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Listeria monocytogenes factsheet

L. monocytogenes is a bacterium that occurs widely in the environment. It is a Gram-positive, non-spore forming, facultative anaerobic, short rod.

It is a pathogenic bacterium which causes a group of diseases which are collectively known as listeriosis. It is of major concern because of:

  • Its widespread nature in the environment,  
  • Its ability to survive and grow at refrigeration temperatures (i.e. ≤5 °C), 
  • The severity of the disease it causes, and
  • The high human fatality rate.

Common questions

See answers to common questions below.

  • Why is L. monocytogenes in ready-to-eat food a potential food safety risk?

    Ready-to-eat food poses a particular microbiological risk for clinical infection with L. monocytogenes (listeriosis) because it is consumed without any further treatment which would kill the pathogen, such as cooking. Due to this potential risk, Commission Regulation (EC) No 2073/2005 as amended, sets microbiological criteria for L. monocytogenes that are specific to ready-to-eat food along with additional rules on environmental monitoring and shelf-life studies. 

    Ready-to-eat food is defined in the Regulation as: 

    “…food intended by the producer or the manufacturer for direct human consumption without the need for cooking or other processing effective to eliminate or reduce to an acceptable level, microorganisms of concern;”

    It is the responsibility of the food business operator to determine if a food is ready-to-eat or not, and to ensure the presence and accuracy of mandatory food information to consumers through appropriate labelling of the packaging. For certain food products, there is strong evidence that some consumers could reasonably be expected to normally eat them raw or without thoroughly cooking or reheating them first (e.g. spinach, raw cookie dough, frankfurters, minced meat burgers). While some consumers may believe that particular foods do not require thorough reheating because of their appearance (e.g. flash fried chicken nuggets, ready meals, pre-cooked sausage rolls and quiches, soup). Even when there are specific instructions labelled on the packaging with respect to cooking or reheating, food businesses should establish and document if these foods could be reasonably foreseen to be used by the consumer as a ready-to-eat food. 

    See FSAI Guidance Note 27 for a food business operator’s decision tree for determining the ready-to-eat status of the food they manufacture or produce.

    Annex II of the Commission Regulation (EC) No 2073/2005 describes the shelf-life studies that the food business operator shall conduct, as necessary, in order to ensure compliance with the relevant L. monocytogenes criterion for ready-to-eat food products throughout their shelf-life. Under Article 3.1 of the Regulation, these studies must be carried out under reasonably foreseeable conditions of distribution, storage and use.

    Food businesses should take the worst-case scenario into consideration when carrying out shelf-life studies to determine the potential growth of L. monocytogenes in ready-to-eat food. This includes considering the most favourable physico-chemical properties of the product under real-world production conditions (e.g. pH, water activity [aw]) that would enable L. monocytogenes to grow in the food during its shelf-life, as well as taking into account the worst-case storage temperatures that could reasonably occur during distribution, storage, and use throughout the product's shelf life.

  • What are reasonably foreseeable conditions of distribution, storage and use?

    Reasonably foreseeable conditions of distribution, storage and use of food refer to the conditions that the food is exposed to after it has left the immediate control of the food business that has produced it. For example, for chilled ready-to-eat food products, the storage conditions foreseen during the shelf-life should consider reasonable variations in storage temperatures during distribution of the product, and storage of the product at the manufacturer, retail and consumer level throughout the shelf-life of the food. The food business must also take into account the instructions and any serving suggestions provided on the packaging, as well as reasonably foreseeable conditions of misuse by consumers.

    The control of temperature during all stages of food manufacture, storage, distribution and use should be carefully considered as it can significantly affect the potential growth of L. monocytogenes during the shelf-life of the food. For example, the recommended temperature for refrigerated storage during distribution, catering and retail in Ireland is ≤5 °C but food business operators should consider the possibility of a degree of temperature abuse along this food chain and by the final consumer. If a more conservative storage temperature is used in setting the shelf-life of the food compared to realistic worst-case temperatures during storage, distribution and use, there may be an underestimation of L. monocytogenes growth in the food throughout its shelf-life and thus an overestimation of a safe shelf-life for that product.

    For more information on determining and validating a safe shelf-life for food, see FSAI Guidance Note 18.

    The possible unintended use of food products by consumers should also be taken into consideration when determining the potential risk from L. monocytogenes in food products. For example, certain food products which are not intended as ready-to-eat foods by the manufacturer may be used by the consumers without following the cooking instructions to eliminate L. monocytogenes before consumption. Reasons for unintended use vary. It may be a matter of preference, but several other reasons exist, including problems with consumers fully understanding the label and any instructions that may be provided. This underlines the importance of clear and valid cooking instructions on the label as otherwise there may be ambiguity about whether the food needs to be thoroughly cooked before consumption.

