Listeria: The Hidden Food Safety Threat You Can’t Ignore

The genus Listeria comprises several bacterial species, many of them harmless, but one stands out for its impact on food safety: Listeria monocytogenes. This species demonstrates high environmental resilience and can grow even at refrigeration temperatures, making it a silent threat in various food industry segments, such as meat, dairy, minimally processed vegetables, refrigerated desserts, and even chocolate.

General characteristics of the Listeria genus

• Gram-positive, rod-shaped, motile bacteria
• Grow between 0°C and 45°C, even under refrigeration
• Tolerate pH up to 9.9, salinity up to 10%, and low water activity (Aw down to 0.94) [1]
• Form biofilms that hinder removal by conventional detergents
• Survive for long periods on poorly sanitized surfaces

Listeria monocytogenes: The Primary Pathogen of Concern

• The only relevant pathogenic species in the genus
• Responsible for listeriosis, a serious disease with a high mortality rate (up to 30%) [2]
• Long incubation period (1 to 70 days), making traceability difficult
• Symptoms: fever, vomiting, muscle aches, meningitis, and miscarriage
• Risk groups: pregnant women, newborns, the elderly, immunosuppressed 

Relevant outbreaks: 

Table 1: Relevant Listeria outbreak cases worldwide (Source: WHO, CDC, EFSA)

High-risk food products

• Dairy: fresh, aged, or creamy cheeses
• Meat and cold cuts: deli meats, pâtés, sausages
• Minimally processed vegetables: ready-to-eat salads, sprouts
• Fish: smoked, marinated, vacuum-packed
• Refrigerated desserts and moist fillings: cakes, chocolates, pies
• Ready-to-eat sauces: mayonnaise, pastes, and MAP products

Sources of industrial contamination

• Drains and hard-to-reach channels
• Equipment with poor hygienic design (cracks, crevices)
• Cold surfaces with condensation (e.g., chillers and evaporators)
• Biofilms formed within 24–48h [3]
• Cross-contamination via utensils, PPEs, packaging, and workers' hands

Effective environmental control

• Rigorous cleaning with process-appropriate products
• Validated sanitizers (e.g., PAA, iodine, quaternary ammonium, chlorine)
• Use of biofilm control products
• Hygienic design: smooth surfaces, disassemblable equipment
• Moisture control and elimination of water puddles
• Segregation of dirty and clean areas
• Mid-shift cleaning and entry control programs
• Environmental monitoring using swabs (zones 1 to 4)

Process and product control measures

• Effective heat treatment (pasteurization at 72°C/20s) [4]
• Control of Aw and pH
• Use of preservatives: nisin, lactate, nitrate (where permitted)
• Modified atmosphere packaging (MAP) with temperature < 4°C
• Reduction of shelf life
• Shelf-life and challenge testing

Biofilms: an invisible challenge

Biofilms are structured communities of microorganisms encapsulated in an extracellular matrix of polysaccharides, proteins, and DNA, adhering to living or inert surfaces. In industrial environments, this matrix protects Listeria from temperature changes, chemical agents, and mechanical forces.

Sanitization resistance can be up to 100 times greater than planktonic forms, requiring combined actions of specific detergents, mechanical action, and validated disinfectants [5]. Biofilms can form in under 48 hours, especially in humid, hard-to-reach, or poorly cleaned areas. Studies show that Listeria can remain viable for months inside biofilms in drains and equipment.

They also hinder microbiological detection, as the bacteria may enter a VBNC (viable but non-culturable) state, impairing standard testing results. Effective control requires validated anti-biofilm products and corrective actions based on periodic validations, such as targeted swabs, RODAC plates, and ATP testing.

Economic impact and regulatory requirements

An outbreak may lead to mass recalls, brand damage, fines, and lawsuits

• The FDA requires zero tolerance for Listeria monocytogenes in ready-to-eat foods [6]
• In Brazil, ANVISA’s RDC 331/2019 also sets strict limits
• The average cost of a Listeria outbreak for a company can exceed US$10 million [7]

Conclusion

Listeria contamination can occur in any food industry segment — even in unexpected ones like chocolates and desserts. Prevention requires an integrated approach focusing on environmental hygiene, process control, hygienic design, and continuous staff training. Fighting Listeria is not a one-time task — it’s a constant vigilance.

Want to assess Listeria control in your facility? Contact a Diversey specialist.

References:

[1] FAO/WHO. Risk Assessment of Listeria monocytogenes in Ready-to-Eat Foods.

[2] CDC - Centers for Disease Control and Prevention.

[3] Carpentier B. and Cerf O. Review: Persistence of Listeria monocytogenes in food industry environments. Int J Food Microbiol.

[4] FDA - Pasteurization process standard.

[5] Bridier A. et al. Resistance of bacterial biofilms to disinfectants: a review.

[6] FDA. Bad Bug Book – Listeria monocytogenes. Foodborne Pathogenic Microorganisms.

[7] GMA (Grocery Manufacturers Association). Capturing recall costs: Measuring and recovering the losses.