Novel control measures for Listeria in food processing facilities

Listeria monocytogenes is the bacterium responsible for causing listeriosis, an illness that can cause fever, chills, headache, gastroenteritis, and in some cases can lead to hospitalization or death.  Pregnant women, newborns, and immunocompromised adults are at greatest risk of severe illness. One source of exposure to Listeria includes ready-to-eat (RTE) foods, which can be contaminated at any point in the food processing system. Some examples of RTE foods that have triggered recalls in Canada include salad kits, deli-meats, plant-based beverages, and frozen waffles. Hospitalizations and deaths have occurred in listeriosis outbreaks related to RTE foods. This blog provides information on how people may be exposed to L. monocytogenes, and the current and novel approaches used to control this bacterium in food processing settings.

Background

L. monocytogenes is ubiquitous in nature, thriving in moist environments such as soil, water, animal digestive tracts, and decaying vegetation. Produce can be contaminated by soil or manure used as fertilizer. Transmission can occur via several routes: mother-to-fetus in utero or during childbirth, infant-to-infant, animal-to-human, and consumption of contaminated food. In utero infections can result in miscarriage, stillbirth, or perinatal septicemia and meningitis.

In Canada, listeriosis is a nationally notifiable disease and the leading cause of foodborne illness-related deaths, with approximately 178 cases and 35 deaths reported annually. The National Enteric Surveillance Program (NESP) reported a higher incidence rate in 2022 compared to 2021, but similar to 2019. Data from the Public Health Agency of Canada shows that while the incidence rate has been relatively stable since 2011, it remains higher than in 1991. From 2019 to 2023, there were 123 recalls of food products contaminated with L. monocytogenes, with 23 recalls issued to date in 2024.  The US Centers for Disease Control estimates that about 1,600 people are infected with Listeria annually, with 260 deaths. Data from the US National Outbreak Reporting System recorded 126 outbreaks with 1,462 cases of listeriosis, 1,141 hospitalizations, and 214 deaths from 2009-2022. Reported cases of listeriosis in Europe have increased in recent years. In 2022, the European Surveillance System recorded 2,770 confirmed cases. A report by the European Food Safety Authority and the European Centre for Disease Control noted 2,621 confirmed cases of invasive listeriosis in 2019 with 300 fatalities, making it one of the most serious foodborne illnesses in Europe.

High-risk foods

Listeria can contaminate food at any point in the food processing system, from contaminated raw ingredients, processing and production, to packaging and storage. Listeria forms biofilms that adhere to surfaces, making it difficult to remove. It is a facultative anaerobic bacterium and can survive and grow with or without oxygen. Unlike many other foodborne pathogens, L. monocytogenes can survive and multiply at temperatures ranging from -0.4 to 45°C, pH values of 4.4 or higher, and water activity (Aw) values of 0.92 or higher. These factors make the pathogen a significant threat to food safety, especially for RTE foods that may be stored in packaging for extended periods under refrigeration and not typically cooked before consumption, such as deli meats and salads.

Outbreaks of L. monocytogenes are more commonly associated with RTE foods, which do not require additional cooking or heating before consumption. Examples include cold pre-cooked meats, deli meats, smoked and cured fish, cooked shellfish, soft mold-ripened cheeses, raw unpasteurized milks and cheeses, pre-washed salads, pre-prepared sandwiches, and pre-cut fruits. Past outbreaks in Canada and the US have been linked to raw sprouts, unpasteurized milks and cheeses, ice cream, raw or processed vegetables, raw or processed fruits, raw or undercooked poultry, sausages, hot dogs, deli meats, and raw or smoked fish and other seafood. Pets can also spread L. monocytogenes if they consume contaminated raw food.

Recent outbreaks

Notable outbreaks in Canada and the US in recent years:

Standard control and risk mitigation measures

Standard control measures to monitor, eliminate, or reduce L. monocytogenes in RTE foods are applied to foods and food processing environments. The pervasive nature of L. monocytogenes in the environment and its ability to spread and multiply throughout the food processing chain necessitate a multi-faceted approach to control. Key factors influencing the likelihood of L. monocytogenes introduction include:

• Infrastructure (e.g. water supply, drainage and waste disposal, ventilation, overhead fans)
• Plant layout (e.g. separating raw and RTE areas, traffic control)
• Equipment design and maintenance
• Sanitation and disinfection practices
• Employee training and practices

RTE food manufacturers must implement Good Manufacturing Practices (GMPs) to mitigate contamination risks, along with supply chain controls on raw ingredients and process validation to ensure L. monocytogenes is reduced or eliminated. The Canadian Food Inspection Agency provides detailed recommendations on control measures in food processing facilities and in foods. Manufacturers must adhere to Health Canada’s sampling methods and microbiological criteria. In Canada, RTE foods are categorized into two broad categories based on the likelihood of L. monocytogenes growth during processing and shelf-life, which influences the frequency of process monitoring, environmental sampling, and end-product testing that is conducted for a specific food product.

