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Food poisoning by Listeria monocytogenes – Listeriosis

What is Listeriosis?

Listeriosis, also known as listeria infection, is a foodborne illness caused by the bacterium Listeria monocytogenes. It is a significant cause of foodborne illnesses worldwide. Here are some key points about listeriosis:

  1. Causative Agent: Listeriosis is primarily caused by the bacteria Listeria monocytogenes, which is commonly found in soil, water, and the gastrointestinal tracts of animals. It can contaminate various foods, including raw and processed meats, dairy products, and produce.
  2. Symptoms: After consuming contaminated food, individuals may experience mild flu-like symptoms, such as fever, muscle aches, and gastrointestinal problems. However, in highly susceptible individuals, such as pregnant women, immune-compromised patients, and the elderly, listeriosis can lead to severe systemic (invasive) infections.
  3. Risk Groups: Pregnant women are particularly vulnerable to listeriosis because the bacteria can cross the placenta and cause severe complications, including spontaneous abortion, stillbirth, or serious illness in newborns. Immune-compromised individuals and the elderly are also at higher risk of developing severe listeriosis.
  4. Pathogenicity: Listeria monocytogenes can cause sepsis (blood infection) and meningoencephalitis (inflammation of the brain and its protective membranes) in both animals and humans. It can invade various tissues and organs, leading to life-threatening complications.
  5. Other Listeria Species: While L. monocytogenes is the most concerning human pathogen, there are other species of Listeria, such as L. ivanovii, that primarily affect animals. L. innocua, L. weishimeri, L. seeligeri, and L. grayi are considered non-pathogenic to humans.
  6. Food Production Risk: Listeria monocytogenes poses a significant threat in food production facilities, as it can contaminate a wide range of food products. Ready-to-eat foods, such as deli meats, soft cheeses, and smoked fish, are particularly susceptible to contamination. The bacterium can survive and multiply even under refrigeration temperatures.
  7. Quality Control: Due to the risks associated with Listeria monocytogenes, it is crucial for risk and quality control managers in food production facilities to prioritize constant vigilance and implement stringent controls throughout the production process. This includes careful monitoring and control of raw materials, production equipment, hygiene practices, packaging, and storage conditions.
  8. Prevention: Preventing listeriosis requires strict adherence to food safety protocols. This includes maintaining good hygiene practices in food handling and preparation, thorough cooking of foods, proper refrigeration and storage, and regular cleaning and sanitization of food contact surfaces.
  9. Regulatory Measures: Food safety regulatory agencies in many countries have established guidelines and regulations to control the presence of Listeria monocytogenes in food products. These regulations may include testing requirements, product recalls, and enforcement of strict hygiene practices in food establishments.
  10. Awareness and Education: Raising awareness among consumers about the risks of listeriosis and providing education on safe food handling practices, especially for high-risk individuals, is crucial in preventing infections and reducing the impact of this foodborne illness.

It is important to note that the information provided here is a general overview of listeriosis. For specific guidance and advice, individuals should consult their healthcare professionals or refer to local food safety authorities and guidelines.

Source of contamination

The source of contamination for Listeria monocytogenes, a gram-positive bacterium that causes listeriosis, can be attributed to various factors. Here are some key points about the sources of contamination:

  1. Environmental Reservoir: Listeria monocytogenes is commonly found in the environment, particularly in soil, water, sewage, and decaying plant matter. As saprophytic bacteria, they can survive and proliferate in these environments.
  2. Salt Tolerance and pH Range: Listeria monocytogenes exhibits tolerance to high salt concentrations, enabling its survival in environments with up to 10% salt content. Additionally, these bacteria can thrive in a broad pH range of 4.1 to 9.6, allowing them to adapt to various conditions.
  3. Psychrophilic Nature: Listeria monocytogenes is psychrophilic, meaning it can grow at refrigeration temperatures (1-45°C). This characteristic poses a challenge for the control of Listeria contamination in refrigerated food products.
  4. High-Risk Food Products: Ready-to-eat foods are considered high-risk products for Listeria monocytogenes contamination. These include items such as hotdogs, smoked fish, raw seafood, sausages, pre-prepared or pre-packaged fruits and vegetables, deli meats, and unpasteurized milk and milk products.
  5. Potential Source of Contamination: The aforementioned high-risk food products have been linked to numerous Listeria monocytogenes outbreaks. Contamination can occur during various stages, including production, processing, packaging, and distribution. Improper handling, inadequate sanitation practices, and cross-contamination can introduce the bacteria into these food products.

