Listeria Invasion – How is it Creeping into Our Beloved Foods?

By Traci Slowinski
1 Comment

As a result of several recent high-profile Listeria-related recalls, interest in the foodborne pathogen is increasing and food processors must take preventive measures to keep it out of facilities.

Listeria monocytogenes is a gram-positive bacteria. It has 13 serotypes, three of which have been associated with the vast majority of foodborne infections (called listeriosis). Although it is not a leading cause of foodborne illness, it is among the leading causes of death from foodborne illness. This hardy pathogen is salt tolerant, thrives in cold, moist environments and can grow under refrigeration temperatures. Listeria is found throughout our environment including in soil, water, sewage, decaying vegetation and even some animals. Its presence has most often been identified in foods such as raw or under-pasteurized milk, smoked fish, deli meats, soft cheeses and raw vegetables.

For a healthy individual, Listeria may cause mild symptoms or have no effect at all. Fever, muscle aches, nausea, vomiting and diarrhea are common signs of infection. For the e  immunocompromised (the very young or old, pregnant women, or adults with weakened immune systems), symptoms can be more severe and include septicemia and meningitis; in pregnant women, it can cause miscarriage. Symptom-onset ranges from a few hours to two–three days, with durations lasting from days to several weeks, depending on the severity of infection.

Keep Listeria Out of Your Plant

The first line of defense against Listeria contamination is to keep it out of a facility, but that may not always be possible, thus a Listeria prevention plan should be integrated into a company’s food safety program, considering the following areas:

    • Employees—can be brought in on shoes and clothing, or through infected workers. Prevention includes:
      • Good Manufacturing Practices
        • Personnel Hygiene—proper hand washing, wearing clean clothes and shoes, wearing proper hair restraints
        • Employee Illness Policy—restriction/exclusion of ill employees
    • Raw Materials—introduction into the environment from raw ingredients (milk, fish, seafood, produce), pallets, cardboard, packaging material. Prevention includes:
      • Supply Chain Management
        • Supplier Approval Program—having strategic partners that ensure only safe, high-quality raw materials
        • Ingredient Management—requesting COAs, letters of guarantee, allergen control
        • Receiving/Storage Procedures—completing incoming inspections, proper nonconforming material handling
    • Processing Aids—harborage in ice, brine solutions, improperly filtered compressed air and HVAC units. Prevention includes:
      • Sanitation Program—proper cleaning/sanitizing of equipment
      • Preventative Maintenance—regular replacement/maintenance on filters/motors
      • Water, Air, and Steam—utilizing potable water, properly filtered air, properly treated steam
    • Equipment Design—contamination of conveyor belts, filling and packaging equipment, slicers/dicers, ice makers, hoses, equipment framework, condensate drip pans, maintenance tools, trash cans, tools for cleaning equipment (brushes and scouring pads). Prevention includes:
      • Sanitary Equipment Design – ensuring that all equipment can be broken down as far as possible and properly cleaned and sanitized to remove dirt, debris and micro-organisms
    • Process Flow—not maintaining segregation of high vs. low risk, clean vs. dirty. Prevention includes:
      • Separation of high-risk vs low-risk areas through time, space or physical barriers
      • Proper garb (smocks, hair restraints, captive shoe policy) and sanitary measures (hand wash and sanitize, shoe sanitizer) to reduce introduction into high-risk areas
      • Proper personnel flow or movement to prevent cross-contamination
    • Plant Environment—common pathogen harborage areas. Prevention includes:
      • Floors/Drains – splash back, biofilms
      • Overhead Structures – condensate, dust/debris
      • Waste Areas – trash buildup
      • Wash Areas – standing water
    • Sanitation Program—insufficient cleaning/sanitizing to remove pathogens. Prevention includes:
      • SSOP’s – comprehensive sanitation SOPs with special focus on hard-to-clean areas and equipment.
      • Drain Cleaning—proper frequency, chemicals and procedures
      • Clean-In-Place Systems—accessibility to hard-to-reach areas
      • Sanitizing Agents—quaternary ammonium compounds, peroxyacetic acid sanitizers (for biofilms)
    • Environmental Monitoring Program—tool to identify gaps and risk used improperly resulting in missed problems. Prevention includes:
      • Robust Sampling Plan—identify zones and sampling areas
      • Effective Track & Trend Tool—identify gaps or risk that require corrective/preventive action
      • Timely Corrective Action— ensure proper follow-up on any issues that arise

First Person: The Listeria Experience and Lessons Learned

The above list is by no means exhaustive when it comes to all the areas you need to consider when completing a gap analysis within your facility. Listeria can be very insidious, and you will need to be ever-vigilant to ensure it does not take hold in your environment. Believe me, I have been there when it has.

