What is Listeria?

Listeria Monocytogenes, abbreviated as Listeria, is a type of bacterium that causes a serious infection known as listeriosis, particularly dangerous for pregnant women, newborns, elderly individuals, and those with weakened immune systems. This bacterium is known for its adaptability to a variety of environments, including those with low temperatures, acidic pH, and high salt concentrations, which can make it a persistent threat in food processing environments.

Listeria is unique among foodborne pathogens for its ability to continue to grow at temperatures as low as 0°C, making it a significant concern in refrigerated food environments. When ingested, the bacterium can lead to severe illness or even death, particularly in high-risk populations.

The molecular mechanisms that contribute to Listeria’s virulence include its ability to invade host cells, escape from the phagocytic vacuole, and spread from cell to cell, thus evading the host’s immune response. Understanding these mechanisms is crucial for developing strategies to control Listeria in food processing environments and reduce foodborne listeriosis.

What Foods Can Be Contaminated?

Listeria monocytogenes can contaminate a wide range of food products. Here are some of the foods most commonly associated with Listeria contamination:

  • Deli Meats and Hot Dogs: These products can become contaminated after processing. Listeria can survive and sometimes continue to grow at refrigerator temperatures.
  • Unpasteurized (Raw) Milk and Dairy Products: Listeria can be present in raw milk and can be transferred to products made from raw milk, including soft cheeses.
  • Soft Cheeses: Particularly those made with unpasteurized milk, can harbor Listeria. However, infections can also occur from soft cheeses made with pasteurized milk if they are contaminated after pasteurization.
  • Seafood: Smoked fish and other seafood can become contaminated with Listeria during processing.
  • Fresh Produce: Contamination can occur in the field through soil or manure, or during processing or retail handling.

The risk of Listeria contamination extends to various food-processing environments, due to the bacterium’s ability to form resilient biofilms. These biofilms can be particularly challenging to eradicate once established, providing a persistent source of potential contamination in food processing facilities.

The ability of Listeria to survive and multiply at low temperatures further exacerbates the risk associated with refrigerated foods. This trait makes the control of Listeria a significant challenge in the food safety domain.

How Does it Affect Human Health?

Listeria monocytogenes is notorious for causing listeriosis, a severe infection that can be fatal if left untreated. Here are the impacts of listeriosis on human health:

Clinical Manifestations:

  • Mild Symptoms: In individuals with robust immune systems, listeriosis may manifest as a mild, flu-like illness or produce no symptoms at all.
  • Invasive Listeriosis: In at-risk populations such as pregnant women, the elderly, newborns, and individuals with compromised immune systems, the infection can become invasive, affecting the bloodstream and central nervous system.
  • Neonatal Listeriosis: Pregnant women infected with Listeria can transmit the bacterium to their fetuses or newborns, leading to neonatal listeriosis, which can result in stillbirth, premature birth, or severe infection in the newborn.

Long-Term Effects:

  • Neurological Damage: Listeriosis can lead to meningitis, encephalitis, or other neurological disorders which may result in long-term neurological damage.
  • Death: Listeriosis has a high fatality rate, especially among high-risk groups.

Listeriosis is a prime example of how foodborne pathogens can cause severe health impacts. Its potential to cause serious illness underscores the importance of robust food safety practices to mitigate the risk of Listeria contamination in the food supply chain.

How Common is Illness?

Listeriosis is relatively rare, but its severe consequences make it a significant public health concern. The Centers for Disease Control and Prevention (CDC) estimates that Listeria infection affects about 1,600 people in the United States each year, with about 260 cases resulting in death. The mortality rate, therefore, is quite high, standing at about 16% which is notably higher than many other foodborne illnesses.

The real burden of listeriosis is likely higher than reported due to under-diagnosis or misdiagnosis. Many mild cases might not be diagnosed at all, while others might be misattributed to more common illnesses with similar symptoms.

Global Occurrence:

Listeriosis occurs worldwide. However, the incidence rate can vary significantly between regions and countries, often depending on the local dietary habits and the effectiveness of food safety controls in place.


Outbreaks of listeriosis are often associated with ready-to-eat foods that have been contaminated with Listeria. These outbreaks can affect multiple states or countries when contaminated products are widely distributed.

The relatively low incidence but high severity of listeriosis underscore the importance of effective Listeria control measures in the food industry to protect public health.

Where Does It Come From?

Listeria monocytogenes is ubiquitously present in the environment, which is a primary reason behind its potential to contaminate food products. Here’s how the contamination pathway usually unfolds:

Environmental Reservoirs:

  • Soil and Water: Listeria is commonly found in soil and water, which can serve as a direct source of contamination for foods like fruits and vegetables.
  • Animal Carriers: Many animals carry Listeria without appearing ill and can contaminate foods of animal origin such as meats and dairy products.

Food Processing Environments:

  • Equipment Contamination: Food processing equipment and surfaces can become contaminated with Listeria, especially if the bacterium has formed biofilms which are difficult to eradicate.
  • Human Handling: Improper handling of food by workers can also be a source of Listeria contamination.


  • Post-Processing Contamination: Cross-contamination can occur after the food has been cooked or processed, especially in deli settings where the same slicing equipment may be used for different foods.

Understanding the various routes of contamination is crucial for devising strategies to control Listeria in the food processing environment and reduce the risk of listeriosis.

How Is It Affected By Environmental Factors?

