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Definition: Listeria from The Penguin Dictionary of Science

A genus of rod-shaped, motile ➤bacteria. It includes Listeria monocytogenes, whose name is derived from the appearance of large numbers of monocytes in the blood of infected organisms. One species causes the disease listeriosis, the clinical symptoms of which include a type of meningitis. Infections in the uterus can cause birth defects. The bacterium is highly resistant to both physical and chemical treatment. Potential sources of listeriosis include contaminated faeces and some foodstuffs such as soft cheese. Named after Joseph Lister (1827-1912).


Summary Article: Listeria from The SAGE Encyclopedia of Food Issues

Listeria refers to a group of bacteria of which one species, Listeria monocytogenes, is predominant in causing a serious and potentially fatal disease known as listeriosis. The bacterium poses particular problems to both the food industry and regulator because it is widespread, can grow in chilled foods, and is relatively resistant to control measures when compared with other non-spore-forming foodborne pathogens. Characteristically, it infects the young, old, pregnant, and immunocompromised and generally causes its most significant clinical outcomes (e.g., septicemia, meningitis, fetal death) in these populations.

Listeriosis as a Disease

There are two types of listeriosis infection. The most serious invasive form is rare in the general population, with an incidence (cases per 100,000 population per annum) usually less than 1. However, the consequences of infection can be severe and include (not exclusively) meningoencephalitis and septicemia in nonpregnant cases and fetal death in pregnant women. Around 20% to 30% of cases are fatal, with an estimated 255 listeriosis deaths occurring in the United States every year, and 90% require hospitalization. Most fatalities involve the elderly with predisposing conditions leading to lower immunity, fetuses, or newborn babies. Infection in healthy people is likely only when a very large number of cells is consumed, but infection in at-risk groups occurs when foods are contaminated at lower concentrations. However, the concentration may be very low; in an outbreak involving frankfurters, L. monocytogenes was present at <0.3/gram in the food. Modeling suggests that the subtype of L. monocytogenes consumed also affects the probability of disease occurring.

The other type of disease, noninvasive febrile gastroenteritis, presents as diarrhea, muscle pain, and headache. Outbreaks have been associated with the consumption of foods containing very high concentrations of the bacterium. As an example, in one outbreak, the implicated chocolate milk contained 109/milliliter.

The period between ingestion and the onset of disease is typically lengthy (range 3–70 days), making investigation difficult (e.g., foods consumed are forgotten, potentially implicated foods have been consumed).

Most cases are thought to be sporadic, with case–control studies identifying as risk factors foods that are similar to those causing outbreaks.

Pertinent Characteristics of the Organism and Its Transmission

L. monocytogenes is widespread in the environment, having been isolated from water, vegetation, silage, and household sites (e.g., refrigerators, toothbrushes). It is a pathogen of animals and can cause mastitis in dairy cows. It may be present on raw food ingredients, so controls must be in place to ensure that it does not survive processing to be present in ready-to-eat foods. Its ability to grow at low temperatures and form biofilms enables it to colonize food-processing plants, increasing the likelihood of cross-contamination of ready-to-eat foods. It can also grow in the absence of oxygen, so vacuum packaging does not prevent growth. Therefore, long-shelf-life refrigerated foods may pose a particular problem if contaminated prior to packaging, as L. monocytogenes may then grow with the potential to reach a high concentration.

The organism can tolerate high salt concentrations, growing in 10% to 11% NaCl (sodium chloride), and it has been suggested that reduced use of salt in foods may result in increased listeriosis. It can grow at pH values as low as 4.4, but only under otherwise optimal conditions.

Examples of foods that have been associated with transmitting the organism and causing outbreaks and incidents are given in Table 1. Most human cases are thought to be foodborne, but other transmission routes are known, including hospital-acquired infections not involving food (e.g., via contaminated mineral oil).