Once upon a time, I worked for a Ready-to-Eat processing plant. We had robust food safety and quality assurance programs. We employed two microbiologists and had a good environmental monitoring program. The sanitation team did a thorough job of cleaning and sanitizing every night, and we completed preoperational sanitation inspections (including ATP testing) every morning.

Then we had a Listeria recall.

It started out small. One sample tested by FDA came back positive. Then another…and another. More intensive environmental testing found Listeria in the plant—in the drains, in the hard to reach areas of the old slicers and MAP packaging equipment, and even in the production room cooling units.

After many, many hours of cleaning, sanitizing, testing and retesting, we determined that the current layout of the facility would never allow for complete elimination of the Listeria. We had one big production room where raw material was brought in and broken down, fed into the processing lines and assembled, and lastly, finished product was placed into the packaging, which then went into cases and onto wooden pallets. There was no separation of high-risk areas from low risk.

So, what did we do? We set up a temporary location for production (which was a major project in and of itself), redesigned the process flow, shut down the plant, and remodeled our production area.

Highlights of the redesign included:

      • Building a cleanroom for the production area. We walled off the raw material handling area by the receiving warehouse, enclosed the packaging area by the shipping warehouse, and made the room that housed the processing lines a cleanroom/high-risk room. Entry into the room required appropriate garb (clean smocks, hair restraints, gloves, captive shoes), use of the hands-free hand wash and sanitizer, and a pass through the boot sanitizer. The temperature of the room was reduced from 50oF to 38oF to discourage pathogen growth.
      • Setting up a raw material handling room. A separate “dirty” area was built to break down raw material components and remove them from their packaging (corrugate cases, plastic wrap). Raw material was then fed through a wall opening where a UV light was mounted to “sanitize” the outside of the material. This helped to reduce the introduction of cardboard packaging and wood pallets into the cleanroom and drive down any pathogens that might be on the surface of the raw material.
      • Adding a packaging room. All sealed, packaged finished product passed through a wall opening into the packaging room where it was boxed up and placed on pallets. This also helped to reduce the introduction of cardboard and wood into the clean room.

The new process flow allowed for employees to move from the “clean” processing room to the raw material or packaging rooms without any extra precautions, but they were required to go back through the clean room procedures prior to going back to the processing area. Raw material and packaging employees were excluded from the cleanroom to minimize potential of contamination from personnel flow.

We also reevaluated our Hazard Analysis and Critical Control Points (HACCP), Sanitation Standard Operating Procedures (SSOPs), and Sanitation and Environmental Monitoring (EM) programs to ensure all potential risk was identified and addressed either through the new facility design or other control measures. One key takeaway was to use the EM program to identify gaps and areas of opportunity rather than to just try to prove that everything is fine. We learned that having a comprehensive EM program that can capture all the necessary data points, analyze trends and drive corrective action helped our team use the program to drive food safety and continuous improvement. It wasn’t good enough to just have an EM program in place; we needed to use the data to address gaps and mitigate identified risks.


Foodborne pathogens are one of the biggest risks to the safety of our foods. Listeria poses a threat to a number of food categories (dairy, protein, and produce) and should be highlighted as a significant hazard to be assessed when developing and implementing your food safety programs. Using risk-based preventive controls within your facility will help prevent adverse events related to Listeria.

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  1. Faylinn

    It truly helped me a lot when you explained that discourage pathogen growth the room has to be at a temperature of above 50 degrees Fahrenheit. The other night my husband and I were talking about how UVM Modified listeria enrichment broths work and how they could benefit the manufacturing of food. Hopefully, we will be able to find a supplier soon that can help us get rid of this problem, not only as a family but as a community and that will eliminate food poisoning.

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