Listeria monocytogenes is a bacterium that causes listeriosis, a serious foodborne illness. It thrives in cold environments and can contaminate dairy products, including cheese. Pregnant women, older adults and those with weakened immune systems face the highest risk.
This post explores the science behind Listeria, its survival mechanisms and its impact on cheese safety.
What is Listeria monocytogenes?
Listeria monocytogenes is a Gram-positive, rod-shaped bacterium found in soil, water and animal faeces. Unlike many bacteria, it can grow at refrigeration temperatures. This makes it a major concern in chilled foods.
Listeria has several survival advantages:
- It thrives at 0–45°C, making refrigeration ineffective in stopping its growth.
- It tolerates high salt concentrations, allowing survival in processed foods.
- It forms biofilms, making it difficult to remove from food surfaces.
- It can cross the intestinal, blood-brain, and placental barriers, leading to severe infections.
How does L. monocytogenes cause illness?
Listeriosis occurs when contaminated food is ingested. The bacteria invade the intestinal cells and spread through the bloodstream. Severe infections affect the brain, leading to meningitis or encephalitis.
- Ingestion and survival in the gut: Listeria enters the body through contaminated food. It survives stomach acid and bile, allowing it to reach the intestines.
- Cell invasion: Listeria binds to intestinal cells using proteins like Internalin A and B. It tricks the cells into engulfing it through a process called phagocytosis.
- Escape from the immune system: Inside the cell, Listeria escapes the phagosome by producing Listeriolysin O, a toxin that breaks down the surrounding membrane. This allows it to enter the cytoplasm, where it can move freely.
- Spreading between cells: Listeria uses a protein called ActA to hijack the host’s actin filaments. This forms “actin tails,” propelling the bacteria into neighbouring cells. It spreads without exposing itself to the immune system.
- Crossing protective barriers: Listeria can cross critical barriers in the body: Blood-brain barrier (Causes meningitis or encephalitis) and placental barrier (Can infect the foetus, leading to miscarriage or stillbirth).
Symptoms of listeriosis
Listeriosis symptoms vary depending on the person’s immune response. Common symptoms include:
- Mild cases: Fever, muscle aches, nausea and diarrhoea.
- Severe cases: Sepsis, meningitis and neurological complications.
- In pregnancy: Mild flu-like symptoms but severe risks to the unborn baby, including miscarriage or stillbirth.
Why is Listeria in cheese a problem?
Cheese provides an ideal environment for Listeria growth. It contains moisture, nutrients and sometimes lacks preservatives.
Which cheeses are high-risk?
Certain cheeses carry a higher risk of Listeria contamination:
- Soft cheeses made with unpasteurised milk – Brie, Camembert, Queso Fresco and some blue cheeses.
- Washed-rind cheeses – Reblochon, Taleggio and Munster, which have high moisture levels.
- Raw milk cheeses – Any cheese made from unpasteurised milk can carry Listeria if contaminated.
Why is soft cheese more dangerous?
Soft cheeses contain more moisture than hard cheeses. This allows Listeria to grow more easily. Hard cheeses, like Cheddar or Parmesan, have lower moisture and a more acidic environment, reducing bacterial survival.
Listeria & Pasteurisation
Pasteurisation is a key defence against Listeria in dairy products.
How pasteurisation works
Heat treatment kills bacteria by damaging their cellular structures. In dairy processing, milk is heated to at least 72°C for 15 seconds (high-temperature, short-time pasteurisation). This eliminates Listeria, Salmonella and E. coli.
Is pasteurised cheese always safe?
While pasteurisation destroys bacteria in milk, contamination can occur later. If cheese is handled in unsanitary conditions, Listeria can still grow. Factory hygiene plays a crucial role in preventing outbreaks.
How does Listeria survive in cheese factories?
Listeria adapts well to food production environments. Once it contaminates a facility, it becomes difficult to remove.
Common sources of contamination
- Drains and floors – Biofilms allow bacteria to persist despite cleaning.
- Processing equipment – Listeria can survive on slicing machines and packaging lines.
- Raw ingredients – Unpasteurised milk or contaminated cultures introduce bacteria.
- Employee handling – Poor hygiene can spread Listeria to finished products.
Biofilms: a hidden threat
Listeria forms biofilms, which are protective layers of bacteria. These attach to surfaces and resist cleaning chemicals. Once a biofilm develops, it continuously sheds bacteria, contaminating food.
Notable Listeria outbreaks in cheese
Several outbreaks have been linked to contaminated cheese. These incidents highlight the risks of improper food handling.
- Listeria in soft cheeses (2019, USA): In 2019, an outbreak in the USA linked to soft cheeses caused 24 illnesses and two deaths. Investigators traced the bacteria to unsanitary cheese-making conditions.
- Listeriosis in France (2022): A 2022 outbreak in France was linked to raw milk Brie. Authorities recalled multiple batches after detecting high levels of Listeria.