Antigenic structure and virulence factors

The antigenic structure and virulence factors of Listeria monocytogenes play a significant role in its pathogenicity and ability to cause infections. Here is some information regarding its antigenic structure and virulence factors:

  1. Antigenic Structure: Listeria monocytogenes possesses antigens that help classify and identify different strains of the bacterium. It has 13 known distinct serotypes of somatic (O) antigens and 5 serovars of flagellar (H) antigens. However, the serotypes 1/2a, 1/2b, and 4b are responsible for the majority (98%) of human and animal outbreaks caused by Listeria monocytogenes.
  2. Survival in Harsh Conditions: Listeria monocytogenes exhibits the ability to survive in harsh environmental conditions. It can tolerate thermal variations, including both high and low temperatures. The bacterium can also withstand acidic pH levels, enabling it to survive in the stomach’s acidic environment. Additionally, Listeria monocytogenes can resist the effects of antimicrobial agents.
  3. Virulence Factors: Listeria monocytogenes possesses various virulence factors that contribute to its pathogenicity. One important virulence factor is the sigma factor B (sigB gene), which is responsible for the formation of virulence genes. These genes help the bacterium adapt and survive in different environments, including within host cells.
  4. Invasion and Intracellular Replication: Listeria monocytogenes has the ability to invade and replicate within host cells. It can survive the acidic pH of the stomach and reach the intestinal cells by entering phagocytic vacuoles. The bacterial surface protein called internalin (Inl) plays a crucial role in attaching to and entering host cells. Once inside, Listeria monocytogenes promotes intracellular replication, allowing the bacterium to evade the immune system.
  5. Gene Regulation: The expression of virulence genes in Listeria monocytogenes is tightly regulated by various regulatory genes. These genes control the timing and level of gene expression, allowing the bacterium to modulate its virulence factors as needed during infection.

Epidemiology of Listeriosis

The epidemiology of listeriosis, referring to the study of its occurrence and distribution, reveals important insights into the impact and patterns of this disease. Here is some information on the epidemiology of listeriosis:

  1. High Mortality Rate: Listeria monocytogenes infection, particularly in vulnerable populations, such as newborn infants and immunocompromised patients, can lead to severe illness and high mortality rates. Newborns are particularly susceptible to listeriosis, and the mortality rate in infected infants is notably high.
  2. First Reported Outbreak: The first documented outbreak of listeriosis was reported in Canada, where individuals consumed cabbage contaminated by Listeria from infected sheep manure. This outbreak helped highlight the association between contaminated food and listeriosis.
  3. Impact on Pregnant Women and Immunocompromised Patients: Pregnant women and immunocompromised individuals are at higher risk of listeriosis. This population is more susceptible to severe forms of the disease, and it has caused significant morbidity and mortality rates in the United States and European countries.
  4. Disease Burden: Each year in the United States, approximately 2,500 serious illnesses and 500 deaths are reported due to listeriosis. This highlights the significance of the disease as a public health concern and emphasizes the need for prevention and control measures.
  5. Recent Outbreak in South Africa: A notable outbreak of listeriosis occurred in South Africa in early 2017. The outbreak was linked to the consumption of ready-to-eat (RTE) meat products contaminated with Listeria monocytogenes. The outbreak resulted in a fatality rate of 27%, with 1,060 confirmed cases and 216 deaths.
  6. Association with Production Process: Many outbreaks of listeriosis have been linked to issues in the production process of food products. This emphasizes the importance of conducting thorough risk assessments and implementing appropriate control measures to identify and mitigate potential routes of contamination.

