fish.
Where does Listeria contamination come from?
Listeria is generally transmitted when food is harvested, processed, prepared, packed, transported or stored in environments contaminated with the bacterium per the USDA. Factors such as cleanliness of incoming raw materials, personnel delivering raw materials, cleanliness of transport vehicle and contaminated raw materials being introduced into a food processing facility are not insignificant. Once there is introduction of Listeria to a food processing facility, the bacterium, given the right environmental conditions which a food processing facility is likely to provide, has a probability of survival. Testing over the last 20 years has indicated that contamination from processing equipment by Listeria at processing facilities for RTE, as being the primary source of introduction [3]. Further, the virulence and survivability of the bacterium pose a significant challenge to the sanitation teams.
How can Listeria be controlled in the processing environment ?
Detecting and combating a Listeria contamination at a Food Processing Facility requires an effective Environmental Monitoring Program (EMP). In-process and finished product testing are not an effective or efficient means of detecting and then creating a corrective action to address the presence of the bacterium in the product. The creation of a truly beneficial EMP requires three distinct requirements to be effective:
1) design of the program, not only including sites and frequency for sample collection but also the discipline to collect and process samples routinely
2) the ability to process hundreds of data points and transform the data into actionable insights
3) a timely and effective response/corrective action to a positive result to work towards eradication of the bacterium.
Mitigating Listeria risks with the Novolyze Platform
With the amount of data flowing to an Environmental Coordinator or facility Quality Manager, Novolyze has created an EMP solution which is a vital step in turning that volume of data into actionable information. By using the Novolyze proprietary processes, data can be trended for each sampling point, for each type of test conducted, for all positive test results over time, etc. within the EMP. In the platform, the data displayed on the factory map show the sensitive areas that required specific attention. Through that information facility management is able to assess the opportunity, provide the necessary resources to resolve the issue and maintain overall confidence in the facility sanitation program. By using the Corrective Action Module, within the tool, the QA staff is able to document exactly the steps to be taken and the consequence of those steps, further providing feedback to the facility about the effectiveness of the corrective action. More importantly, the Novolyze Tool enables facility management a method of validating that the facility EMP is having all samples collected / analyzed and the EMP managed correctly by environmental/QA staff leading to regulatory compliance.
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- Jordan K, McAuliffe O. Listeria monocytogenes in Foods. Adv Food Nutr Res. 2018;86:181-213. doi: 10.1016/bs.afnr.2018.02.006. Epub 2018 Apr 3. PMID: 30077222.
- Siyun Wang, Renato Orsi. (2021). Foodborne Infections and Intoxications (Fifth Edition, Chapter 11 Listeria). Academic Press
- Tompkin RB Control of Listeria monocytogenes in the Food-Processing Environment. Jnl Food Protection (2002) 65 (4): 709-725.
