A genus of aerobic to microaerophilic, motile, peritrichous bacteria containing small, coccoid, Gram-positive rods; these organisms tend to produce chains of 3-5 cells and, in the rough state, elongated and filamentous forms. Cells 18-24 hours old may show a palisade arrangement with a few V or Y forms; the bacteria produce acid but no gas from glucose and are found in the feces of humans and other animals, on vegetation, and in silage and are parasitic on poikilothermic and warm-blooded animals, including humans. The type species is Listeria monocytogenes.

Listeria monocytogenes is a Gram-positive rod-shaped bacterium. It is the agent of listeriosis, a serious infection caused by eating food contaminated with the bacteria. Listeriosis has been recognized as an important public health problem in the United States. The disease affects primarily pregnant women, newborns, and adults with weakened immune systems.

Listeriosis is a serious disease for humans; the overt form of the disease has a mortality greater than 25 percent. The two main clinical manifestations are sepsis and meningitis. Meningitis is often complicated by encephalitis, a pathology that is unusual for bacterial infections.

Microscopically, Listeria species appear as small, Gram-positive rods, which are sometimes arranged in short chains. In direct smears they may be coccoid, so they can be mistaken for streptococci. Longer cells may resemble corynebacteria. Flagella are produced at room temperature but not at 37°C. Hemolytic activity on blood agar has been used as a marker to distinguish Listeria monocytogenes among other Listeria species, but it is not an absolutely definitive criterion. Further biochemical characterization may be necessary to distinguish between the different Listeria species.

Until about 1960, Listeria monocytogenes was thought to be associated almost exclusively with infections in animals, and less frequently in humans. However, in subsequent years, listeriaae, including the pathogenic species L. monocytogenes and L. ivanovii, began to be isolated from a variety of sources, and they are now recognized to be widely distributed in Nature. In addition to humans, at least 42 species of wild and domestic mammals and 17 avian species, including domestic and game fowl, can harbor listeriae. Listeria monocytogenes is reportedly carried in the intestinal tract of 5-10% of the human population without any apparent symptoms of disease. Listeriae have also been isolated from crustaceans, fish, oysters, ticks, and flies.

The term listeriosis encompasses a wide variety of disease symptoms that are similar in animals and humans. Listeria monocytogenes causes listeriosis in animals and humans; L. ivanovii causes the disease in animals only, mainly sheep. Encephalitis is the most common form of the disease in ruminant animals. In young animals, visceral or septicemic infections often occur. Intra-uterine infection of the fetus via the placenta frequently results in abortion in sheep and cattle.

The true incidence of listeriosis in humans is not known, because in the average healthy adult, infections are usually asymptomatic, or at most produce a mild influenza-like disease. Clinical features range from mild influenza-like symptoms to meningitis and/or meningoencephalitis. Illness is most likely to occur in pregnant women, neonates, the elderly and immunocompromised individuals, but apparently healthy individuals may also be affected. In the serious (overt) form of the disease, meningitis, frequently accompanied by septicemia, is the most commonly encountered disease manifestation. In pregnant women, however, even though the most usual symptom is a mild influenza-like illness without meningitis, infection of the fetus is extremely common and can lead to abortion, stillbirth, or delivery of an acutely ill infant.

In humans, overt listeriosis following infection with L. monocytogenes is usually sporadic, but outbreaks of epidemic proportions have occurred. In 1981, there was an outbreak that involved over 100 people in Canada. Thirty-four of the infections occurred in pregnant women, among whom there were nine stillbirths, 23 infants born infected, and two live healthy births. Among 77 non pregnant adults who developed overt disease, there was nearly 30% mortality. The source of the outbreak was coleslaw produced by a local manufacturer.

In 1985, in California, 142 people developed overt listeriosis. Of these, 93 cases were perinatal, and among the 49 cases that were in non pregnant individuals, 48 were immunocompromised. Thirty fetuses or newborn infants died and 18 adults died. The source of the bacteria was a certain brand of "pasteurized" soft cheese that apparently had gotten contaminated with non pasteurized (raw) milk during the manufacturing process.

In 2002, a multistate outbreak of Listeria monocytogenes infections with 46 culture-confirmed cases, seven deaths, and three stillbirths or miscarriages in eight states was linked to eating sliced turkey deli meat. One intact food product and 25 environmental samples from a poultry processing plant yielded L. monocytogenes. Two environmental isolates from floor drains were indistinguishable from that of outbreak patient isolates, suggesting that the plant might be the source of the outbreak.

A peculiar property of L. monocytogenes that affects its food-borne transmission is the ability to multiply at low temperatures. The bacteria may therefore grow and accumulate in contaminated food stored in the refrigerator. So it is not surprising that listeriosis is usually associated with ingestion of milk, meat or vegetable products that have been held at refrigeration temperatures for a long period of time.

About 2500 cases of listeriosis occur each year in the United States. The initial symptoms are often fever, muscle aches, and sometimes gastrointestinal symptoms such as nausea or diarrhea. The illness may be mild and ill persons sometimes describe their illness as flu-like. If infection spreads to the nervous system, symptoms such as headache, stiff neck, confusion, loss of balance, or convulsions can occur. Most cases of listeriosis and most deaths occur in adults with weakened immune systems, the elderly, pregnant women, and newborns. However, infections can occur occasionally in otherwise healthy persons. Infections during pregnancy can lead to miscarriages, stillbirths, and infection of newborn infants. Outbreaks of listeriosis have been linked to a variety of foods especially processed meats (such as hot dogs, deli meats, and paté) and dairy products made from unpasteurized milk.


