Hazard Identification

What is Listeria?

The genus Listeria are gram positive, non-spore forming rod shaped bacteria. The genus contains a number of species including L. monocytogenes, L. innocua, L. welshimeri, L. seeligeri, L. ivanovii and L. grayi. Although the first four of these have all been implicated in human infection nearly all cases of Listeria infection are caused by L. monocytogenes.

At least 13 different serotypes of L. monocytogenes are known. All can cause human listeriosis, but most cases are caused by serotypes 1/2a, 1/2b and 4b. The majority of significant reported foodborne outbreaks have been caused by serotype 4b.

Occurrence in foods

Listeria monocytogenes has the potential to be present in all raw foods. Cooked foods can also be contaminated, usually as the result of post-process contamination. L. monocytogenes has been isolated from a very wide range of processed foods including pâtés, milk, soft cheeses, ice cream, ready-to-eat cooked and fermented meats, smoked and lightly processed fish products and other seafood products. L. monocytogenes is usually found only in low numbers (<10 CFU/g) in foods. However, some product such as pâtés and soft cheese have occasionally been found to contain populations of > 10,000 CFU/g.

Hazard characterization

Effects on health

Listeria monocytogenes causes one of the most severe forms of foodborne infection and it is fortunate that listeriosis is a relatively rare disease. The overall mortality rate associated with the disease is 30 %, although it can be as high as 40 % in susceptible individuals. Those most at risk at acquiring the disease are pregnant women (20 times greater risk than healthy individuals), the elderly and the immunocompromised, although healthy individuals can develop listeriosis particularly if the food is heavily contaminated. Monitoring in the USA suggests that Listeria infections are more likely to result in the hospitalisation of affected individuals in comparison with those affected by other foodborne pathogens such as Salmonella (hospitalization rate 95% for Listeria compared with 21% for Salmonella).

The incubation period is 1 to 90 days (mean 30 days). The onset of illness is typically marked by flu-like symptoms (fever and headache), and sometimes by nausea, vomiting and diarrhoea. In some cases these symptoms can lead on to meningitis and septicaemia. Symptoms in pregnant women can lead to infection of the foetus, which can result in miscarriage, stillbirth, or the birth of an infected infant, although the mother usually survives.

The infective dose is unknown, although it is generally considered to be >10³ CFU/g for healthy individuals. Due to the length of the incubation period, it can be difficult to determine the numbers of organisms in foodstuffs at the time of consumption and an outbreak associated with frankfurters in the USA in 1998 is thought to have been caused by product containing less than 0.3 cfu/g, although it is thought that the causative strain may have carried enhanced virulence.

Incidence and outbreaks

The first outbreak of L. monocytogenes that could be definitely linked to food was caused by commercially prepared coleslaw in Canada in 1981 (at least 41 cases with 7 deaths). Manure from Listeria-infected sheep had been used as a fertilizer when growing the cabbages used to prepare the salad.

The incidence of reported Listeria infections increased dramatically during the 1980's as did the number of food-related outbreaks. An outbreak in Los Angeles County during 1985 was caused by Mexican-style cheese (142 cases with 48 deaths) and during the late 1980's an outbreak in the UK was associated with pâté (>350 cases with >90 deaths).

Notable outbreaks occurring in the 1990s were linked to smoked mussels (1992; New Zealand); 'rillettes' or potted pork (1993, France); pasteurized chocolate milk (1994, USA); raw milk soft cheese (1995, France); frankfurters (1998-9, USA); butter (1998-9, Finland) and pork tongue in jelly (1999 - 2000, France).

During the first few years of the 21^st Century there have been a number of large Listeria outbreaks caused by ready-to-eat (deli) poultry products in the USA. In 2000 a multi-state outbreak (29 cases, with 7 deaths) was linked to turkey deli meat, and during 2002 another outbreak (at least 46 cases with 11 deaths) was linked to poultry deli products produced by the Pilgrims Pride Corporation. This outbreak resulted in the recall of 27.4 million pounds of product, the largest meat recall in US history.

Strategies to reduce the incidence of Listeria infections were implemented in many countries during the 1990s resulting in a reduction in the incidence of the disease. However, outbreaks have continued to occur and incidence has again risen in some countries in recent years. For example, in the UK 278 cases were reported in 1988 and this number fell to only 87 in 1995. But by 2003 the number of reported case had risen to 237. A similar pattern has been reported in the EU, but the USA has reported a downward trend in recent years and the incidence of infection in 2005 was 0.27 cases per 100,000 people, despite some significant outbreaks.