Clinical signs and symptoms of Listeriosis

Listeriosis, caused by the bacterium Listeria monocytogenes, can present with various clinical signs and symptoms. Here is some information on the clinical signs and symptoms associated with listeriosis:

  1. Fetomaternal and Neonatal Listeriosis: Listeriosis in pregnant women can lead to serious complications in both the mother and the fetus. It may result in abortion or the birth of a baby with a condition called granulomatosis infantiseptica, which has a high mortality rate. Pyrogranulomatous microabscesses may be present throughout the body in affected individuals.
  2. Symptoms in Pregnant Women: In pregnant women, symptoms of listeriosis are often mild or asymptomatic. If symptoms do occur, they can resemble a mild flu, including headache, fever, chills, and muscle pain. These symptoms typically last for one to two weeks.
  3. Neonatal Listeriosis: Neonates born to mothers with listeriosis can develop various symptoms, including meningitis, gastroenteritis, and in some cases, pneumonia.
  4. Symptoms in Adults: In adults, listeriosis can manifest as severe changes in consciousness, movement disorders, and paralysis of the brain. Common symptoms may include headache, vomiting, malaise, visual disorders, and rhombencephalitis (inflammation of the hindbrain).
  5. Atypical Clinical Signs: Listeriosis can present with atypical clinical signs in different individuals. These may include endocarditis (infection of the heart lining), myocarditis (inflammation of the heart muscle), arthritis, pneumonia, hepatitis (inflammation of the liver), sinusitis, conjunctivitis, and mastitis (inflammation of the mammary glands) in cattle.

Mechanism of Pathogenesis

The mechanism of pathogenesis of Listeria monocytogenes involves several steps and virulence factors. Here is some information on the mechanism of pathogenesis:

  1. Ingestion and Entry: Contaminated food is the major source of Listeria monocytogenes infection. When ingested, the bacteria reach the gastrointestinal tract and initiate the primary entry of pathogens. The incubation period for gastroenteritis is approximately 20 hours after consuming heavily contaminated food, while systemic illness has a longer incubation period of around 20 to 30 days.
  2. Attachment and Colonization: The attachment of Listeria monocytogenes to the intestinal cell wall is mediated by a surface protein called InlA. Once attached, the bacteria colonize the intestinal wall and rapidly translocate through the mucosal barrier to the bloodstream and lymphatic system.
  3. Survival in the Gastrointestinal Tract: Listeria monocytogenes employs several mechanisms to survive in the gastrointestinal tract. The glutamate decarboxylase (GAD) system protects the bacterium from stomach acid, and bile salt hydrolase (BSH) and the bile exclusion system (BilE) provide protection against bile salts. This allows the bacteria to bypass these barriers and penetrate the intestinal epithelial barrier.
  4. Dissemination: Listeria monocytogenes can enter host cells and use them as a means of dissemination. Helper T-cells (CD4+) or cytotoxic T-cells (CD8+) transport the bacteria to systemic sites such as the liver, spleen, lymph nodes, and the placenta in pregnant women.
  5. Intracellular Survival and Escape: Once inside host cells, Listeria monocytogenes is engulfed by a phagosome. However, the bacterium escapes this compartment with the help of virulence factors such as listeriolysin O (LLO) and phospholipase. These enzymes form pores in the phagosome membrane, leading to the lysis of the host cell and the release of intracellular enzymes and DNA.
  6. Tissue Infection and Spread: During infection, Listeria monocytogenes primarily infects the liver (90%) and spleen (10%). In the liver, neutrophils, Kupffer cells, and macrophages control the infection by targeting infected hepatocytes and forming discrete microabscesses. The Actin polymerization protein (ActA) aids bacterial movement within the cytoplasm and facilitates cell-to-cell spread, contributing to systemic listeriosis.
  7. Placental Infection and Fetal Complications: Listeria monocytogenes can colonize the uterus, leading to placental infection. Hematogenous penetration of the placental barrier allows the bacteria to access the fetus, resulting in placental infection characterized by microabscesses and focal necrotizing villitis. This can lead to abortion, premature childbirth, or death of the fetus.

