Listeria monocytogenes in Fresh and Ready-to-Eat Produce: common sources and potential hazards

Have you heard of Listeria monocytogenes? This tricky bacterium can cause severe illnesses, especially for those of us who fall into the YOPI group (young, old, pregnant, immunosuppressed). What makes Listeria monocytogenes so concerning? It has some impressive abilities to survive and multiply in conditions that would typically hinder most other bacteria, like cold temperatures, acidic environments, and high salt content. Out in the field where the organisms come from, it poses no threat and is referred to as a saprophyte. However, most food manufacturers unknowingly allow Listeria monocytogenes into their facilities daily. As a result, every batch of raw materials has the potential to harbour this bacterium. Despite this, most raw and fresh produce is consumed without causing illness.  

Listeria monocytogenes primarily affects individuals with weakened immune systems and require specific conditions to grow in food to reach an infectious level. Nevertheless, there is reason for concern: Listeria mono can even make itself at home in refrigerated environments, making it tricky to control during food processing and storage. And it can form biofilms on surfaces, protecting the bacteria from sanitation efforts and allowing them to persist in food processing facilities. 

Common Sources of Contamination 

So where do Listeria monocytogenes come from, and how can we protect ourselves? One of the most common sources of Listeria contamination is food products that have not been properly handled or stored. Fresh and ready-to-eat produce are all potential sources of Listeria, and the bacteria can spread through cross-contamination. 

A great example is leafy vegetables. They have a limited lifespan, and once harvested, they become highly perishable. To prolong their freshness, produce is stored in cold temperatures and sometimes packaged in modified gas atmospheres to inhibit microbial growth. Throughout this storage and refrigeration process until consumption, it is crucial to be aware of the presence of Listeria monocytogenes. This bacterium has a competitive advantage over other pathogens in the refrigerated environment where fresh and ready-to-eat produce is handled. NEMIS Technologies AG has created a concise and informative video that sheds light on the common areas where Listeria monocytogenes is typically found in the production of fresh produce. 

Listeria monocytogenes in fresh and ready-to-eat p

Listeria monocytogenes in fresh and ready-to-eat produce: Hotspots of Swabbing 

Essential considerations for critical control points: 

  1. Water re-use systems: Using chilled water for washing ready-to-eat fresh and raw products, which are often recycled, poses a critical control point because water re-use systems create an optimal environment for the growth of Listeria monocytogenes. 
  1. Air conditioning systems: Chilling, refrigeration, and air conditioning systems are crucial in maintaining the cold temperature required for food preservation. These systems draw in warm, moist air, which is then converted into cold, dry air. However, the condensation of water on the metal coils of these systems can provide a favourable niche for the growth of Listeria monocytogenes. Contamination of an air conditioning system can also act as a vector, spreading the pathogen throughout the factory. If these systems are located directly above vulnerable areas of the production process, any condensation could drop onto exposed surfaces, leading to cross-contamination of products. 
  1. Biofilms: Biofilms are thin, resilient layers of microorganisms and food residue that adhere to solid surfaces. They typically form on moist and wet surfaces. In producing fresh and raw produce, equipment involved in washing, draining, and cutting vegetables can provide ideal conditions for biofilm accumulation. If microorganisms like Listeria monocytogenes thrive within biofilms, they become more resistant to disinfectants and cleaning efforts, posing a higher risk of re-contaminating the processed fresh vegetables. Special attention should be given to effectively controlling and eliminating biofilms in these areas. 

How to prevent Listeria contamination? 

Proper food handling, storage, and preparation are essential in preventing Listeria contamination. In addition to that, adequate storage and refrigeration are also important in preventing Listeria. Foods that are at high risk for contamination, such as deli meats or ready-to-eat salads, should be kept refrigerated at or below 40 °F (4 °C), and should be consumed within the recommended time frame. 

It is also important to follow food safety guidelines and regulations set by local authorities to ensure food products are free from Listeria monocytogenes and other harmful contaminants. This includes proper food handling and preparation techniques and regular testing and monitoring of food products. 

Environmental testing using rapid pathogen detection methods is also crucial to prevent Listeria monocytogenes contamination in fresh produce. These methods can quickly identify the presence of harmful pathogens in the food production facilities preventing potential contamination and outbreaks. For instance, the NEMIS screening test can be utilized to monitor and control various food safety-related factors. N-LightTM Listeria monocytogenes is suitable for use in food processing areas and equipment. By leveraging this technology, food manufacturers can proactively identify and address potential sources of contamination to prevent the spread of Listeria monocytogenes in their products. 


Listeria monocytogenes is a serious foodborne pathogen that can cause illness and even death in vulnerable populations. Proper food handling, storage, and preparation are essential in preventing Listeria contamination, and consumers should be vigilant in following food safety guidelines. The use of rapid pathogen detection methods and advanced technologies such as N-LightTM Listeria monocytogenes can also help monitor and control food processing areas and equipment to prevent product contamination before it reaches the consumer. Understanding the risks associated with Listeria and taking steps to against contamination can help protect yourself and your loved ones from the potentially deadly effects of this dangerous bacteria.  

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