Listeria monocytogenes, ready-to-eat foods and public health

Listeria monocytogenes and listeriosis

To date, the genus Listeria consists of nine species, of which only L. monocytogenesis essentially pathogenic to humans. The majority (99%) of human infections caused by L.monocytogenesare foodborne, i.e. caused bythe consumption of foods containing high numbers of L. monocytogenes[1]. Serotyping of L. monocytogenesis of limited epidemiological value given that the majority of the strains involved in human listeriosis cases and outbreaks belong to just three(1/2a, 1/2b and 4b) of the 13 known L. monocytogenes serotypes[2].

The infectious dose for humans remains unknown and most probably varies depending on an individual’s immune status. In most cases, the symptoms of infection in healthy adults resemble those of gastrointestinal or influenza-like infections (febrile gastroenteritis). However, in individuals belonging to high-risk groups, the disease often progresses to the more severe form (invasive disease, listeriosis), which is characterized by septicemia, meningoencephalitis, late-term abortions and a mortality rate of c. 30%. High-risk population groups are elderly people, neonates, pregnant women and immunocompromised individuals, such as human immunodeficiency virus (HIV) patients, diabetics and people undergoing chemotherapy [3].

The occurrence of L. monocytogenesin the environment

Listeria monocytogenesis widely distributed in nature. It can be found in soil, vegetation, surface waters, sludge, animal feeds and the digestive tract of domestic and wild animals. Therefore low levels of L. monocytogenes can easily contaminate raw foods such as meat, milk, fish and vegetables. In addition, the pathogen is frequently isolated from the environment of food-processing plants (floors, drains, processing equipment), where it oftenresides for prolonged periods of time because of its ability to form biofilms [1].

Characteristics of L. monocytogenes–important for food safety

Listeria monocytogenesis has a mesophile displaying optimum growth at 37°C and neutral pH. However, L. monocytogenes possesses elaborate physiological adaptation mechanisms towards various environmental stresses. In foods such stress conditions include low temperature (refrigeration), increased osmotic strength (salting or addition of sugars) and increased acidity (fermentation). Undoubtedly, its psychrotrophic nature, i.e. the ability of L. monocytogenes to adapt and proliferate at low temperatures (up to -0.5°C),causes the most concern for the food industry, because refrigeration is the most common method of food preservation in developed countries.

Pasteurization or more intense heat treatments effectively eliminate levels ofL. Monocytogenes typically encountered in raw foods. However, post-pasteurization contamination of foods during subsequent processing steps, such as slicing or packaging, as a result of cross-contamination from contaminated surfaces, equipment and packaging materials, is the main reason for the occurrence of the pathogen in ready-to-eat (RTE) foods. The ability of L. monocytogenes to proliferate in certain categories of RTE foods (permissive RTE foods) stored for a prolonged period of time under refrigeration is the main cause of L. monocytogenes foodborne infections in humans.

The inter-strain variation in the behavior of L. monocytogenes under stress conditions, as well as the effect of the history (i.e. previous environmental adaptations) of the L. monocytogenes cells contaminating RTE foods, are factors that hinder attempts to predict the behavior of the pathogen safely during storage of contaminated RTE foods. For instance, previous exposure of L. monocytogenes to sub-lethal, moderately acidic conditions usually renders the organism more tolerant to subsequent exposure to more extreme stresses of the same (e.g. higher acidity) or a different nature (adaptation or cross-protection) [4].

Human foodborne listeriosis outbreaks

Extensive phylogenetic and subtyping studies have shown that L. monocytogenes isolates can be classified into four divergent phylogenetic lineages (I-IV) [2]. Most of the L. monocytogenes strains isolated from foods and human listeriosis cases belong to lineages Ι and ΙΙ. Listeria monocytogenes has caused foodborne epidemics via many different foods, such as pasteurized milk, soft cheeses, meat products, coleslaw, smoked salmon, RTE salads and cantaloupes. Contaminated RTE foods can lead to foodborne infections affecting hundreds or thousands of consumers, because of the frequently extensive food production and distribution schemes. Currently, there is an ongoing L. monocytogenes epidemic in the USA that appears to be related with the consumption of contaminated soft cheese.As of 25 October 2012, 22 peoplehave been affected across 13states and two deaths have been reported.


