Listeria monocytogenes in Ready to Eat Operations

What is Listeria monocytogenes?

L. monocytogenes, commonly known as Listeria, is bacteria naturally found in soil, water, air, sewage, domesticated and wild animals and humans. Consuming foods contaminated with L. monocytogenes can lead to the life threatening foodborne illness listeriosis, which may primarily affect pregnant women, newborns, the elderly and people with weakened immune system. Common symptoms include headache, nausea, vomiting, diarrhea, cramps, constipation and persistent fever. These symptoms can occur two to 70 days after consuming contaminated food. For more information, visit the Canadian Food Inspection Agency food safety fact sheet on Listeria .

Find more about how to control L. monocytogenes in the following sections:

L. monocytogenes in Ready to Eat Food Operations

Foods eaten without any prior washing, cooking or reheating, (ex: deli meats and pre-cut or pre-washed fruits and vegetables) are considered ready to eat (RTE) foods. RTE foods may become contaminated with L. monocytogenes during processing because of:

  • improper sanitation techniques
  • poor personnel practices
  • improper flow patterns

By not properly washing or cooking food products before consumption, consumers are unable to eliminate any possible L. monocytogenes contamination, so RTE food operations have a higher risk of causing foodborne illnesses.

Growth Environments and Hiding Places

L. monocytogenes can survive the following conditions:

  • freezing
  • drying
  • temperatures up to approximately 74 °C

L. monocytogenes can grow under the following conditions:

  • refrigeration temperatures
  • moist conditions

Unlike L. monocytogenes, most pathogens (ex: E. coli and Salmonella) do not have the unique and dangerous ability to grow at refrigeration temperatures. L. monocytogenes thrives especially well in moist conditions which puts wet operations at an increased risk of contamination. Contamination is usually line or equipment specific where L. monocytogenes become established in an area where cleaning and sanitizing is ineffective. As equipment is operated, the bacteria move out of their established areas and onto the surfaces of the equipment. Food products travelling on the equipment will then spread the bacteria into subsequent processing areas. The only way to correct this problem is to find and eliminate L. monocytogenes from their hiding places.

Some common areas where L. monocytogenes is found include:

  • equipment framework
  • hollow conveyor rollers
  • standing water in production areas
  • floors and walls with cracks that retain moisture
  • drains
  • rubber seals around doors
  • ceilings, overhead structures, catwalks
  • condensate (the liquid product from condensation)
  • wet insulation
  • trolleys and forklifts
  • cleaning tools such as sponges, brushes, floor scrubbers
  • maintenance tools

Controlling Contamination

The highest risk of contamination is between processing (cooking, pasteurizing, etc.) and packaging. Common sites where cross-contamination occurs include:

  • filling or packaging equipment
  • fibrous or porous conveyor belts
  • slicers, dicers, shredders, blenders (used after cooking and before packaging)
  • collators used for assembling/arranging product for packaging
  • racks for finished products
  • hand tools, gloves, aprons, etc. that contact exposed finished product
  • spiral freezer/blast freezers
  • containers used to hold food before further processing or packaging

You can control L. monocytogenes in the environment and reduce the risk of food or food contact surface contamination by following Good Manufacturing Practices (GMPs) such as sanitation, personal practices, and traffic patterns.


The best way to combat L. monocytogenes in your plant is with proper sanitation techniques including:

  • Clean in a manner that prevents re-contamination of areas previously cleaned (ex: cleaning from top to bottom). Visit the Sanitation page for proper daily sanitation steps and further sanitation information.
  • Floor drains are the most common area for L. monocytogenes because they are moist and cool. Clean and sanitize floor drains in a manner that prevents contamination of other surfaces in the room. Do not use high-pressure hoses to clean drains, because the aerosols created will spread contamination throughout the room. Design and maintain floor drains to prevent backups.
  • Select equipment that can be easily and fully disassembled and cleaned. Damaged, pitted, corroded, or cracked equipment should be repaired or replaced to eliminate areas where microbial growth can occur. Second-hand equipment, although visually clean, may still harbour bacteria. Before using it, take apart all the pieces and properly clean and sanitize them.

For a complete guide on sanitation, see the OMAFRA Cleaning and Sanitation Guidebook.

Personnel Practices

Personnel are a high risk factor in contamination of food and equipment, especially where food is handled significantly. Following are examples of policies that could be used:

  • Employees handling raw ingredients should not come into contact with finished ready to eat products.
  • Proper hand washing must be done when entering food processing areas and when hands become contaminated (ex. after touching a garbage container or the floor).

Flow Patterns

Consider the following:

  • Maintain product and traffic flow of personnel and equipment, from raw ingredients to the finished product, to avoid cross contamination.
  • Personnel handling raw ingredients should not be allowed to enter the finished product area, as they may track in possible contaminants. If this is not possible, sanitizing foot baths should be used before entering the finished product area.
  • L. monocytogenes can be found in the air and contaminate products or surfaces through air currents in your facility. Control airborne contamination with proper air flow in the plant. For example, supply your finished products area with filtered air at a higher pressure than your raw area. This will control the entrance of airborne L. monocytogenes and other contaminants into your finished products area.

Environmental Testing for Listeria

An environmental monitoring program can be used to assess the risk of product contamination. In general, focus the environmental testing on the generic Listeria species, because they are found more frequently in the environment than L. monocytogenes. Both food contact and non food contact areas should be tested. If Listeria is found in your processing environment, you can make the decision whether further product testing and holding of products is required. However, if a food contact surface shows positive results for Listeria, products should be tested and held.

For further information on environmental testing, see Validation of GMP Programs (PDF 257 KB).

To test for Listeria in your plant, you can use a rapid Listeria test or send your samples to an accredited lab for testing. For more information:


For more information, email the Food Safety and Inspection Branch or call 204-795-8418 in Winnipeg.