    For more information on determining reasonably foreseeable conditions of distribution, storage and use for food products see the European Food Safety Authority (EFSA) Guidance on date marking and related food information - Part 1 (date marking).

  • Growth and Survival

    L. monocytogenes is highly adaptable and can grow or survive across a wide range of environmental conditions. It grows best at moderate temperatures, neutral pH, high moisture, and low–moderate salt levels, but it can also survive under much harsher conditions where growth is not possible. The survival period will vary depending on nature of food and other factors. 

    Table 1 outlines some of the key growth characteristics of L. monocytogenes that can be used by food business operators to assess the potential risk of the food product(s) supporting the growth of L. monocytogenes under various conditions. 

    Table 1. Growth and survival limits for L. monocytogenes under laboratory conditions (variable according to the strain)

    Table 1. Growth and survival limits for L. monocytogenes under laboratory conditions (variable according to the strain)
    Conditions Minimum Optimuma Maximum Can survive but no growth
    Temperature (°C) -1.5 30-37 45 -18
    pH 4.2 7 9.5 3.3 – 4.2
    Water Activity (aw) 0.92 0.97 Not Applicable < 0.90
    Salt (%)b < 0.5 Not Applicable 12 ≥20

    a When growth rate is highest
    b Percent sodium chloride, water phase

    Of the foodborne pathogens that do not form spores, L. monocytogenes is regarded as the most heat resistant. Thoroughly cooking food, so that its core reaches a temperature of 75˚C (or equivalent time/ temperature combination), is considered effective to achieve a 6-D reduction in the number of L. monocytogenes cells. Food business operators may use alternative time/temperature combinations if they achieve the same lethal effect as 75˚C instantaneously. Scientifically accepted alternative time/temperature combinations include: 70˚C for 2 minutes, 67˚C for 5 minutes and 64˚C for 12 minutes and 37 seconds.

    Typical D-values for L. monocytogenes in chicken breast and beef are shown in Table 2. The D-value is the time in minutes at a given temperature to achieve a 90% reduction in the number of viable cells.

    Table 2. D-values for L. monocytogenes in chicken breast and beef 

    Table 2. D-values for L. monocytogenes in chicken breast and beef
    Temperature Chicken Breast Beef
    50°C 100 85
    60°C 8.7 3.5–4.5
    70°C 0.13 0.14

     

    L. monocytogenes possesses multiple mechanisms that enable it to survive and grow under adverse conditions. It can withstand environmental stress, allowing for swift responses and adaptations to changing environments. As consumer preferences shift toward less heavily preserved and more convenient foods, processing methods within the food industry are becoming milder. The ability of L. monocytogenes to adapt to these less harsh conditions presents a significant challenge for food safety and industry stakeholders. 
    For example, the pathogen can become resistant to heat if it has been exposed to ‘heat shock’ immediately before heating. The heat-shock response is a common phenomenon among pathogenic bacteria that enables them to survive a wide variety of stresses, in particular heat stress. L. monocytogenes can also have increased heat resistance if there are elevated levels of solutes (such as salts or sugars) present, or if the pH conditions of the food product are more alkaline (pH of 9.0–12.0). This stress response enables L. monocytogenes to become more resilient in food products such as processed meats, cheese, and seafood, as the pathogen can better withstand processing steps intended to eliminate it.

  • The Illness (listeriosis)

    Symptoms of infection with listeriosis can include mild flu-like symptoms, or gastrointestinal symptoms such as nausea, vomiting and diarrhoea. In rare cases, the infection can be more severe, causing serious complications. Some people are more vulnerable to L. monocytogenes infections, including pregnant women, babies, and people with weakened immune systems, including adults aged 65 years and older. The incubation period (time between initial infection and first symptoms appearing) is on average 3 weeks but can range between 3 and 70 days.

    For more details on symptom of clinical infection, see the Health Protection Surveillance Centre (HPSC) website.

  • Sources of L. monocytogenes and Routes of Transmission

    L. monocytogenes is found widely in the environment, including in soil, plants, silage, sewage, water, and in the faeces of healthy animals and people. It is often found in many types of food, both animal- and plant-based. It can also become established in food factories, where certain strains can persist for long periods. Research has shown that individual processing establishments often harbour their own unique population of L. monocytogenes and that these populations are capable of persisting in the environment over time. Environmental niches, harbourage sites, and other potential vectors in the food processing facility may allow the persistent survival and growth of L. monocytogenes. Cooked foods may present a risk of L. monocytogenes contamination if they are not cooked thoroughly or if cross-contamination occurs through contact with contaminated surfaces or equipment, especially when the food is exposed to the production environment after cooking.