Listeria monocytogenes is known for its robustness in facing adverse environmental conditions, which significantly contribute to its persistence in food processing environments and the subsequent risk of food contamination. Here’s how environmental factors impact Listeria:


  • Cold Tolerance: Listeria can survive and even multiply at low temperatures, which is unusual among bacteria. It can proliferate in refrigerated conditions where other bacteria cannot, making refrigerated ready-to-eat foods particularly vulnerable.

Acidity and Salinity:

  • Acid Tolerance: Listeria can survive in acidic conditions, a trait that allows it to persist in foods and the human gastrointestinal tract.
  • Salt Tolerance: It is also tolerant to high salt concentrations, enabling it to survive in salted food products.

Moisture and Oxygen Levels:

  • Moisture: Listeria requires moisture for growth, which is why it thrives in wet environments often found in food processing facilities.
  • Oxygen Levels: Listeria is a facultative anaerobe, meaning it can grow in the presence or absence of oxygen, further broadening the range of environments in which it can survive.

Biofilm Formation:

  • Resilience: Listeria can form biofilms on various surfaces, including food processing equipment. These biofilms provide protection from adverse environmental conditions and cleaning and disinfecting agents, making Listeria particularly hard to eradicate once established.

These environmental tolerances pose significant challenges for controlling Listeria in food processing environments and underline the necessity of stringent food safety measures.

How Can It Be Controlled?

Controlling Listeria monocytogenes necessitates a multi-faceted approach encompassing good hygiene practices, stringent monitoring, and effective cleaning and disinfection procedures. Here are the key strategies to control Listeria:

Good Manufacturing Practices (GMP):

  • Hygiene: Ensuring good personal hygiene among food handlers, including hand washing and the use of clean clothing and gloves.
  • Equipment Cleaning: Regular cleaning and sanitizing of food-contact surfaces and equipment to prevent Listeria contamination and biofilm formation.

Environmental Monitoring:

  • Regular Testing: Routine environmental testing for Listeria to identify contamination sources and assess the effectiveness of cleaning and sanitation procedures.
  • Corrective Actions: Prompt corrective actions in response to positive Listeria findings, including re-cleaning, re-sanitizing, and, if necessary, product recalls.

Process Controls:

  • Temperature Control: Maintaining cold food storage temperatures as low as possible to slow down Listeria growth.
  • Food Processing: Employing processes like pasteurization or cooking to kill Listeria, and avoiding post-processing contamination.

Training and Education:

  • Staff Training: Educating food industry personnel on the risks associated with Listeria and the importance of prevention measures.
  • Consumer Education: Educating consumers on safe food handling practices, particularly for ready-to-eat and refrigerated foods.

Research and Innovation:

  • Emerging Technologies: Exploring novel food processing and packaging technologies that can help control Listeria.
  • Predictive Microbiology: Utilizing predictive microbiology tools to understand and control Listeria growth in different food and environmental conditions.

The combination of these strategies, adapted to specific food processing environments and products, can significantly reduce the risk of Listeria contamination and the incidence of listeriosis.

Are There Rules and Regulations?

Ensuring food safety concerning Listeria monocytogenes requires a regulatory framework that sets forth standards and guidelines for food producers and processors. Here are some of the key rules and regulations pertaining to Listeria control:

International Guidelines:

Codex Alimentarius: The Codex Alimentarius Commission provides international guidelines for the control of Listeria in foods, advocating for the application of Good Hygiene Practices (GHP) and Hazard Analysis and Critical Control Points (HACCP) systems.

United States:

  • Food and Drug Administration (FDA): The FDA has established regulations and provides guidance for controlling Listeria in ready-to-eat foods.
  • United States Department of Agriculture (USDA): The USDA’s Food Safety and Inspection Service (FSIS) oversees the safety of meat, poultry, and egg products, setting standards for Listeria control in these products.

European Union:

The European Food Safety Authority (EFSA) provides scientific advice to the EU on Listeria control, and the European Commission sets regulatory standards for Listeria in ready-to-eat foods.

Other National Regulations:

Various countries have their own regulatory standards for Listeria control in foods, often based on international guidelines and tailored to local conditions and food supply chains.

Industry Standards:

Industry standards like those benchmarked by the Global Food Safety Initiative (GFSI) also play a crucial role in setting food safety requirements regarding Listeria control.

Certification Programs:

Third-party certification programs audit and certify food safety practices, including Listeria control measures, in food processing facilities.

These regulatory frameworks and industry standards aim to create a food safety culture that prioritizes the control of Listeria monocytogenes, thereby reducing the risk of foodborne listeriosis and enhancing public health protection.

Resources and Further Readings

CDC – Prevent Listeria – a valuable resource for preventing Listeria as provided by the Centers for Disease Control and Prevention (CDC).

FDA – Control of Listeria monocytogenes in Ready-To-Eat Foods – a draft guidance provided by the Food and Drug Administration (FDA) regarding the control of Listeria monocytogenes in Ready-To-Eat Foods.

USDA – Best Practices Guidance for Controlling Listeria monocytogenes –  provides a PDF guidance for controlling Listeria monocytogenes in retail delicatessens.

EFSA – Listeria – provides an overview and useful informations about Listeria

Codex Alimentarius – Guidelines on the Control of Listeria Monocytogenes in Foods – another PDF resource providing guidelines on the application of general principles of food hygiene for controlling Listeria monocytogenes.