European legislation, food safety criteriaand compliance of RTE foods

According to the European Union (EU) food safety criteria outlined in Regulation (EC) 2073/2005 [5], RTE foods intended for consumption by infants or people with special medical conditions are required to be free of L. monocytogenes (absence of the pathogen in 25 g, in a 10-unit sampling plan). RTE foods other than those intended for infants or special medical purposes are subdivided into those that are able to support the growth of L. monocytogenes and those that are not. Products ‘with pH ≤ 4.4 or aw ≤ 0.92, products with pH ≤ 5.0 and aw ≤ 0.94, and products with a shelf-life of less than five days are automatically considered to belong to the category of RTE foods that are unable to support the growth of L. monocytogenes. The Regulation also states that ‘other categories of products can also belong to this category, subject to scientific justification’. Lastly, the food safety criteria for L. monocytogenes are adjusted according to their temporal stage in the food chain. Thus, for RTE foods that are able to support the growth of L. monocytogenes, the Regulation demands the absence of the pathogen (in 25 g) ‘before the food has left the immediate control of the food business operator, who has produced it’, but allows for up to 100 colony-forming units (CFU)/g for ‘products placed on the market during their shelf-life’. The 100 CFU/g limit also applies throughout the shelf-life of marketed RTE foods unable to support L. monocytogenes growth. For RTE foods that are able to support the growth of L. monocytogenes, Regulation 2073 specifies that the ‘100 CFU/g’ criterion ‘applies if the manufacturer is able to demonstrate, to the satisfaction of the competent authority, that the product will not exceed the limit of 100 CFU/g throughout the shelf-life’ and the ‘absence in 25 g’ criterion applies only when the manufacturer is ‘not able to demonstrate, to the satisfaction of the competent authority, that the product will not exceed the limit of 100 CFU/g throughout the shelf-life’. It is therefore the responsibility of the manufacturer to engage in research and generate product-specific data in order to provide scientific proof to meet the above requirements.

Challenge testing can provide data on the behavior of L. monocytogenesin RTE foods.Alternatively, mathematical models can be used. These models predict the behavior of L. monocytogenesin foods as a function of their physicochemical characteristics (pH, aw, preservative concentration, etc.) and of the storage temperature. However, in my opinion, neither approach is entirely adequate, in the sense of being capable of providing unequivocal data regarding the behavior of L. monocytogenes in all types of RTE foods, under all possible circumstances. This is because the circumstances of challenge testing on a given lot of an RTE food product and the resulting data and interpretations may not be representative for all future product lots because of, for instance, small, yet significant differences in the product’s physicochemical characteristics (e.g. pH) between lots. Also, with some exceptions in recent years, most available mathematical models for the prediction of the behavior of L. monocytogenes in foods have been constructed based on data derived from experiments in laboratory media and not actual foods. Such predictive models often tend to overestimate the ability of L. monocytogenesto grow in RTE foods, because they do not account for variables such as the intrinsic microbial flora of foods or the effect of natural antimicrobial components present in some foods. The non-constant or unforeseeable temperature conditions during the transport and storage of RTE foods at both retail and consumer levels further hampers the ability to draw definite conclusions regarding the potential and/or extent of growth of L. monocytogenes during the shelf-life of RTE foods. Compliance of RTE foods with the Regulation’s criteria should not be considered as a discrete characteristic; expression of the L. monocytogenes safety criteria for RTE foods in probabilistic terms might be a more realistic alternative[6].

EU data

According to the latest report of the European Food Safety Authority (EFSA) [7], 26 EU member states had reported 1,601confirmed cases of human listeriosis, with an incidence of 0.35 cases per 100,000 individuals. The incidence was higher in those over 65 years old (1.21 cases per 100,000). The outcome of the disease was known for only 1,036 of these cases (181 were reported as diseased). RTE foods that were retailed(single samples) in non-compliance with the criteria of Regulation 2073 (≤ 100 CFU/g) were mainly fishery products (1%), RTE meat products other than fermented sausage (0.4%)and soft and semi-soft cheeses (0.2%).

In 2010 and 2011 a large baseline survey was conducted in the EU regarding the occurrence of L. monocytogenesin certain categories of RTE foods (smoked and gravid fish, soft and semi-soft cheeses and heat-treated meat products that receive additional handling between heat treatment and packaging). The results of this study are expected to be published by EFSA in 2013.


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  7. European Food Safety Authority, European Center for Disease Prevention and Control. The European Union Summary Report on Trends and Sources of Zoonoses, Zoonotic Agents and Food-borne Outbreaks in 2010.EFSA J2012;10:2597.

Apostolos S. Angelidis, Assistant Professor, Laboratory of Milk and Dairy Products, University of Thessaloniki