    Even with an effective factory hygiene programme in place, there may be harbourage sites and growth niches within the food processing environment where L. monocytogenes can evade hygiene procedures, persist, and potentially proliferate. For example, this may be due to the poor implementation of the hygiene programme and/or an inadequate hygiene programme, poor training of staff, maintenance work being carried out, damaged areas in the fabrication of the buildings, and/or poor hygienic design of processing equipment. These areas may pose a risk of cross-contamination to ready-to-eat food products exposed to the environment and to food-contact surfaces in the processing facility. 

  • Foodborne transmission of L. monocytogenes

    The majority of listeriosis cases come from eating food contaminated with L. monocytogenes. Infection can also be transmitted directly from infected animals to humans or spread from person to person. Ready-to-eat foods pose a higher risk for L. monocytogenes contamination because they are consumed without additional cooking, which could otherwise eliminate the bacteria. 

    Ready-to-eat foods are a broad and diverse food category, that can be prepared and stored in different ways. Some ready-to-eat food products support the growth of L. monocytogenes to >100 colony forming units per gram (cfu/g) throughout shelf-life¸ while others do not. The potential growth of L. monocytogenes throughout the shelf-life of food products depends on the physico-chemical characteristics of the food. Growth will also vary according to the reasonably foreseeable temperatures the food is stored at and the shelf-life duration of the product. 

    L. monocytogenes contamination can occur at any stage of the food production and handling process, including post-cooking handling of prepared foods. While many ready-to-eat products have the potential to be contaminated, the incidence and levels of contamination are typically low and vary across products. Foods that support the survival and/or growth of L. monocytogenes pose a higher risk of transmitting listeriosis, particularly when storage conditions (such as temperature and duration) or preparation instructions provided on product labels are not properly followed.

    Foods most often associated with listeriosis include those that: 

    • Support the growth of L. monocytogenes 
    • Have a long shelf-life under refrigeration (L. monocytogenes can grow to significant numbers in food held at refrigeration temperatures when given sufficient time) 
    • Are consumed without further listericidal treatments (i.e. treatments that would kill L. monocytogenes, such as thorough cooking). 

    Examples of food products that have been involved in previous listeriosis outbreaks include certain meat, poultry and fish products (e.g. frankfurters, pâté, smoked salmon); dairy products (e.g. soft cheese, unpasteurised milk); and prepared salads (e.g. coleslaw, bean sprouts). It is estimated that levels of L. monocytogenes below 100 cfu/g of food represent a very low risk of listeriosis for all population groups. 

  • L. monocytogenes and non-ready-to-eat foods

    Occasionally, non-ready-to-eat food products have been linked to outbreaks of listeriosis where consumers have deviated from labelled storage, handling and heating instructions (e.g. opening and portioning before heating, high storage temperatures in domestic fridges, heating to eating temperature (i.e. warm) rather than following labelled cooking instructions, not stirring meal/mixing meal components during microwave heating).

    The greatest risk is for those people who are susceptible to illness from lower infectious doses of L. monocytogenes (e.g. adults aged 65 and older, immunocompromised people, and pregnant women and their unborn children). Although these products are not classified as ready-to-eat foods under EU legislation, food businesses have a responsibility to ensure all food placed on the market is safe to eat. Food businesses must take account of the ‘reasonably foreseeable use’ of these products by recognising that some consumers do not always handle them or fully cook them in accordance with the manufacturer’s instructions on the packaging.

    Considering the reasonably foreseeable use of the product by the customer and by the consumer target groups for which the product is intended, it may be appropriate to control L. monocytogenes as a hazard in the production of certain types of non-ready-to-eat under HACCP-based procedures.

    For more information on how food businesses producing non-ready-to-eat foods can strengthen their food safety management system, see Guidance Note 46: Control of Listeria monocytogenes and Ensuring Food Safety in the Production of Certain Cook/Chilled Ready-To-Heat Meals.