Sources

Listeria is ubiquitous in the environment. It is found in soil, where it can survive for extended periods and leads to the contamination of plant material. Listeria has been isolated from a wide variety of fresh produce. It is also found in marine environments and the organism is often associated with fish and seafood products. Animals such as sheep, goats and cattle are recognized carriers of the organism, often acquired from the consumption of contaminated (usually poor quality) silage. Healthy humans can also be carriers of the organism.

Kitchen and food processing environments, particularly those that are cold and wet or moist, can be reservoirs for Listeria. The organism can be particularly persistent and difficult to control because of its psychrotrophic nature and resistance to environmental conditions. The efficacy of hygiene standards in food production facilities producing ready-to-eat products is usually monitored and this can include environmental swabbing for L. monocytogenes. Although other Listeria species are not normally associated with human disease, a positive test for Listeria species other than L. monocytogenes can be a useful indicator that there is the potential for L. monocytogenes to be present.

Growth and survival in foods

Listeria monocytogenes is psychrotrophic and the ability to grow at chill temperatures is the reason why it is a particular risk in extended shelf life chilled foods that can support its growth. Extremely slow growth of L. monocytogenes has been recorded at temperatures as low as -1.5 ^ºC and the maximum temperature for growth is generally accepted as 45 ^ºC. The organism survives well in frozen foods, but survival times can be adversely affected under acid conditions.

The pH range for the growth of L. monocytogenes is 4.3 - 9.4 under otherwise ideal conditions. These values are affected by the specific acid in the product, and the minimum pH is likely to be higher in real foods and at low temperatures. However, L. monocytogenes can survive for extended periods in acid conditions, particularly at chilled temperatures.

The minimum water activity for the growth of L. monocytogenes is 0.92. The organism is tolerant of high sodium chloride levels and is able to grow in environments of up to 10 % salt, and to survive in concentrations of 20 - 30%. L. monocytogenes is also able to survive for some time in low water activity environments, and may survive drying processes. Survival times are extended at chilled temperatures.

Listeria monocytogenes grows well in aerobic and anaerobic conditions. Its growth is unaffected by many modified atmospheres even at low temperatures. High concentrations of carbon dioxide are necessary to inhibit growth.

Although Listeria monocytogenes is not especially resistant to antimicrobials, it can prove difficult to control on food contact surfaces such as stainless steel because the bacteria can form persistent biofilms. It is important to clean equipment prior to using sanitizers because organic matter can affect their efficacy at inactivating the pathogen.

Thermal resistance

Although Listeria monocytogenes is not particularly heat resistant it is more heat resistant than some other foodborne pathogens, such as Salmonella and E. coli O157:H7. It is readily inactivated at temperatures above 70 ^ºC and heat processes such as normal commercial milk pasteurisation will destroy numbers typically found in milk.

Typical D-values in food substrates are: between 5-8 mins at 60 ^ºC, and 0.1-0.3 mins at 70 ^ºC. Concern about the pathogen in particular food product categories has lead to heating guidelines been issued by various heath authorities. The UK Department of Health advised that ready meals or similar products should receive a heat treatment of at least 2 min at 70 ^ºC, or equivalent, to ensure the destruction of L. monocytogenes. For consumers, terms such as heating till 'piping' hot in the UK, and 'steaming' hot in the USA are used to describe heat processes required to ensure the safety of foods identified as being a potential risk of causing Listeria food poisoning.

Control options

The control of Listeria in foods relies largely on a HACCP approach and the establishment of effective critical control points in the process.

Processing

The careful design and layout of processing equipment in conjunction with the implementation of regular, thorough cleaning regimes of the processing environment can significantly reduce the level of Listeria-contamination in many processed foods. However, because of its ubiquitous nature it is virtually impossible to totally eliminate the pathogen from many food products. The organism should be inactivated by heat applied during the cooking process and the presence of Listeria in cooked products can indicate poor hygiene either during manufacture, distribution or at retail.

Other critical controls include strict temperature control, the prevention of cross-contamination between raw and processed foods and between the processing environment and processed foods, as well as the use of a restricted shelf life for potentially contaminated products that could support the growth of the pathogen.

Product use

Appropriate scientifically-based methods should be used to devise safe shelf lives for 'at risk' chilled foods and these restricted shelf lives should be rigorously implemented and adhered to in order to reduce the risk from L. monocytogenes. Clear cooking instructions are needed on the packaging of many chilled foods requiring reheating prior to consumption, to ensure that all parts of the product receive a listericidial process.