Laboratory Diagnosis of Listeria monocytogenes

Laboratory diagnosis of Listeria monocytogenes involves various techniques that aid in the detection and identification of the bacterium. Here is some information on the laboratory diagnosis of Listeria monocytogenes:

  1. Serology Testing: Serology testing involves the detection of antibodies against Listeria monocytogenes in patient serum. The Widal test is a classical agglutination reaction that yields results based on O and H antigens. Listeriolysin O (LLO) antibodies can also be used for diagnosis. In this method, LLO antibodies are serially diluted and incorporated into nitrocellulose fibers. The absorbed anti-LLO is then detected using an enzyme-labeled anti-human IgG. Although this test has a sensitivity and specificity of around 90%, it is not commercially available.
  2. Organism Cultivation: Listeria monocytogenes can be cultured from various clinical samples, including venous blood, urine, placenta, amniotic fluid, and biopsy tissues. Gram staining is also performed on sterile specimens such as cerebrospinal fluid and tissue smears. It is worth noting that Listeria monocytogenes can sometimes be mistaken for pneumococci or corynebacterium during diagnosis.
  3. Molecular Detection: Polymerase Chain Reaction (PCR)-based methods are widely used for the rapid and sensitive detection of Listeria monocytogenes. Clinical samples, such as cerebrospinal fluid, can be used for PCR amplification of specific target genes. Additionally, the 16S ribosomal RNA gene sequence of Listeria can be targeted for the screening of Listeria in food samples.

Laboratory diagnosis plays a crucial role in confirming the presence of Listeria monocytogenes in clinical samples and food products. The choice of diagnostic method depends on the type of sample being tested and the resources available in the laboratory. It is important to follow appropriate guidelines and quality control measures to ensure accurate and reliable results.

Treatment Listeriosis

The treatment of listeriosis depends on the severity of the infection. Here is some information on the treatment options for listeriosis:

  1. Mild Infection: In cases of mild listeriosis with minor symptoms, specific medication may not be required. Supportive care, such as rest and adequate hydration, can help the body fight off the infection.
  2. Severe Infection: If the infection is serious or if it affects vulnerable individuals such as pregnant women, newborns, or immunocompromised patients, prompt antibiotic therapy is essential. Antibiotics are effective in treating Listeria monocytogenes infections.
  3. Choice of Antibiotics: The antibiotics commonly used to treat listeriosis include ampicillin, amoxicillin, penicillin G, and gentamicin. These antibiotics are often used in combination to enhance effectiveness and cover a broad spectrum of bacterial strains.
  4. Intravenous Antibiotics: In cases of severe listeriosis, such as meningitis or septicemia, intravenous administration of antibiotics is necessary. Intravenous antibiotics allow for high drug concentrations in the bloodstream and central nervous system, providing optimal treatment for these invasive infections.
  5. Duration of Treatment: The duration of antibiotic treatment for listeriosis is typically prolonged compared to other bacterial infections. In cases of meningitis or septicemia, treatment should be continued for at least 6 weeks to ensure complete eradication of the bacteria and prevent relapse.
  6. Fluid Therapy: In cases of listeriosis-associated gastroenteritis, fluid therapy may be required to restore hydration and balance electrolytes in the body. Gastroenteritis can cause fluid loss due to vomiting and diarrhea, and replacing fluids is crucial for maintaining adequate hydration.

Prevention and Control Listeriosis

Prevention and control measures are essential in reducing the risk of listeriosis. Here is some information on the prevention and control of listeriosis:

  1. Proper Heat Treatment and Pasteurization: Proper heat treatment and pasteurization of processed foods before consumption is crucial in preventing listeriosis. Adequate cooking temperatures ensure that any Listeria monocytogenes present in the food are killed, reducing the risk of infection.
  2. Thorough Cooking of Meat and Seafood: Thoroughly cooking refrigerated meat and seafood is important to eliminate any potential Listeria contamination. Cooking temperatures should reach a level that kills the bacteria and ensures food safety.
  3. Farm and Processing Plant Cleanliness: Maintaining cleanliness and hygiene in farms, including proper handling of animal feed and silage, helps prevent Listeria contamination in livestock. Similarly, maintaining hygiene in processing plants is crucial to prevent cross-contamination and the introduction of Listeria monocytogenes into food products.
  4. Sanitation and Routine Surveillance: Implementation of proper sanitation practices in processing plants is vital for preventing Listeria contamination. Regular cleaning and sanitization of equipment, surfaces, and utensils reduce the risk of bacterial growth and transmission. Routine surveillance of production facilities can help identify potential sources of contamination and implement corrective actions promptly.
  5. High-Risk Food Awareness: Pregnant women and individuals who are susceptible to listeriosis should be aware of high-risk foods and take precautions. Avoiding consumption of certain foods, such as salads, meats, and soft cheeses from deli counters, can help reduce the risk of Listeria infection.
  6. Food Safety Education: Educating the general public, food handlers, and healthcare professionals about the risks associated with listeriosis and the importance of proper food safety practices is crucial. This includes understanding safe food handling, storage, and preparation techniques to minimize the risk of Listeria contamination.