  • Control of L. monocytogenes in the food chain

    Control of L. monocytogenes is required at all stages in the food chain. An integrated approach is required to prevent the growth of this pathogen in the final product. The challenges for controlling L. monocytogenes are considerable given its ubiquitous nature, high resistance to heat, salt and acidic pH and its ability to grow and survive at or below normal refrigeration temperatures. The following general recommendations apply to all sectors of the food chain:

    • Implementation of prerequisite programmes (PRPs). These are good hygiene practices that are the basic conditions and activities necessary to maintain a hygienic environment.
    • Implementation of a food safety management system based on the principles of Hazard Analysis and Critical Control Point (HACCP).
    • Testing against microbiological criteria, as appropriate, when validating and verifying the correct functioning of the HACCP-based procedures and other hygiene control measures.
    • Implementation of a robust Listeria Environmental Monitoring (LEM) programme to detect, investigate, manage and/or eliminate potential sources of L. monocytogenes contamination in the environment of a ready-to-eat food business operation.

  • Incidence of the Disease and Foodborne Outbreaks

    Listeriosis is a notifiable disease in Ireland. This information is used to investigate cases thus preventing spread of infection and further cases. The information will also facilitate the early identification of outbreaks. The most up-to-date data on notifiable infectious diseases is available on the HPSC’s National Notifiable Disease Hub. This hub uses aggregated data to display a dynamic series of dashboards which provide an overview of notifiable infectious diseases in Ireland. The dashboard is updated on a weekly basis and is managed by HPSC.

    The European Food Safety Authority (EFSA) and the European Centre for Disease Prevention and Control (ECDC) also produce an annual One Health Zoonoses reports on zoonotic trends in the European Union which provides information on clinical cases, foodborne outbreaks and implicated sources of listeriosis. The One Health Zoonoses report is typically published at the end of each calendar year and reports zoonoses data at a European level for the previous year. The One Health Zoonoses reports are available to view at:

    Ireland’s national zoonoses report is also published annually by the FSAI, in collaboration with the Department of Agriculture, Food and the Marine (DAFM), the Health Service Executive (HSE), the Sea-Fisheries Protection Authority (SFPA) and the Health Protection Surveillance Centre (HPSC). The report brings together the results of thousands of tests carried out on samples of food and feed, as well as tests on material of animal or human origin, in an effort to determine the pattern and extent of infection by zoonotic pathogens transmitted to humans from animals. 

    View the Irish national zoonoses report.

  • Legislation

    All food business operators have a legal responsibility to produce safe food (Regulation 178/2002, as amended). Safety is ensured through a preventative approach by implementing a food safety management system based on HACCP principles. All food business operators, except primary producers, are legally obliged to implement and maintain permanent procedures based on HACCP (Article 5 of Regulation 852/2004, as amended). 

    All food business operators, including primary producers, must also implement good hygiene practices (GHP). Regulation 852/2004, as amended, lays down hygiene requirements for all foodstuffs. Regulation 853/2004, as amended, lays down more specific hygiene requirements for foods of animal origin. 

    Commission Regulation (EC) No 2073/2005, as amended, lays down microbiological criteria for combinations of food commodities and microorganisms, their toxins, or metabolites. It requires operators to ensure compliance as part of GHP/HACCP, including testing when validating and verifying procedures. The Regulation differentiates between food safety criteria (defining acceptability of a foodstuff) and process hygiene criteria (indicating whether production operates hygienically).

    Microbiological food safety criteria for L. monocytogenes in ready-to-eat foods
    The specific food safety criteria for L. monocytogenes in ready-to-eat foods are laid down in food categories 1.1, 1.2 and 1.3 of Annex I, Chapter 1. Food Safety Criteria under the Regulation. The food category for ready-to-eat foods with regard to L. monocytogenes is chosen by its ability to support, or not support, the growth of L. monocytogenes to greater than the limit of 100 cfu/g throughout the shelf-life of the food, depending on the intended use of the food product. The most stringent L. monocytogenes microbiological criteria apply for foods targeted to infants and foods for special medicinal purposes (Food Category 1.1).

    The determination of the appropriate food category is needed to identify the correct microbiological criterion for L. monocytogenes. To substantiate that the food belongs to one of the three specified categories, it is necessary to identify the target consumers population, and to investigate and characterise the growth potential of L. monocytogenes throughout the shelf-life based on: 

    • the physico-chemical characterisation of the food, 
    • scientific literature, 
    • historical data, 
    • predictive microbiology predictions, 
    • challenge tests, 
    • durability studies.

    The three categories, their sampling plans, stages of application and criteria limits are listed in Commission Regulation (EC) No 2073/2005 of 15 November 2005 on microbiological criteria for foodstuffs. Note that any amendments to the legislation since it was initially published in 2005 must be taken into account when determining compliance with the requirements of the Regulation. 

