Listeria monocytogenes is a ubiquitous microorganism. It is found in the intestine of animals and people that act as carriers and, also, widely distributed in natural environments such as soil, water, effluents, pastures and silages where they survive for extended periods of time. It is also found in the soil, walls, ceilings, and equipment of food processing plants and has been isolated from a variety of ready-to-eat (RTE) foods of vegetable, dairy, marine or meat origin and in salads and fruits. Ready-to-eat (RTE) foods are those prepared for direct consumption without the need for cooking or other culinary treatments that reduce the presence of worrisome microorganisms to an acceptable level.
What Are The Food Sources Of Listeria?
Therefore, foods potentially contaminated with Listeria are:
- The Meat of Ruminants, Other Food, and Poultry: Although the prevalence of listeria in meat differs significantly, some studies have cited prevalence rates of up to 92% of the analyzed products. A large number of animal’s supplies (11-52%) are considered healthy carriers. In abattoirs, the presence of listeria can be endemic, particularly in effluents and soils.
- Milk and Dairy products: Listeria has been described in milk and dairy products, as well as in pasteurized products and fresh derivatives, probably due to a recontamination after its pasteurization. In the case of cheeses made with pasteurized or non-pasteurized milk, the absence of listeria depends on intrinsic parameters (acidity, antimicrobial peptides, etc.) and extrinsic (temperature, humidity, recontamination, etc.), while its presence is always problematic due to its extraordinary capacity for survival in hostile conditions.
- Fresh, Frozen and Smoked Fish: The presence and survival of listeria in fresh and frozen fish are unlikely, while in smoked fish it is variable depending on the used smoking method (cold or hot), but in any case, it is always worrying.
- Fruits and Vegetables: It is known that up to 10-20% of ready-to-eat vegetable products can be contaminated with listeria including sprouted seeds, lettuce, radishes, tomatoes, onions, cucumbers, cultivated cauliflowers and mushrooms and that some outbreaks of listeriosis have been due to the consumption of this type of products. Of special concern are the organic products fertilized with manure possibly contaminated with listeria from sick animals.
- Eggs and Egg Derivatives: The contamination of eggs and egg derivatives with listeria is very rare.
- Food Ready for Consumption (RTE): The first outbreaks of listeriosis associated with the consumption of vegetables and fresh cheeses at the beginning of the 1980´s not only led to the recognition of listeria as a microorganism, responsible for food poisoning but also the role of some foods in the transmission of the disease. Therefore, the foods most commonly associated with outbreaks of listeriosis are: Ready-to-eat (RTE) of vegetables, dairy products, meat and smoked fish origin, foods that are kept for a long time in refrigeration, food not subjected to bactericide treatments during storage and processing, food with a risk of contamination after processing, foods whose ingredients facilitate the development of listeria and food consumed by immunocompromised individuals and pregnant women. In the case of chopped meat derivatives and emulsified ones with a lot of preparation and handling, the possible contamination with listeria is very high. In fermented meat derivatives, possibly, the presence of listeria is lower although its survival is possible.
What Is The Food Industry Doing For Control?
Governments are encouraging the food industry (production, manufacturing, processing, food services, and warehouses) to: “make every effort to commercialize Listeria monocytogenes-free food”. This may require the industry to modify the means and practices of operation (such as a more severe thermal treatment, increase the frequency, thoroughness of cleaning, sanitization of the equipment and the environment, and improve the temperature controls).
Listeria monocytogenes in foods cannot be seen, savored, or smelled. But common sense and simple precautions, which apply to any disease of food origin, must be taken by the consumer. Good personal hygiene, storage, cooking, and proper methods of serving, when applied at home, stores and food service environments can reduce the risk of problems with this bacterium.
- Centers for Disease Control and Prevention. (2021). Listeria (Listeriosis). https://www.cdc.gov/listeria/index.html
- European Food Safety Authority. (2019). Scientific opinion on the maintenance of the list of QPS biological agents intentionally added to food and feed. https://www.efsa.europa.eu/en/efsajournal/pub/5555
- de Oliveira, M. M., et al. (2019). Listeria monocytogenes: Incidence in food industries, detection methods and preventive measures. Food Control, 95, 196-208.
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