Listeria monocytogenes (L. monocytogenes) is a persistent challenge in food processing environments, particularly for small and medium-sized plants that may struggle with fully implementing pathogen control processes. While L. monocytogenes is the primary species responsible for foodborne illness, the broader Listeria genus includes multiple species, not all of which are pathogenic. However, in sanitation programs, it is important to address all Listeria species, as the presence of non-pathogenic Listeria often indicates conditions that could also support L. monocytogenes.

Throughout this paper, we will refer to both pathogenic and non-pathogenic species collectively as "Listeria." While the industry has well-established best practices for Listeria prevention, the real issue lies in execution. Failing to properly assess and mitigate Listeria risks can have serious consequences. The lesson is not that best practices are ineffective—it is that best practices must actually be put into practice consistently and rigorously.

Understanding Listeria: A Persistent Threat

L. monocytogenes can cause severe illnesses, particularly in pregnant people, newborns, the elderly, and immunocompromised individuals, with the highest mortality rate of any foodborne bacterial pathogen. Unlike many other foodborne pathogens, Listeria is able to grow at refrigeration temperatures (40 °F), making it a significant risk in ready-to-eat (RTE) foods that will not undergo heat treatment before consumption.¹ Listeria monocytogenes is widely found in soil, water, and decaying vegetation where it can persist.^2,3 Its ubiquity in these environments make for an easy contamination route into food processing environments, where persistence can lead to significant risk of foodborne illness.

Listeria contamination is particularly concerning in post-processing environments, where already-cooked products can be re-contaminated through contact with contaminated surfaces, equipment, or hands. After cooking and chilling, but before packaging, is an especially vulnerable step for Listeria to contaminate products. This is why Listeria control measures must be focused on post-processing contamination prevention rather than solely on cooking interventions.^4,5

Key characteristics of Listeria include:

• Grows at refrigeration temperatures
• Biofilm-forming: Can attach to surfaces and resist standard cleaning, especially in environments with other biofilm-formers present^6,7
• Can grow with or without oxygen
• Survives extreme conditions: Freezing does not kill it, and it can persist in high-salt (10 percent NaCL) environments
• Heat-sensitive: Cooking to 165 °F (74 °C) kills it; thus, recontamination is the primary concern.

Prevention: Keep Listeria Out and Control Growth and Spread

To effectively manage Listeria, plants must take a two-pronged approach: (1) keep Listeria out in the first place and (2) assume Listeria is present, thereby taking proactive measures to control further growth and spread.

Keep Listeria Out

• Control environmental entry points: Listeria can be introduced through raw ingredients, shoes, equipment, air handling systems, and personnel. Implementing dedicated shoes and outerwear for RTE areas and managing airflow to prevent cross-contamination are critical.⁸
• Separation of raw and cooked product: Physical barriers, dedicated equipment, and controlled movement of personnel minimize the risk of cross-contamination.
  • This can be especially challenging for smaller processors that may not have space to create separation, such as packaging cooked product in a dedicated cooked area.
  • Facilities that have common pass-through spaces before and after cooking should still maintain separate chilling, slicing, and packaging spaces whenever possible.
• Supplier controls: Ensure that raw materials are sourced from suppliers with robust Listeria control programs, including environmental monitoring and sanitation validation.
• Proper waste disposal: Ensure that waste materials, including organic food residues and used packaging, are managed effectively to prevent microbial growth and contamination.

Lynn Delmore, a food safety consultant who advises food processors, highlights a common concern: "I walk into plants and regularly see overlooked issues—like employees moving from raw to RTE zones without changing aprons or gloves. Smaller processors often don't have the luxury of a fully separated clean room, which makes it even more important to maintain physical separation and strict traffic control. A poorly designed workflow can unravel even the best sanitation plan."

Control Growth and Spread: Seek and Destroy

The "seek and destroy" philosophy means actively trying to find Listeria—not avoiding it. During cleaning, inspection, and pre-operational checks, the goal is to identify potential harborage points where the organism might survive. Similarly, environmental swabbing should target areas most likely to yield positive results—not just clean surfaces—so any contamination can be effectively eliminated before production begins.⁹ Seek and destroy Listeria by identifying "hot spots" where Listeria may thrive to both clean and swab for test results.

• Floors and drains: Listeria thrives in damp environments like drains.¹⁰ These areas must be cleaned daily, and drains must be part of a regular cleaning routine that includes a combination of practices such as manual scrubbing, detergents and alkaline cleaners, a rotation of sanitizers, and possibly enzymatic or foam cleaning systems to reach deep into drains.
• Equipment crevices: Listeria can hide in seals, gear housings, and under cover panels. Disassemble equipment regularly for cleaning. For clean-in-place (CIP) systems, ensure proper maintenance and chemical sanitizer programs.
• Condensate and moisture control: Drip pans, ceiling pipes, and overhead structures should be routinely checked and cleaned to prevent contamination from condensation.
• Walls, ceilings, and overhead structures: Listeria can settle on ceiling beams or drip from condensation. Regular inspections and sanitation prevent contamination.
• Cold storage areas: Even in refrigerated environments, Listeria can persist and grow. Cold storage units should be cleaned and monitored for temperature fluctuations that might promote bacterial survival.¹¹

Environmental Monitoring Programs

Environmental monitoring programs (EMPs) are essential for detecting and preventing contamination in food processing environments. These programs rely on systematic swabbing, corrective actions, and data analysis to maintain food safety and regulatory compliance.