Prevention and control of listeriosis require a comprehensive approach involving all stages of food production, from farm to table. By implementing proper hygiene practices, following recommended cooking and processing guidelines, and promoting awareness among the public, the incidence of listeriosis can be significantly reduced. It is important to stay updated on food safety guidelines and regulations provided by local health authorities to ensure effective prevention and control measures are in place.


What is listeriosis?

Listeriosis is a foodborne illness caused by the bacterium Listeria monocytogenes. It can lead to severe infections, particularly in vulnerable populations such as pregnant women, newborns, the elderly, and individuals with weakened immune systems.

How does Listeria monocytogenes contaminate food?

Listeria monocytogenes can contaminate food through various sources, including contaminated raw materials, cross-contamination during food processing, improper storage and handling, and unhygienic conditions in food production facilities.

What are the common sources of Listeria monocytogenes contamination?

Ready-to-eat foods such as deli meats, smoked fish, soft cheeses, raw seafood, pre-packaged fruits and vegetables, and unpasteurized milk and dairy products are commonly associated with Listeria monocytogenes contamination.

What are the symptoms of listeriosis?

The symptoms of listeriosis can vary, but commonly include fever, muscle aches, gastrointestinal problems (such as nausea and diarrhea), and flu-like symptoms. In severe cases, it can lead to sepsis, meningitis, and other serious complications.

How long does it take for symptoms to appear after consuming contaminated food?

The incubation period for listeriosis can range from a few days to several weeks. It depends on factors such as the individual’s immune system, the amount of bacteria ingested, and the strain of Listeria monocytogenes.

Who is most at risk of developing severe listeriosis?

Pregnant women, newborns, the elderly, and individuals with weakened immune systems (such as those with HIV/AIDS or undergoing cancer treatment) are at higher risk of developing severe forms of listeriosis.

How is listeriosis diagnosed?

Diagnosis of listeriosis is typically done through laboratory testing, which may involve culturing samples from blood, cerebrospinal fluid, placenta, or other affected tissues. Molecular methods, such as PCR, can also be used for rapid detection.

What is the treatment for listeriosis?

Treatment of listeriosis usually involves the use of antibiotics, such as ampicillin or a combination of antibiotics. Prompt diagnosis and early initiation of treatment are crucial in managing the infection and reducing complications.

How can listeriosis be prevented?

Prevention of listeriosis includes proper food handling, storage, and cooking practices. This involves thorough cooking of meat and seafood, practicing good hygiene during food preparation, avoiding high-risk foods during pregnancy, and ensuring proper sanitation in food processing facilities.

Are there any specific recommendations for pregnant women to prevent listeriosis?

Pregnant women are advised to avoid consuming high-risk foods, such as deli meats, soft cheeses, and unpasteurized dairy products. It is important to follow safe food handling practices, thoroughly cook food, and maintain good personal hygiene to reduce the risk of listeriosis during pregnancy.


  1. Vazquez-Boland, J. A., Kuhn, M., Berche, P., Chakraborty, T., Dominguez-Bernal, G., Goebel, W., … Kreft, J. (2001). Listeria Pathogenesis and Molecular Virulence Determinants. Clinical Microbiology Reviews, 14(3), 584–640. 
  2. Buchanan, R. L., Gorris, L. G. M., Hayman, M. M., Jackson, T. C., & Whiting, R. C. (2017). A review of Listeria monocytogenes : An update on outbreaks, virulence, dose-response, ecology, and risk assessments. Food Control, 75, 1–13.
  3. Bortolussi, R., & Mailman, T. L. (2010). Listeriosis. Infectious Diseases of the Fetus and Newborn, 470–488.

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