    For guidance on determining which food category applies for certain ready-to-eat food products, food businesses are advised to consult the specific guidance on complying with legal requirements in relation to L. monocytogenes in the European Commission (2025) GUIDANCE DOCUMENT on Listeria monocytogenes monitoring and shelf-life studies for ready-to-eat foods under Commission Regulation (EC) No 2073/2005 of 15 November 2005 on microbiological criteria for foodstuffs.

    Commission Regulation (EU) 2024/2895 
    Commission Regulation (EU) 2024/2895 amended Commission Regulation (EC) No 2073/2005 in December 2024 and is applicable from 1st July 2026. The amendment to food criterion 1.2 related to “ready-to-eat foods able to support the growth of L. monocytogenes, other than those intended for infants and for special medical purposes” in Commission Regulation (EC) No 2073/2005 has been introduced to ensure a consistent level of public health protection from production through to distribution for ready-to-eat foods. 

    To support this objective, the food safety criterion of ‘L. monocytogenes not detected in 25 g’ will apply to all situations where those foods are placed on the market during their shelf-life, unless the producing food business operator can demonstrate, to the satisfaction of the competent authority, that the level of L. monocytogenes will remain below the limit of 100 cfu/g throughout their shelf-life. To demonstrate this, the food business operator responsible for the manufacture of the product shall conduct studies in accordance with Annex II of the Regulation in order to investigate compliance with the 100 cfu/g L. monocytogenes criterion limit throughout the shelf-life.

    Commission Regulation (EU) 2024/2895 of 20 November 2024 amending Regulation (EC) No 2073/2005 as regards Listeria monocytogenes.

    Compliance with shelf-life studies listed in Annex II of Commission Regulation (EC) No 2073/2005
    Annex II of the Regulation describes the shelf-life studies that the food business operator shall conduct, as necessary, in order to ensure compliance with the relevant L. monocytogenes criterion for ready-to-eat products throughout their shelf-life. The studies should be carried out under reasonably foreseeable conditions of distribution, storage and use. 

    These shelf-life studies include: 

    • determination of physico-chemical characteristics of the product (such as pH, water activity (aw), salt content, concentration of preservatives and the type of packaging system) taking into account the processing steps and storage conditions, the possibilities for re-contamination, the foreseen shelf-life, and
    • consultation of available scientific literature and research data regarding the growth and survival characteristics of the micro-organisms of concern in the product of interest. 
    • When the studies mentioned above are not able to give the necessary confidence to validate the shelf-life and comply with the relevant criterion, the food business operator should conduct additional studies. These additional studies consider the inherent variability linked to the product, processing and storage conditions and may include one or more of the following: 
    • studies to investigate the ability of the micro-organism of concern to grow or survive in the product using predictive mathematical modelling established for the food in question, using parameters that are representative of microbial behaviour (survival or growth) in the product under reasonably foreseeable conditions of distribution, storage and use, and/or; 
    • studies to investigate the ability of the appropriately inoculated micro-organism of concern to grow or survive in the product under reasonably foreseeable conditions of distribution, storage and use (referred to as challenge tests), and/or; 
    • studies to evaluate the growth or survival of the micro-organisms of concern that may be naturally present in the product during the shelf-life under reasonably foreseeable conditions of distribution, storage and use (referred to as durability studies).

    Environmental monitoring for L. monocytogenes
    Article 5.2 of Commission Regulation (EC) No 2073/2005 requires food business operators manufacturing ready-to-eat foods, which may pose a Listeria monocytogenes risk for public health, to sample the processing areas and equipment for Listeria monocytogenes as part of their sampling scheme. This means that food business operators making ready-to-eat food products that are able to support the growth of L. monocytogenes are legally required to sample the processing area and equipment for L. monocytogenes as part of their sampling scheme.

    However, while Commission Regulation (EC) No 2073/2005 specifies that food business operators must sample the processing area and equipment for L. monocytogenes as part of their sampling scheme, it does not provide a microbiological criterion to assess the test results, nor does it provide information on what actions to take when L. monocytogenes is detected in the food business operation’s environment. FSAI Guidance Note 45 offers recommendations for ready-to-eat food business operators on establishing a risk-based framework to identify sampling sites for monitoring the presence of L. monocytogenes. It also provides guidance on the appropriate timing and frequency of sampling activities. Additionally, it outlines how to interpret test results and describes the necessary steps to be taken should L. monocytogenes be detected in the environment of a ready-to-eat food business operation.

  • Related

    For specific guidance on complying with legal requirements in relation to L. monocytogenes see:

    For guidance on compliance with food safety legislation see:

    For further information on L. monocytogenes see: