Listeria monocytogenes: Introduction, Morphology, Pathogenicity

Introduction

Table of Contents

Listeria monocytogenes is a gram-positive, rod-shaped bacterium that is widely distributed in nature and can cause a serious foodborne infection known as listeriosis. It is one of the few bacterial species capable of growing at refrigeration temperatures, making it a significant concern for food safety.

It was first discovered in 1926 by the British surgeon Albert Gray in the spleen of a patient who died from an infection. The name “Listeria” was given in honor of Joseph Lister, a pioneer in antiseptic surgery.

Key features of Listeria monocytogenes include:

Pathogenicity: It is a facultative intracellular pathogen, meaning it can survive and multiply inside host cells. This property allows the bacterium to evade the host immune response and cause systemic infections.
Foodborne Transmission:It is primarily transmitted through contaminated food. It can be found in various food sources, including raw and processed meats, unpasteurized dairy products, soft cheeses, and ready-to-eat foods like deli meats and salads. Once ingested, the bacterium can survive harsh acidic conditions in the stomach and invade the intestinal lining, leading to systemic infection.
Listeriosis: Listeriosis is the disease caused by Listeria monocytogenes infection. It primarily affects vulnerable populations, including pregnant women, newborns, the elderly, and individuals with weakened immune systems. Symptoms of listeriosis can range from mild flu-like symptoms to severe systemic infections, meningitis, and septicemia. Pregnant women are at particular risk as the infection can lead to miscarriage, stillbirth, or severe illness in the newborn.
Environmental Resilience: Listeria monocytogenes is remarkably resilient and can survive and grow in various environmental conditions, including refrigeration temperatures and high-salt environments. This resilience makes it challenging to control and eradicate from food processing environments.
Food Industry Concern: Due to its ability to grow at low temperatures and cause severe foodborne illness, L. monocytogenes is a significant concern for the food industry. Proper sanitation practices, good manufacturing practices, and routine testing are crucial to preventing contamination and ensuring food safety.

In response to the public health risks associated with Listeria monocytogenes, food safety regulations and guidelines are in place in many countries to prevent, detect, and control its presence in food products. Regular monitoring and testing of food products, particularly those known to be at higher risk of contamination, are essential for minimizing the risk of listeriosis outbreaks.

Morphology

Listeria monocytogenes is a gram-positive, rod-shaped bacterium with distinct morphological characteristics. Under a microscope, it appears as follows:

Shape: L. monocytogenes is a bacillus, which means it has a rod-like shape.
Size: The bacterium is typically 0.5 to 2.0 micrometers in width and 0.5 to 4.0 micrometers in length. The size can vary slightly depending on growth conditions.
Arrangement: L. monocytogenes cells are usually single, meaning they exist as individual cells rather than forming chains or clusters.
Gram Staining: Listeria monocytogenes is gram-positive, meaning it retains the crystal violet stain during the Gram staining procedure. When viewed under a microscope after Gram staining, the cells will appear purple.
Motility: It is motile, possessing flagella that allow it to move. This motility is one of its unique features among gram-positive bacteria.
Endospores: Listeria monocytogenes does not form endospores. Endospores are a protective survival structure formed by some bacterial species but are not observed in Listeria monocytogenes.
Capsule: Some strains of Listeria monocytogenes may produce a capsule, which is an outer layer that surrounds the bacterial cell. The capsule can play a role in the bacterium’s ability to resist the host’s immune response.

Listeria monocytogenes colony morphology on blood agar-Upper side — Fig. *Listeria monocytogenes* colony morphology on blood agar-Upper side

It’s important to note that the morphology of Listeria monocytogenes can be affected by growth conditions, age of the culture, and other factors. Therefore, microscopic examination should be complemented with other identification methods, such as biochemical tests and molecular techniques, to confirm the presence of L. monocytogenes in clinical or food samples accurately.