• Routine swabbing: Swab high-risk areas regularly and trend results over time to detect patterns. This can be completed as part of sanitation standard operating procedures (SSOPs) separately from the Listeria control plan for alternative 1 and 2 facilities (as defined by USDA-FSIS Listeria control options).
• Corrective actions: If Listeria is detected, immediate corrective actions must include root cause analysis and intensified sanitation.
• Data logging and trend analysis: Use software to track patterns of contamination and help predict future risks.¹²

Enhanced Sanitation Techniques

An effective sanitation program is more than just a checklist of tasks—it is a science-driven system that must be tailored to the plant's environment, product type, and microbial risks. Enhanced sanitation goes beyond basic cleaning by incorporating strategies designed specifically to combat persistent pathogens like Listeria monocytogenes. In high-risk areas, such as RTE zones and post-lethality environments, sanitation procedures must be validated, regularly reviewed, and adapted to address microbial resistance, facility design limitations, and evolving pathogen behavior. A layered approach combining chemical, physical, and procedural interventions creates the strongest defense.

The following tools and practices support this enhanced framework:

• Use of dry cleaning methods: For low-moisture environments, dry steam cleaning can help remove biofilms.
• Rotation of sanitizers: Changing disinfectants regularly helps prevent Listeria resistance. Quaternary ammonium compounds (QACs) and peracetic acid are effective. A good sanitation plan may also include specific anti-biofilm agents.
• Validation of cleaning procedures: Conduct environmental swabbing post-cleaning to verify sanitation effectiveness.
• Foam and gel-based cleaners: Some advanced foaming agents help penetrate hard-to-reach areas where Listeria can hide.
• Automated cleaning systems: In large plants, automated sanitation systems can provide more consistent and effective coverage.
• Sanitary equipment design: Work with manufacturers to ensure machinery is designed for easy cleaning, reducing bacterial harborage points.
• Employee hygiene and training: Routine training programs should ensure that all personnel understand best practices for preventing contamination.

Disrupting Listeria Biofilms

Physical cleaning, especially scrubbing with soap, brushes, and mechanical action, is one of the most effective ways to remove biofilms.⁶ Biofilms comprise extracellular polymeric substances (EPS) that form a protective matrix around bacterial cells, making them highly resistant to sanitizers alone. This matrix must be mechanically broken down to expose and eliminate the bacteria underneath. Abrasive cleaning tools like stiff-bristled brushes, scrub pads, and scouring surfaces help disrupt this layer and physically detach microbial buildup from equipment and environmental surfaces.^13,14

Repeated, targeted scrubbing of problem areas—such as joints, drains, gaskets, and hard-to-clean seams—should be routine and reinforced during employee training. Relying on foam or fog systems without manual action can result in a false sense of sanitation security. Even in automated systems, hand-scrubbing remains essential for "nooks and crannies" that automated systems cannot reach. For small and medium-sized processors, investing in proper brushes and dedicating time to thorough scrubbing can make a substantial difference in breaking the chain of contamination.

Aeriel Belk, Ph.D., Assistant Professor of Animal Science at Auburn University, whose laboratory studies Listeria biofilms, adds: "Biofilms are becoming more adaptive and resistant to cleaning compounds. In smaller plants with aging infrastructure, these harborage points are harder to eliminate without hands-on scrubbing and validated swabbing protocols."

Ingredient-Based Interventions as Complementary Listeria Controls

While sanitation remains the cornerstone of controlling Listeria monocytogenes in (RTE) meat and poultry products, targeted ingredient interventions offer an important secondary defense, especially given Listeria's persistence in cold environments and during post-lethality processing. Clean-label antimicrobials like buffered vinegar, cultured sugar, bacteriocins, and fruit/spice extracts can inhibit pathogen growth without compromising product quality. These interventions are especially valuable in pre-packaged deli meats, where controlled manufacturing environments reduce the chance of cross-contamination.

Advances such as pediocin-based processing aids—now generally recognized as safe (GRAS) in the U.S.—add further protection without altering sensory attributes or requiring label changes. Although not substitutes for robust sanitation, these ingredient technologies help extend shelf life, reduce recall risk, and provide data-driven assurance when paired with predictive modeling tools. In today's food safety landscape, integrating such approaches supports both regulatory compliance and consumer trust.

Regulatory Considerations: USDA-FSIS Actions and Industry Implications

Regulatory agencies have reviewed past Listeria outbreaks and found that long-standing food safety violations often precede major contamination events.^4.5 In response, several regulatory changes have been implemented to modernize Listeria oversight:

• Strengthening Listeria testing requirements for RTE facilities
• Enhancing documentation requirements for Listeria prevention measures
• Expanding RTE sampling programs, incorporating more environmental surface testing
• Establishing a new, inspector-led routine sampling program using advanced detection methods
• Introducing digital recordkeeping requirements to ensure consistency in compliance.

Many small- and medium-sized facilities that fall under state inspection may find their regulatory oversight increasing in response to recent Listeria monocytogenes outbreaks. This will likely require the implementation and modernization of control programs. The recommendations included here are intended to help with compliance.

These measures reflect a shift toward proactive rather than reactive food safety regulation. Small- and medium-sized plants must take note—stronger regulatory scrutiny is coming, and compliance will require consistent, well-documented pathogen control efforts.

Moving Forward: Implementation Over Theory

The industry does not need more advanced training on Listeria control—the industry needs implementation-focused training for technical personnel wearing multiple hats in small- and medium-sized plants. These individuals must be equipped not only with technical knowledge, but also with the ability to catalyze food safety culture within their facilities. Food safety must be the foundation of the business, not an afterthought.

For small- and medium-sized plants, the goal is clear: move beyond theoretical best practices and focus on rigorous, routine, and verifiable implementation. The only way to prevent Listeria contamination, avoid devastating recalls, and, most importantly, protect consumers is through strict adherence to these principles.

Acknowledgments

The authors thank Aeriel Belk, Ph.D., Assistant Professor at Auburn University, for her expertise and thoughtful review of this article.

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