How to prevent Listeria contamination in cheese
Both consumers and manufacturers play a role in reducing Listeria risks.
For cheesemakers
- Strict hygiene protocols – Regular cleaning and sanitising prevent contamination.
- Regular testing – Routine Listeria testing ensures safety in production.
- Pasteurisation enforcement – Using pasteurised milk reduces bacterial risks.
For consumers
- Choose pasteurised cheese – This is crucial for pregnant women and the immunocompromised.
- Store cheese correctly – Keep cheese below 5°C and consume it before expiration.
- Practise good hygiene – Wash hands and use clean utensils when handling cheese.
Can hard cheese contain Listeria?
Hard cheese can contain Listeria monocytogenes, but the risk is much lower than in soft cheeses. The low moisture content, high salt levels and acidic environment of hard cheeses, such as Cheddar and Parmesan, make it difficult for Listeria to grow.
Additionally, the long ageing process further reduces the bacteria’s chances of survival. However, Listeria is a resilient pathogen that can still persist under certain conditions.
Cross-contamination is a key risk factor, as Listeria can spread from contaminated surfaces, utensils or infected soft cheeses stored nearby. Improper storage, such as keeping cheese at unsafe temperatures, may also allow bacteria to survive.
Hard cheeses made from raw milk pose a slightly higher risk, though regulations often require ageing for at least 60 days to reduce bacterial contamination. While rare, food safety inspections have occasionally detected Listeria in aged cheeses, showing that even hard cheese is not entirely risk-free.
Does freezing kill Listeria?
Freezing does not kill Listeria monocytogenes; it merely halts its growth. The bacterium can survive freezing temperatures and resume activity upon thawing. Studies have shown that Listeria experiences greater injury and death when frozen and stored at -18°C compared to -198°C.
However, some bacteria survive and can repair themselves under suitable conditions. Therefore, freezing should not be relied upon to eliminate Listeria in contaminated foods.
Does cooking reduce the risk of contamination with Listeria?
Yes, cooking does kill Listeria monocytogenes, but only at sufficiently high temperatures. The bacteria die when food is heated to at least 74°C (165°F). Proper cooking ensures food safety by eliminating Listeria from contaminated products.
Key temperature guidelines
- Pasteurisation (72°C for 15 seconds) – Effectively kills Listeria in dairy products.
- 74°C (165°F) or higher – Listeria is destroyed.
- Reheating leftovers – Should also reach 74°C (165°F) to ensure safety.
Future strategies to combat Listeria
Scientists continue to research ways to control Listeria in cheese production.
Bacteriophage therapy
Bacteriophages are viruses that infect bacteria. Some phages specifically target Listeria, offering a natural method for control.
Natural antimicrobials
Cheesemakers are testing plant extracts and bacterial cultures that inhibit Listeria growth. These include:
- Nisin – A natural antimicrobial found in some cheese cultures.
- Rosemary extract – Contains compounds that slow bacterial growth.
- Lactic acid bacteria – Compete with Listeria in cheese environments.
Improved detection methods
Faster DNA-based tests help cheesemakers detect Listeria before products reach the market. This reduces outbreaks and recalls.
Conclusion
Listeria monocytogenes poses a serious risk to cheese safety. Its ability to survive cold temperatures and form biofilms makes it difficult to control. Soft cheeses, unpasteurised varieties, and poor hygiene increase the risk.
Preventing Listeria requires strict hygiene, pasteurisation and safe handling practices. Scientists are developing new methods to combat this persistent bacterium. Consumers should choose pasteurised cheese, store it properly, and handle it safely.
By understanding the science behind Listeria, we can reduce its risks and enjoy cheese safely.
References
- Listeria monocytogenes Cross-Contamination of Cheese: Risk Assessment and Control Strategies: This study discusses the prevalence of Listeria monocytogenes in cheese and highlights the risk of cross-contamination during cheese production. PMC
- Outbreak Investigation of Listeria monocytogenes: Brie and Camembert Soft Cheese Products: The FDA investigated a multistate outbreak linked to Brie and Camembert cheeses, underscoring the potential for contamination in soft cheeses. U.S. Food and Drug Administration
- Listeria Outbreak Linked to Queso Fresco and Cotija Cheese: The CDC provides details on an outbreak associated with Queso Fresco and Cotija cheeses, emphasizing the risks even in pasteurized products. CDC
- Listeriosis Caused by Persistence of Listeria monocytogenes in Cheese Production Environment: This article examines a listeriosis outbreak traced to persistent environmental contamination in a cheese dairy, highlighting the challenges of eradicating Listeria once established. PMC
- Listeria monocytogenes in Cheese Products: The Centre for Food Safety discusses the risk of L. monocytogenes in cheese products and provides guidelines for prevention. FEHD
Cheese lover. Scientist. Created a website and a Youtube channel about cheese science because he could not find answers to his questions online.