A person with listeriosis (listeria infection) has fever, muscle aches, and sometimes gastrointestinal symptoms such as nausea or diarrhea. If infection spreads to the nervous system, symptoms such as headache, stiff neck, confusion, loss of balance, or convulsions can occur.

Infected pregnant women may experience only a mild, flu-like illness; however, infections during pregnancy can lead to miscarriage or stillbirth, premature delivery, or infection of the newborn.

Causes and Risk factors:

Cause of Listeriosis: You get listeriosis by eating food contaminated with Listeria. Babies can be born with listeriosis if their mothers eat contaminated food during pregnancy. Although healthy persons may consume contaminated foods without becoming ill, those at increased risk for infection can probably get listeriosis after eating food contaminated with even a few bacteria. Persons at risk can prevent Listeria infection by avoiding certain high-risk foods and by handling food properly.

Contaminated food: Rare cases of nosocomial transmission have been reported.

Cause details for Listeriosis: Two food-borne bacteria, Listeria monocytogenes and Yersinia enterocolitica can actually grow at refrigerator temperatures. High salt, high sugar or high acid levels keep bacteria from growing, which is why salted meats, jam, and pickled vegetables are traditional preserved foods.

Listeria monocytogenes is found in soil and water. Vegetables can become contaminated from the soil or from manure used as fertilizer.

Listeria is found in soil and water. Vegetables can become contaminated from the soil or from manure used as fertilizer. Animals can carry the bacterium without appearing ill and can contaminate foods of animal origin such as meats and dairy products. The bacterium has been found in a variety of raw foods, such as uncooked meats and vegetables, as well as in processed foods that become contaminated after processing, such as soft cheeses and cold cuts at the deli counter. Unpasteurized (raw) milk or foods made from unpasteurized milk may contain the bacterium.

Risk factors:

    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.


Listeriosis can be diagnosed with a blood test. Further testing is only required if it is thought the infection has spread to your nervous system. These tests may include:

A computerized tomography (CT) scan - a series of X-rays pieced together to produce cross sectional images.

A magnetic resonance imaging (MRI) scan - which uses a strong magnetic field and radio waves to produce detailed images of the inside of your body.

A lumbar puncture: A lumbar puncture (also known as a spinal tap) is used to test a sample of fluid from the spinal cord. The fluid is called cerebrospinal fluid (CSF) and surrounds the brain and spinal cord. It helps support and protect the brain and spinal cord.

A hollow needle is inserted into the lower part of the spinal canal to draw out a sample of CSF. If the infection has spread to the nervous system, listeria will be present in the CSF.


If diagnosed early enough, antibiotic treatment of pregnant women or immunocompromised individuals can prevent serious consequences of the disease. Antibiotics effective against Listeria species include ampicillin, vancomycin, ciprofloxacin, linezolid and azithromycin. However, early diagnosis is the exception rather than the rule, since the first signs of a case or an outbreak are reports of stillbirth or serious infections resembling listeriosis. By then, any cohorts who have become infected from eating the same food are likely recovered from an inapparent or flu-type infection, or they themselves may have developed serious disease. However, processed foods known to be the source of Listeria that may still be in the market place, restaurant or home should obviously not be used, and recalls should be imperative. It must also be constantly recognized that L. monocytogenes is able to grow at low temperatures.

Protect yourself from listeriosis by following these food safety tips:

    Avoid eating soft cheeses (e.g., Brie, Camembert) or refrigerated meat spreads or pâtés.

  • Avoid eating unpasteurized dairy products.

  • Clean utensils and working area used to prepare food with a diluted bleach solution. To prepare the solution, mix 1 teaspoon (5 mL) of bleach with 3 cups (750 mL) of water.

  • Defrost food in cold water, the microwave, or the refrigerator, but never at room temperature.

  • Keep raw meat separate from other foods in your shopping cart while at the store, in the refrigerator, and during preparation. This will help prevent cross contamination.

  • Regularly clean and disinfect your refrigerator. Listeria can survive and continue to grow even when in the refrigerator. Cleaning the refrigerator more frequently will reduce the risk of transferring Listeria from contaminated food and surfaces to other foods.

  • Thoroughly cook meat before serving. Thoroughly cooking food can kill Listeria, although it does not guarantee that it is safe to eat if proper food handling processes were not followed (e.g., the cooked food is cross-contaminated by infected, raw food). Wash all raw fruits and vegetables before eating.

  • Wash your hands frequently, especially before and after handling food.

Medicine and medications:

The following drugs and medications are in some way related to, or used in the treatment. This service should be used as a supplement to, and NOT a substitute for, the expertise, skill, knowledge and judgment of healthcare practitioners.

The therapy of choice is penicillin G 75,000 to 100,000 U/kg IV every 4 hours, continuing for 10 to 14 days after the temperature has returned to normal. This may often take a total of 4 or even 6 weeks. Ampicillin 12 gram/day IV is a good alternative. Cephalosporins are not effective and it is best to avoid chloramphenicol. If there is endocarditis, ampicillin or penicillin G should be combined with tobramycin or gentamicin. Cotrimoxazole or erythromycin can be used if there is an allergy to penicillin and in oculoglandular and dermal listeriosis. Despite aggressive therapy, granulomatosis infantisepticum has a high mortality rate.

Ampicillin in combination with an aminoglycoside such as gentamicin is the therapy of choice. Listeria is not susceptible to cephalosporins of any generation. Therefore, cephalosporins should not be used to treat Listeria infections.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.


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