Vulnerable individuals, especially pregnant women, the elderly and the immunosuppressed are advised to avoid eating specific foods to reduce the risk from listeriosis. Health authorities in the UK advise these groups not to eat soft mould-ripened or blue-veined cheeses, pâté and unpasteurised dairy products. These groups are advised that they may also choose to avoid cold (pre-cooked) meats and smoked salmon, and that they should thoroughly wash pre-packed salads and heat adequately chilled meals and ready-to-eat chicken before eating. In the US the FDA also includes hot dogs, luncheon meats, cold cuts and smoked seafood (unless thoroughly reheated) to the list of foods that at-risk consumers should definitely avoid.

Legislation

Countries differ in their regulatory approach to the presence of L. monocytogenes in RTE food. In the USA a 'zero tolerance policy' is taken on the presence of L. monocytogenes in any RTE food. Recent European Union regulations generally permit a count of up to 100 cfu/g at the end of shelf life for RTE foods, except those intended for infants and for special medical purposes.

Specific regulatory guidance on Listeria for food manufacturers is also available in a number of countries.

In July 2004, Canadian authorities published a regulatory policy on Listeria monocytogenes in ready-to-eat (RTE) foods, which included the following guidelines:

A refrigerated RTE food not supporting the growth of L. monocytogenes includes the following:

1. pH 5.0 - 5.5 and a_w <0.95
2. pH <5.0 regardless of a_w
3. a_w < or = 0.92 regardless of pH, or
4. frozen foods

In the USA, regulations encourage producers of ready-to-eat (RTE) cold meat or 'deli' products to use HACCP or similar programs to control L. monocytogenes and require these companies to give authorities access to data used to verify procedures, such as environmental and finished product testing for L. monocytogenes. These establishments are encouraged to make food safety enhancement claims on their RTE product labels that describe the processes used to eliminate or reduce Listeria monocytogenes, or suppress its growth in products.

Various countries have standards/legislation for the pasteurisation of ice cream/frozen desserts; these heat processes are more severe than high temperature short time milk (HTST) pasteurisation (which is at least 15 secs at 72 ^ºC) because ingredients such as sugars, fat, emulsifiers and stabilizers in these products protect L. monocytogenes from heat, resulting in an increase in D-value. In New Zealand a heat process of at least 15 s at 79.5 ^ºC (or equivalent) is required for ice cream, and in the US standards require a process of 30 min at 68.3 ^ºC or 25 s for 79.4 ^ºC.

Sources of Further Information

Published

Bell C., Kyriakides A.
Listeria: a practical approach to the organism and its control in foods. 2nd edition. Oxford. Blackwell Publishing. 2005.

Ryser E.T., Marth E.H.
Listeria, listeriosis and food safety. 2nd edition.
New York. Marcel Dekker. 1999.

International Commission on Microbiological Specifications for Foods.
Listeria monocytogenes,
In Microorganisms in Foods, Volume 5, Microbiological Specifications of Food Pathogens, Ed. International Commission on Microbiological Specifications for Foods. Blackie. London, 1996, 141-82.

On the web

Risk profile: Listeria monocytogenes in soft cheeses. Institute of Environmental Science and Research Limited. (November 2005)
http://www.nzfsa.govt.nz/science/risk-profiles/FW0382_L_Mono_in_soft_cheese_November_2005.pdf

Risk profile: Listeria monocytogenes in low moisture cheeses. Institute of Environmental Science and Research Limited. (July 2005)
http://www.nzfsa.govt.nz/science/risk-profiles/FW0440_L_mono_in_low_moisture_cheese_Final_Mar_2007.pdf

Risk profile: Listeria monocytogenes in ready-to-eat salads. Institute of Environmental Science and Research Limited. (July 2005)
http://www.nzfsa.govt.nz/science/risk-profiles/FW0446_L_mono_in_RTE_salads_2005.pdf

Risk assessment of Listeria monocytogenes in ready-to-eat foods, MRA Series 4 & 5 World Health Organization (2004)
http://www.who.int/foodsafety/publications/micro/mra_listeria/en/index.html

Policy on Listeria monocytogenes in Ready-to-Eat Foods. Health Canada (July 2004)
http://www.hc-sc.gc.ca/fn-an/legislation/pol/policy_listeria_monocytogenes_politique_toc_e.html

Risk profile: Listeria monocytogenes in ice cream. Institute of Environmental Science and Research Limited. (October 2003)
http://www.nzfsa.govt.nz/science/risk-profiles/lmono-in-ice-cream.pdf

Quantitative Assessment of Relative Risk to Public Health from Foodborne Listeria monocytogenes Among Selected Categories of Ready-to-Eat Foods (US FDA: September 2003)
http://www.foodsafety.gov/~dms/lmr2-toc.html

Risk profile: Listeria monocytogenes in processed ready-to-eat meats. Institute of Environmental Science and Research Limited. (October 2002)
http://www.nzfsa.govt.nz/science/risk-profiles/listeria-in-rte-meat.pdf