Table 1 Examples of Foods Implicated in Outbreaks and Incidents of Listeriosis

Food Groups

Specific Examples

Dairy products

Milk (including flavored milk), cheese (especially soft cheese), butter

Produce

Coleslaw, melon, fruit salad, salted mushrooms, celery

Seafood

Smoked trout, smoked mussels, imitation crab meat, shrimp

Red meat

Hot dogs, frankfurters, deli meats, pâté, pork rillettes, pork tongue products

Poultry

Ready-to-eat chicken and turkey products

Multicomponent foods

Rice salad, corn and tuna salad, sandwiches

Structured risk assessments have attempted to identify foods particularly at risk. One from the United States categorizes deli meats in the “very high risk” category (on a per annum basis). Pasteurized milk, some high-fat dairy products, and frankfurters cooked without heating were also included in the “high-risk” group.

Control

Control of foodborne pathogens is achieved in a generic sense by the use of the Hazard Analysis Critical Control Point approach. Compared with other non-spore-forming foodborne pathogens, L. monocytogenes is a relatively hardy organism. However, normal cooking temperatures are adequate for its destruction. A typical, but not universally applicable, heat treatment for a ready-to-eat food is 70 °C for 2 minutes. Organic acids such as acetic and lactic acids are effective when present at a sufficiently high concentration. Refrigeration at temperatures effective for the control of most other foodborne pathogens (i.e., 4–5 °C) does not prevent its growth, but can retard growth. Several biologically based controls have been proposed, with bacteriophages and bacteriocins (nisin) permitted for use in foods by some regulators. People in at-risk groups are targeted by health authorities with information advising against the consumption of risk foods.

Regulation

Modeling studies have concluded that most cases of disease are caused when foods containing more than 100 cells/gram are eaten, so many jurisdictions have numerical limits centered on this value. However, the picture is complicated by the ability of the bacterium to grow on some chilled foods. Hence, there is often consideration of the likelihood of growth occurring such that a food with a low level of contamination may become one with a higher level after storage. The Codex Alimentarius Commission advises governments that the criteria for the acceptability of ready-to-eat foods are a maximum of 100 cells/gram in foods on which the organism will not grow, and an absence in a 25-gram sample for foods on which growth can occur, in five 25-gram samples. Recalls of contaminated food can be significant; one frankfurter recall involved 35 million pounds of product.

See also Codex Alimentarius Commission; Food Safety; Food Safety Agencies; Foodborne Illness; Frozen Food; Hazard Analysis and Critical Control Point (HACCP)

Further Readings
  • Allerberger, F.; Wagner, M. (2010). Listeriosis: A resurgent foodborne pathogen. Clinical Microbiology and Infection, 16, 16-23. doi:10.1111/j.1469-0691.2009.03109.x.
  • Centers for Disease Control and Prevention. (n.d.). Listeria (Listeriosis). Retrieved from http://www.cdc.gov/listeria/.
  • Luber, P. (2011). The Codex Alimentarius guidelines on the application of general principles of food hygiene to the control of Listeria monocytogenes in foods. Food Control, 22, 1482-1483. doi:10.1016/j.foodcont.2010.07.013.
  • Swaminathan, B.; Cabanes, D.; Zhang, W.; Cossart, P. (2007). Listeria monocytogenes. In M. P. Doyle; L. R. Beuchat (Eds.), Food microbiology: Fundamentals and frontiers (pp. 457-492). ASM Press Washington, DC.
  • Swaminathan, B.; Gerner-Smidt, P. (2007). The epidemiology of human listeriosis. Microbes and Infection, 9, 1236-1243. doi:10.1016/j.micinf.2007.05.011.
  • Todar, K. (n.d.). Todar's online textbook of bacteriology: Listeria monocytogenes. Retrieved from http://textbookofbacteriology.net/Listeria.html.
  • U.S. Food and Drug Administration. (n.d.). Foodborne illness & contaminants. Retrieved from http://www.fda.gov/Food/FoodborneIllnessContaminants/default.htm.
  • J. Andrew Hudson
    Copyright © 2015 by SAGE Publications, Inc.

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