Pathogenicity

The pathogenicity of Listeria monocytogenes lies in its ability to cause a serious foodborne infection called listeriosis. This bacterium is unique among foodborne pathogens due to its facultative intracellular nature, which enables it to invade and replicate inside host cells. Here are the key aspects of L. monocytogenes‘ pathogenicity:

Foodborne Transmission: Listeria monocytogenes is primarily transmitted through contaminated food. It can be present in various food sources, including raw and processed meats, unpasteurized dairy products, soft cheeses, and ready-to-eat foods like deli meats and salads. When contaminated food is ingested, the bacterium can survive the acidic conditions of the stomach and invade the intestinal lining, allowing it to enter the bloodstream.
Intracellular Pathogen: One of the most critical features of Listeria monocytogenes is its ability to invade and survive within host cells. It has evolved mechanisms to escape from phagosomes, a cellular compartment formed when the bacterium is engulfed by immune cells, and replicate freely in the host cytoplasm. This allows Listeria to evade the host immune response and establish a systemic infection.
Cell-to-Cell Spread: It can spread from one host cell to another, using a unique actin-based motility system. The bacterium propels itself using host cell actin filaments, allowing it to move within the host tissues and disseminate to other organs.
Targeting Vulnerable Populations: Listeria monocytogenes primarily affects vulnerable populations, including pregnant women, newborns, the elderly, and individuals with weakened immune systems (e.g., those with HIV/AIDS, cancer patients, organ transplant recipients). Pregnant women are at particular risk as the bacterium can cross the placental barrier and infect the fetus, potentially leading to miscarriage, stillbirth, or severe neonatal infection.
Clinical Manifestations: The clinical manifestations of listeriosis can vary depending on the patient’s age, health status, and the site of infection. In healthy individuals, symptoms may be mild or flu-like and can include fever, muscle aches, and gastrointestinal symptoms. However, in susceptible populations, listeriosis can lead to more severe manifestations, including septicemia (bloodstream infection), meningitis (inflammation of the brain and spinal cord), and other systemic infections.
High Mortality Rate: Listeria monocytogenes is associated with a relatively high mortality rate, particularly among vulnerable populations. Prompt diagnosis and appropriate treatment with antibiotics are crucial for improving patient outcomes.
Environmental Resilience: Listeria monocytogenes can survive and grow in various environmental conditions, including refrigeration temperatures and high-salt environments. This resilience makes it challenging to control and eradicate from food processing environments.

Lab Diagnosis

The laboratory diagnosis of Listeria monocytogenes infection (listeriosis) involves the detection and identification of the bacterium in clinical samples or food products. Several methods are used for the diagnosis, including both direct and indirect approaches. Here are some common laboratory diagnostic methods for L. monocytogenes:

Listeria monocytogenes in Gram staining of culture microscopy — Fig. *Listeria monocytogenes* in Gram staining of culture microscopy

Microscopic Examination: Microscopic examination of clinical samples, such as blood, cerebrospinal fluid (CSF), or tissue biopsies, can reveal the presence of L. monocytogenes cells. Gram staining is often used to visualize the gram-positive, rod-shaped bacteria.
Culture and Isolation: The gold standard method for identifying L. monocytogenes is through bacterial culture and isolation. Clinical samples, as well as food products, are streaked onto selective media, such as PALCAM agar (Polymyxin, Acriflavine, LiCl, and Ceftazidime). Listeria species can grow on these selective media, and colonies with typical morphological characteristics are further tested for identification.
Biochemical Tests: Biochemical tests are conducted to confirm the identity of Listeria monocytogenes. These tests assess specific metabolic reactions, such as the ability to ferment sugars and the presence of specific enzymes, which are characteristic of Listeria species.
Serological Tests: Serological tests involve detecting specific antibodies against L. monocytogenes in patient serum. These tests are helpful in diagnosing listeriosis, especially in cases of meningitis or septicemia, where bacterial isolation from clinical samples may be challenging.
Molecular Detection: Molecular methods, such as Polymerase Chain Reaction (PCR), are increasingly used for the rapid and specific detection of L. monocytogenes. PCR targets specific genes or DNA sequences unique to Listeria monocytogenes, providing a sensitive and specific diagnosis.
Antigen Detection: Immunoassays, such as enzyme-linked immunosorbent assays (ELISAs), can detect Listeria monocytogenes-specific antigens in clinical samples or food products.
Phage Typing: Phage typing is a method used to differentiate Listeria strains based on their susceptibility to specific bacteriophages (viruses that infect bacteria). This typing method can be useful for epidemiological investigations during outbreaks.
Pulsed-Field Gel Electrophoresis (PFGE): PFGE is a molecular typing method that helps track and compare Listeria strains from different sources to identify potential outbreak sources and patterns of transmission.

