Sources, Characteristics, and Identification

Listeria is the common name for the pathogenic or disease-causing bacterium known as Listeria monocytogenes. It is a foodborne bacterium that, when ingested, causes an infection known as listeriosis.[1] Approximately 2,500 illnesses and 500 deaths are attributed to listeriosis in the United States annually.[2]

Listeria is ubiquitous in the environment, and can be isolated from wild and domestic animals, birds, insects, soil, wastewater, and vegetation. The bacterium easily comes into contact with farm animals, as it has been found to be present in grazing areas, stale water, and poorly prepared animal feed. In addition to being present in the environment, Listeria can live in the intestines of humans, animals, and birds for long periods of time without causing infection. Because Listeria is present in nearly every environment numerous opportunities for contamination exist during the food production process.[3]

Listeria monocytogenes Bacteria

Listeria prevention—particularly in food processing facilities—is a necessary and obvious concern. The food-processing environment is vulnerable to L. monocytogenes entry from a number of sources, including infected employees and contaminated raw materials. This vulnerability persists despite adherence to current good manufacturing practices (GMPs), sanitation standard operating procedures (SSOPs), and Hazard Analysis and Critical Control Point (HACCP) process controls.[4] Control is complicated by the bacterium’s ability to survive and grow under conditions not generally tolerated by similar organisms. To this end, food-processing facilities must be designed properly and must follow sanitary procedures designed to prevent Listeria contamination; the failure to do so is careless in the face of such a pervasive threat to food safety.

Since 1985, FDA has maintained a “zero tolerance” policy for L. monocytogenes in ready-to-eat (RTE) foods, which are foods that may be consumed without further preparation by the consumer. FDA considers RTE foods to be adulterated under section 402(a) of the Food, Drug, and Cosmetics Act if any L. monocytogenes is detected in either of two 25-gram samples. Since 1989, FSIS has maintained a similar “zero-tolerance” policy for RTE meat or poultry products. Meat or poultry products in RTE form in which any L. monocytogenes is detected are deemed adulterated under the Federal Meat Inspection Act and the Poultry Products Inspection Act, 21 U.S.C. §§ 601(m) and 453(g), respectively. The regulatory status of non-RTE products that contain L. monocytogenes is determined on a case-by-case basis, but such products may be subject to “zero tolerance” as well.

Healthcare providers frequently overlook Listeria as a possible cause of illness because of its unusual growth capabilities. First, laboratories sometimes have a difficult time growing Listeria. When it is grown, Listeria can be confused with other less harmful contaminants and disregarded. Second, while most bacteria grow poorly when temperatures fall below 40°F, Listeria survives at temperatures from below freezing to body temperature and grows best at the 0°F to 50°F range, which includes the temperature range used for freezing and refrigeration.

As a result of Listeria’s unusual growth capabilities, the pathogen may be transferred in common ready-to-eat foods that have been kept properly refrigerated. Thus, Listeria presents many challenges because of its ability to grow in diverse environments. These host factors, along with the number of bacteria ingested and the virulence of the strain, determine the risk of disease.

Who is most susceptible to Listeria monocytogenes infection?

Several segments of the population, including pregnant women, persons with compromised immune systems, and the elderly are particularly at risk for listeriosis. The body’s defense against Listeria and other intracellular pathogens is called “cell-mediated immunity” because it depends on the body’s cells (as opposed to antibodies), especially lymphocytes called “T-cells.” Therefore, it is not surprising that individuals whose cell-mediated immunity is suppressed are more susceptible to the devastating effects of listeriosis.

Pregnant women naturally have a suppressed cell-mediated immune system; many thinks that this suppression occurs so that the mother’s immune system will not reject the fetus. In addition, the immune systems of fetuses and newborns are very immature and thus are extremely susceptible to intracellular pathogens. Other adults, especially transplant recipients[5] and lymphoma patients, are given necessary therapies with the specific intent of depressing immune T-cells, and, as a result, these individuals become especially susceptible to Listeria as well. In sum, the kinds of individuals at increased risk for listeriosis include:

  • Pregnant women: They are about 20 times more likely than other healthy adults to get listeriosis. About one-third of listeriosis cases happen during pregnancy.
  • Newborns: Newborns rather than the pregnant women themselves suffer the serious effects of infection in pregnancy.
  • Persons with weakened immune systems.
  • Persons with cancer, diabetes, or kidney disease.
  • Persons with AIDS: They are almost 300 times more likely to get listeriosis than people with normal immune systems.
  • Persons who take glucocorticosteroid medications (such as cortisone).
  • The elderly.