Control measures aim to prevent and monitor the growth and contamination of L. monocytogenes in RTE foods. These components include:

• RTE food control methods: Examples include pasteurization, sterilization, freezing, chilling, acidification, fermentation, drying, and filtration. Some methods may negatively impact the nutritional and sensory attributes of RTE foods.
• Process controls: Certain food additives or food processing aids can inhibit or reduce monocytogenes growth in RTE foods. Post-processing interventions to reduce or eliminate L. monocytogenes on RTE food surfaces before packaging may also be utilized.
• Sanitation controls: A robust cleaning and sanitation program is essential to prevent biofilm formation and mitigate the risk of monocytogenes contamination.
• Growth-limiting parameters: Adjusting product formulations to control pH, temperature, and A_w can limit or prevent monocytogenes growth. These parameters must be maintained throughout the shelf-life to be effective.
• Environmental sampling: Environmental monitoring and sampling are crucial for assessing the effectiveness of control measures and identifying inadequacies in GMPs. The sampling strategy depends on the processing system’s complexity and processing line to identify potential  monocytogenesintroduction sites.  Investigative sampling can identify and eliminate contamination sources if Listeria spp. is found, as presence of Listeria spp. may be indicative of L. monocytogenes.
• End-product testing: End-product testing provides limited information about control measures’ effectiveness due to non-uniform pathogen distribution. It should be coupled with environmental monitoring and sampling to verify effectiveness of control measures, as well as the effectiveness of food additives or food processing aids.
• Shelf-life validation: The shelf-life of a food product is affected by factors such as A_w, pH, temperature during processing, food additives or aids used, storage conditions, etc. The shelf life of the RTE food should be limited to the period during which the food remains safe for consumption.
• Post-lethality treatment: Post-lethality treatments, such as surface heat or high pressure processing applied to RTE foods post-packaging, ensure any  monocytogenes introduced onto the food is reduced or eliminated. If the post-lethality treatment can achieve 3-log reduction or more of L. monocytogenes, the frequency of environmental sampling and end-product testing can be reduced.

Although multiple control measures targeting L. monocytogenes are in place in food processing facilities, outbreaks continue to occur. As such, there is a need to continue to explore and experiment with novel interventions to reduce morbidity and mortality resulting from listeriosis.

Novel control and treatment methods

Additional control and treatment measures can be applied to foods and food processing environments to reduce or eliminate L. monocytogenes. Bacterial tolerance to disinfectants and sanitizers is a concern in food processing environments, so other biocontrols are needed. These can include bacteriophages, competitive bacterial species, and plant-derived products such as essential oils, which are alternatives to combat L. monocytogenes and other pathogens. Some approaches are described below:

Bacteriophages, viruses that infect bacteria, are used to inactivate pathogens on RTE foods and surfaces in food processing environments. Evidence on their effectiveness against L. monocytogenes biofilms is inconclusive. Bacteriophage P100 is approved in Canada to target L. monocytogenes on the surfaces of RTE red meat and poultry products. Endolysins, hydrolytic enzymes used by bacteriophages to propagate from host bacterial cells, have shown effectiveness against staphylococcal biofilms and potentially L. monocytogenes biofilms.  

Another biocontrol method is the use of competitive bacterial species that produce antimicrobials such as bacteriocins, produce organic acids that alter the pH of the growth environment, or physically out-compete the pathogen for nutrients or space.

Plant-derived products such as essential oils from a variety of plant sources such as roots, wood, bark, twigs, leaves, fruits, seeds, and flowers can permeate cell membranes and disrupt bacterial cell functions. However, higher concentrations needed for biocidal effects may interfere with sensory characteristics of RTE food products, such as taste. Research findings on the effectiveness of essential oils against L. monocytogenes and biofilms is mixed. Food debris in food processing environments can also interact with essential oils.

Control methods on RTE foods

Emerging thermal treatments such as microwaves, radio frequency, ohmic heating, and direct steam injection provide rapid heating of foods while maintaining their nutritional and sensory qualities. Some studies have demonstrated their effectiveness against L. monocytogenes in certain foods.

Non-thermal treatments are gaining popularity due to their efficiency and ability to preserve the nutritional quality and physical characteristics of foods. Some examples include high hydrostatic pressure and ultrasound. High hydrostatic pressure (HHP) involves applying pressure above 100 MPa using pressurized liquids such as water. Although high powered ultrasound has been shown to be effective in reducing levels of L. monocytogenes biofilms on stainless steel surfaces, it may impact the quality of foods by producing free radicals and off-flavours, and change the composition of the food matrix.

Another emerging non-thermal treatment is pulsed electric field (PEF), which uses high electric fields for a short duration. Its effectiveness depends on the pH and electric field strength.

The use of organic acids, such as lactic acid, malic acid, citric acid, and acetic acid, in combination with other methods such as ultrasound, ozone, or essential oils is another novel control method. Incorporating organic acids or essential oils and bacteriocins into edible coatings on foods has also been shown to be effective against L. monocytogenes.

Additional resources

L. monocytogenes continues to present significantly challenges to food producers. Further research is needed to explore the effectiveness of novel treatment technologies to eliminate L. monocytogenes in food processing facilities, reduce the occurrence of food recalls, and prevent the potentially life-threatening illness caused by this pervasive bacterium. More information regarding surveillance data, policies and guidance, and detailed control measures can be found below:

• Control measures for Listeria monocytogenes in ready-to-eat foods
• Public Health Ontario listeriosis resources
• Policy on Listeria monocytogenes in ready-to-eat foods (2023)
• Guidelines on the application of general principles of food hygiene to the control of listeria monocytogenes in foods