Treatment

The treatment of L. monocytogenes infection (listeriosis) typically involves the administration of antibiotics to eliminate the bacterium from the body. Prompt and appropriate treatment is essential, especially in severe cases or in vulnerable populations, such as pregnant women, newborns, the elderly, and individuals with weakened immune systems. The choice of antibiotics depends on the severity of the infection and the patient’s clinical condition. Here are some common antibiotics used for the treatment of listeriosis:

Ampicillin: Ampicillin, a penicillin-class antibiotic, is often considered the first-line treatment for listeriosis. It is effective against Listeria monocytogenes and can be administered intravenously for severe cases or orally for milder infections.
Gentamicin: Gentamicin, an aminoglycoside antibiotic, is frequently used in combination with ampicillin to enhance the bactericidal effect. The combination therapy is especially important in severe cases or when the patient is immunocompromised.
Trimethoprim-Sulfamethoxazole (TMP-SMX): TMP-SMX is an alternative antibiotic used for listeriosis, especially in cases of penicillin allergy or resistance to ampicillin.
Other Antibiotics: In certain situations, other antibiotics, such as vancomycin or meropenem, may be considered based on the susceptibility profile of the bacterium or if there are contraindications to the standard treatment options.

It is crucial to administer antibiotics early in the course of the infection, as delayed treatment can lead to severe complications and a higher risk of mortality, especially in vulnerable populations. The duration of antibiotic therapy depends on the severity of the infection, the patient’s response to treatment, and the presence of any underlying conditions.

In addition to antibiotic therapy, supportive care may be necessary to manage complications associated with listeriosis, such as dehydration, organ dysfunction, and neurological complications in cases of meningitis. Supportive measures, such as intravenous fluids, respiratory support, and pain management, are tailored to the patient’s specific needs.

Prevention of listeriosis is also critical, especially for vulnerable populations and pregnant women. This includes practicing good food safety measures, such as thoroughly cooking meat, avoiding unpasteurized dairy products and soft cheeses, and handling ready-to-eat foods with care to prevent cross-contamination.

Prevention

Preventing Listeria monocytogenes infection (listeriosis) is crucial, especially for individuals at higher risk of severe illness, such as pregnant women, newborns, the elderly, and those with weakened immune systems. Here are some key prevention measures to reduce the risk of Listeria infection:

Safe Food Handling: Properly handle, cook, and store food to prevent contamination with L. monocytogenes. Follow these guidelines:
- Thoroughly cook raw meat, poultry, and seafood to their safe internal temperatures.
- Avoid consuming unpasteurized dairy products, soft cheeses, and other foods made from unpasteurized milk.
- Wash raw fruits and vegetables thoroughly before consumption.
- Separate raw and cooked foods to prevent cross-contamination during preparation.
- Store food at safe temperatures: Keep perishable foods refrigerated at or below 40°F (4°C) and frozen foods at or below 0°F (-18°C).
Good Hygiene Practices: Practice good personal hygiene to reduce the risk of Listeria infection:
- Wash hands thoroughly with soap and water before and after handling food, using the restroom, or changing diapers.
- Clean and sanitize kitchen surfaces, utensils, and cutting boards regularly, especially after handling raw meats and vegetables.
Avoid High-Risk Foods: Limit the consumption of high-risk foods known to be associated with Listeria contamination, such as deli meats, hot dogs, and refrigerated pâtés or meat spreads. If consuming these foods, heat them to steaming hot before eating to kill any potential Listeria bacteria.
Pregnant Women Precautions: Pregnant women should take extra precautions to avoid Listeria infection, as it can have severe consequences for both the mother and the unborn baby. They should avoid high-risk foods and unpasteurized products and ensure food safety practices are strictly followed.
Be Cautious with Ready-to-Eat Foods: Ready-to-eat foods, such as salads, sandwiches, and pre-cut fruits or vegetables, can be a source of Listeria contamination. Always check the “use-by” or “best-by” date and follow storage instructions for these foods.
Educational Campaigns: Public health agencies and organizations can conduct educational campaigns to raise awareness about L. monocytogenes infection and the importance of food safety practices. These campaigns can target high-risk populations and the general public.
Food Safety Regulations: Food manufacturers and processors should adhere to strict food safety regulations and implement Hazard Analysis and Critical Control Points (HACCP) systems to minimize the risk of Listeria contamination in food products.
Environmental Monitoring: In food processing facilities, routine environmental monitoring can help detect and control Listeria contamination. This involves regularly testing surfaces, equipment, and food contact areas for the presence of Listeria monocytogenes.

Keynotes

Keynotes of Listeria monocytogenes:

Gram-Positive Bacterium: L. monocytogenes is a gram-positive bacterium, meaning it retains the crystal violet stain during the Gram staining procedure.
Rod-Shaped: The bacterium has a rod-like shape, appearing as bacilli under a microscope.
Intracellular Pathogen: L. monocytogenes is a facultative intracellular pathogen, capable of surviving and multiplying inside host cells. This unique feature allows it to evade the host immune response and cause systemic infections.
Foodborne Transmission: The primary route of transmission is through contaminated food. L. monocytogenes can be present in various food products, including raw and processed meats, dairy products, and ready-to-eat foods.
Listeriosis: L. monocytogenes causes listeriosis, an infectious disease that primarily affects vulnerable populations, such as pregnant women, newborns, the elderly, and individuals with weakened immune systems.
Severe Infection: Listeriosis can lead to severe manifestations, including septicemia (bloodstream infection), meningitis (inflammation of the brain and spinal cord), and other systemic infections. It is associated with a relatively high mortality rate, especially in high-risk individuals.
Pregnancy Risks: Pregnant women infected with L. monocytogenes can transmit the bacterium to the fetus, leading to miscarriage, stillbirth, or severe neonatal infection.
Environmental Resilience: Listeria monocytogenes can survive and grow in various environmental conditions, including refrigeration temperatures and high-salt environments. This resilience poses challenges in controlling its presence in food processing environments.
Preventive Measures: Prevention of Listeria monocytogenes infection involves safe food handling, good hygiene practices, avoiding high-risk foods, and raising awareness about the risks, especially among vulnerable populations.
Laboratory Diagnosis: The diagnosis of listeriosis involves microbial culture and isolation, as well as molecular and serological tests, to identify Listeria monocytogenes in clinical samples or food products.
Antibiotic Treatment: Prompt and appropriate antibiotic therapy, such as ampicillin and gentamicin, is essential in managing listeriosis and improving patient outcomes.
Public Health Importance: Due to its severe consequences and potential for foodborne outbreaks, L. monocytogenes is a significant public health concern, prompting surveillance, food safety regulations, and educational campaigns.

Listeria monocytogenes: Introduction, Morphology, Pathogenicity, Lab Diagnosis, Treatment, Prevention, and Keynotes