Symptoms of Listeriosis

It is believed that the ingestion of fewer than one thousand Listeria bacteria can cause human illness. The incubation period—i.e., the time between ingestion of bacteria and the onset of symptoms—for Listeria infection ranges from three to 70 days, and averages 21 days.[6]

A person with listeriosis usually experiences fever, muscle aches, and gastrointestinal symptoms such as nausea or diarrhea. Five days to three weeks after ingestion, Listeria bacteria can invade all body areas, including the central nervous system, heart, and eyes.[7] If the infection spreads to the nervous system, symptoms such as headache, stiff neck, loss of balance, confusion, reduced alertness (obtundation), and convulsions can occur. With brain involvement, listeriosis sometimes mimics a stroke.

Listeria remains an important threat to public health, especially among those most susceptible to this disease. With an increasing immune-compromised population, the risk multiplies. The fact that Listeria is a disease easily transmitted from mother to fetus through the placenta is worrisome to an expectant mother, especially since pregnant women themselves rarely show outward signs of the devastating infection. Even with prompt treatment, some infections result in death. This is particularly likely in the elderly and in persons with other serious medical problems.

Additional Resources is a comprehensive site with in-depth information about Listeria bacteria and listeriosis.

Listeria Blog provides up-to-date news related to Listeria outbreaks, research, and more.

Listeria Information Video YouTube video outlining what you need to know during a Listeria outbreak.

Listeria Food Safety Guide downloadable Listeria food safety guide.


[1]           Cossart P, Bierne H. (2001). The use of host cell machinery in the pathogenesis of Listeria monocytogenes. Curr Opin Immunol (England). 13(1):96-103.

[2]           CDC. (2005). Listeriosis Technical Information. Retrieved on March 19, 2009 from Centers for Disease Control and Prevention. Website:

[3]           Cossart P, Bierne H. (2001). The use of host cell machinery in the pathogenesis of Listeria monocytogenes. Curr Opin Immunol (England). 13(1):96-103.

[4]           Tompkin, R.B. 2002. Control of Listeria monocytogenes in the food-processing environment. J. Food Prot. 65:709-725.

[5]           Schuchat A, Deaver KA, Wenger JD, Plikaytis BD, Mascola L, Pinner RW, Reingold AL, Broome CV. (1992). Role of foods in sporadic listeriosis. I. Case-control study of dietary risk factors. JAMA. 267(15):2041-5.

[6]           Bryan, FL. (1999). Procedures to Investigate Foodborne Illness Fifth Edition (pp. 119). Des Moines, IA:  International Association for Food Protection.

[7]           FDA/CFSAN. (2003).  Foodborne Pathogenic Microorganisms and natural Toxins Handbook: The ‘Bad Bug Book.’ College Park, MD:  Center for Food Safety and Applied Nutrition, Food and Drug Administration. Retrieved on January 11, 2008 from FDA/CFSAN. Website:

Marler Clark, The Food Safety Law Firm, is the nation’s leading law firm representing victims of Listeria outbreaks. The Listeria lawyers of Marler Clark have represented thousands of victims of Listeria and other foodborne illness outbreaks and have recovered over $650 million for clients.  Marler Clark is the only law firm in the nation with a practice focused exclusively on foodborne illness litigation.  Our Listeria lawyers have litigated Listeria cases stemming from outbreaks traced to a variety of foods, such as cantaloupe, deli meats, cheese, celery and milk.

If you or a family member became ill with a Listeria infection after consuming food and you’re interested in pursuing a legal claim, contact the Marler Clark Listeria attorneys for a free case evaluation.