What is Listeria?
Listeria is a genus of Gram-positive, rod-shaped bacteria, containing 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 the disease listeriosis in humans, but most cases are caused by serotypes 1/2a, 1/2b and 4b. The majority of reported foodborne outbreaks have been caused by serotype 4b.
What foods can be contaminated?
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. The pathogen 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. L. monocytogenes is usually found only in low numbers (less than 10/g) in foods. However, products such as pâtés and soft cheese have occasionally been found to contain levels of 10,000/g or more.
How does it affect human 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 %, but it can be as high as 40 % in vulnerable individuals and hospitalisation is more likely than for other common foodborne infections. Those most at risk of acquiring the disease are pregnant women (20 times greater risk than healthy individuals), the elderly and the immuno-compromised, although healthy individuals can develop listeriosis, particularly if the food is heavily contaminated.
The incubation period varies from 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 develop into meningitis and septicaemia. Infection 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 uncertain, although it is generally considered to be high for healthy individuals, with food contamination rates of more than 1,000 cells/g being required. Due to the length of the incubation period, it can be difficult to determine the numbers of bacteria in foods at the time of consumption. An outbreak associated with frankfurters in the USA in 1998 is thought to have been caused by product containing less than 0.3 cells/g, although it is suspected that the strain involved may have been unusually virulent.
How common is illness?
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 1980s, as did the number of food-related outbreaks. An outbreak in Los Angeles 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 (more than 350 cases with over 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 decade of the 21st Century there were a number of large Listeria outbreaks caused by ready-to-eat (deli) poultry products in North America. 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, then the largest meat recall in US history. In Canada in 2008 an outbreak linked to a number of ready-to-eat (RTE) deli meats resulted in 57 cases of illness and 23 deaths.
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 the incidence in the last decade has again risen in some countries. For example, in the UK 278 cases were reported in 1988. Although this figure decreased in the period 1991 – 2000 to an average annual rate of 110 cases, the number of cases rose again (particularly amongst the over 60s) during 2001 – 2009 to an average annual rate of 192. A similar pattern has been reported in the EU and Canada.
However, the USA has reported a downward trend in recent years: the incidence of infection in 2009 was 0.34 cases per 100,000 people, which compared to 1996 – 1998 rates of Listeria infections is a decrease of 26%.
Where does it come from?
Listeria is common in the environment. It is found in soil, where it can survive for extended periods leading to the contamination of plant material, and has been isolated from a wide variety of fresh produce. It is also found in marine environments and is often associated with fish and seafood products. Animals such as sheep, goats and cattle are recognized carriers of Listeria, often acquired from the consumption of contaminated (usually poor quality) silage. Healthy humans can also be carriers.
Kitchen and food processing environments, particularly those that are cold and wet, can be reservoirs for Listeria. The organism can be particularly persistent and difficult to control because of its resistance to unfavourable environmental conditions and ability to grow at low temperatures, especially within biofilms.
How is it affected by environmental factors?
Listeria monocytogenes is psychrotrophic (able to grow at low temperatures) and this is the reason why it is a particular risk in extended shelf life chilled foods. Extremely slow growth of L. monocytogenes has been recorded at temperatures as low as -1.5ºC and the maximum temperature for growth is generally accepted as 45ºC. The organism survives well in frozen foods, but survival times can be shortened under acid conditions.
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 70ºC and heat processes such as commercial milk pasteurisation will destroy numbers typically found in milk. Typical D-values in food substrates are between 5-8 minutes at 60ºC, and 0.1-0.3 minutes at 70ºC. 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, sometimes referred to as a ‘Listeria cook’, to ensure the destruction of L. monocytogenes. For consumers, terms such as heat till ‘piping hot’ in the UK, and ‘steaming hot’ in the USA are used to describe safe heating regimes for certain foods.
The pH range for the growth of L. monocytogenes is 4.3 – 9.4 under otherwise ideal conditions, but 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 in refrigerated foods.
The minimum water activity for the growth of L. monocytogenes is 0.92. It can tolerate 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 food drying processes. Survival times are extended at chilled temperatures.
Listeria monocytogenes grows well in the presence or absence of oxygen. 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.
How can it be controlled?
The control of Listeria in foods relies largely on a HACCP approach and the establishment of effective critical control points.
For food processors
The careful design and layout of processing equipment in conjunction with regular, thorough cleaning 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 eliminate the pathogen from many food products. Effective cooking processes should inactivate Listeria and its presence in cooked products can indicate poor hygiene during manufacture.
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 Listeria.
For retailers and consumers
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. Effective temperature control during distribution and storage is also a key to safe shelf life. Clear cooking instructions are needed on the packaging of chilled foods requiring reheating prior to consumption, to ensure that all parts of the product reach the temperature required to destroy the bacteria.
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, all types of pâté (including vegetable) 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 adequately heat chilled meals 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.
Are there rules and regulations?
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, and the pathogen should be absent in 25 g of product. However in 2008 the US Food and Drug Administration (FDA) published a draft consultation paper proposing to loosen up these controls to allow a maximum limit of 100/g in frozen and refrigerated RTE foods that do not support the growth of Listeria.
European Union regulations generally permit a count of up to 100/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 (contact local food enforcement authorities for details).
Where can I learn more?
Policy on Listeria monocytogenes in Ready-to-Eat Foods. Health Canada (April 2011)
Risk profile: Listeria monocytogenes in processed ready-to-eat meats. Institute of Environmental Science and Research Limited. (November 2009)
Risk profile: Listeria monocytogenes in soft cheeses. Institute of Environmental Science and Research Limited. (November 2005)
Risk assessment of Listeria monocytogenes in ready-to-eat foods, MRA Series 4 & 5, WHO (2004)