• Listeria monocytogenes is a facultative intracellular bacterium that causes listeriosis, a severe infection primarily affecting pregnant women, newborns, old ages, and individuals with weakened immune systems. 
• Pregnant women are more inevitable than the general population to get listeriosis, and individuals over 65 are four times more likely to get the infection. Other high-risk groups include people with weakened immune systems from illnesses like HIV/AIDS, cancer, or diabetes.  
• Listeria monocytogenes can be identified in soil, water, and animals. Listeriosis is most acquired through consuming contaminated food, particularly soft cheeses, deli meats, and refrigerated ready-to-eat foods. However, infections can also occur through other transmission routes, such as contact with infected animals or exposure to contaminated environments.  
• Outbreaks: Listeria monocytogenes can cause illness outbreaks, particularly in food processing facilities and institutions such as hospitals and nursing homes. In recent years, there have been several large listeriosis outbreaks in the United States, including an outbreak in 2011 linked to cantaloupe that sickened 147 people and caused 33 deaths. 
• Mortality rate: Listeriosis is a serious infection that can lead to severe illness, particularly in vulnerable populations. The case fatality rate (CFR) for listeriosis is estimated to be approximately 20%, which is relatively high compared to other foodborne illnesses. 
• Global impact: Listeriosis is a global health issue, and the incidence of the infection varies widely depending on the region. Listeriosis is often underdiagnosed and underreported in developing countries due to a lack of awareness and laboratory testing capacity. In Europe, there has been a recent increase in listeriosis cases, which has been linked to the consumption of contaminated ready-to-eat foods. 

• Kingdom: Bacteria  
• Phylum: Firmicutes  
• Class: Bacilli  
• Order: Bacillales  
• Family: Listeriaceae  
• Genus: Listeria  
• Species: Listeria monocytogenes 

Structure: 

Listeria monocytogenes is a Gram-positive bacterium with a cell wall comprising a thick peptidoglycan layer surrounded by a cell membrane. 

• The cell membrane of L. monocytogenes is a lipid bilayer that encloses the cytoplasm and regulates the flow of molecules in and out of the cell. 
• The cell wall of L. monocytogenes is composed of peptidoglycan, which provides structural support and protection from the environment. 
• Listeria monocytogenes is motile and has one or more flagella that allow it to move towards nutrients and away from toxins. 
• It has a polysaccharide capsule that helps protect the bacterium from host immune defenses. 
• Inside the cell, Listeria monocytogenes has several vital structures, including the nucleoid (which contains the bacterium’s DNA), ribosomes (which are accountable for protein synthesis), and plasmids (which are small, circular pieces of DNA that can carry genes involved in virulence and antibiotic resistance). 

Listeria monocytogenes has 13 known serotypes based on the antigenic properties of the O-antigens of its cell wall.  

These serotypes are documented based on the reactions of specific antisera with the O-antigens. The 13 serotypes are: 

• Serotype 1/2a, 1/2b and 1/2c.  
• Serotype 3a, 3b, 3c  
• Serotype 4a, 4ab, 4b, 4c, 4d and 4e 
• Serotype 7 

The most common serotypes associated with human infections are 1/2a, 1/2b, and 4b. These serotypes account for over 95% of human listeriosis cases worldwide. 

Listeria monocytogenes is an intracellular pathogen that can infect various host cells, including macrophages, epithelial cells, and endothelial cells. Here is a brief overview of the pathogenesis of L. monocytogenes: 

• Adhesion and invasion: L. monocytogenes use surface proteins called internalins to adhere to and invade host cells. Once inside the cell, the bacterium uses actin “comet tails” to move through the cytoplasm and spread to neighboring cells. 
• Listeria monocytogenes produce a protein called listeriolysin O that forms pores in host cell membranes, allowing the bacterium to escape from the phagosome and move freely within the host cell cytoplasm. 
• Actin “comet tails”: After invading a host cell, L. monocytogenes uses surface proteins to stimulate the host cell’s actin cytoskeleton, causing actin filaments to polymerize and form a tail-like structure that propels the bacterium through the cytoplasm of the host cell. 
• L. monocytogenes produces phospholipase enzymes that break down host cell membranes, allowing the bacterium to access nutrients and other resources within the host cell. 
• Listeria monocytogenes can evade hosting immune defenses by producing proteins that interfere with the host immune response. The bacterium produces a protein called InlB that inhibits the production of inflammatory cytokines, which can help the bacterium to avoid detection and clearance by the immune system. 
• Listeria monocytogenes can form biofilms on surfaces, which allows them to persist and survive in food processing environments and other settings. 

The host has several defense mechanisms against L. monocytogenes infections, including the following: 

• The innate immune response is the first defense against Listeria monocytogenes infections. The innate immune system members, like macrophages and neutrophils, can phagocytose Listeria monocytogenes and kill them through various mechanisms, including producing reactive oxygen species and releasing antimicrobial peptides. 
• The adaptive immune response is a more specific defense mechanism involving antibodies’ production and activation of T cells. During infection, B cells produce antibodies that can bind to and neutralize the bacterium, while T cells can recognize and kill infected host cells. 
• Interferon response: The host can produce interferons responding to Listeria monocytogenes infections. Interferons are signaling molecules that can activate host defenses and inhibit viral replication. They can also stimulate the production of MHC class I molecules, which can help to activate T cells and promote the clearance of infected cells. 
• Autophagy is a process by which the host cell can engulf and degrade intracellular pathogens. Several mechanisms can trigger autophagy, including recognizing L. monocytogenes by pattern recognition receptors and activating the interferon response. 
• The complement system is a series of proteins that can bind to and lyse bacterial cells. The complement system can be triggered by antibodies and other modules of the innate immune system and can contribute to the clearance of Listeria monocytogenes infections. 

• Listeria monocytogenes can cause a range of clinical manifestations, from asymptomatic infection to severe invasive disease. The symptoms and severity of infection can vary depending on the patient’s age, immune status, and underlying health conditions. 
• In healthy adults, L. monocytogenes infection, may cause mild flu-like symptoms, such as fever, muscle aches, and gastrointestinal symptoms.  
• In pregnant women, L. monocytogenes infection can cause mild flu-like symptoms, but it can also lead to severe complications such as miscarriage, stillbirth, or preterm delivery. 
• In immunocompromised individuals, Listeria monocytogenes can cause severe invasive disease, including fever, headache, stiff neck, confusion, and seizures.

The most severe forms of Listeria monocytogenes infection include: 

• Invasive listeriosis: This serious and potentially life-threatening infection can occur when the bacteria spread beyond the gastrointestinal tract and invade the bloodstream, causing sepsis. Invasive listeriosis can lead to meningitis, encephalitis, septicemia, and pneumonia.

• Maternal-fetal listeriosis: L. monocytogenes infection during pregnancy can lead to maternal-fetal listeriosis, which can cause miscarriage, stillbirth, or preterm delivery. Infants born to mothers with listeriosis may also develop severe infections after birth.

• Neonatal listeriosis occurs when a newborn is infected with the pathogen during delivery or breastfeeding. Neonatal listeriosis can cause sepsis, meningitis, and other serious complications.

• Central nervous system listeriosis: This is a rare but severe form of listeriosis that can cause inflammation of the brain and spinal cord, leading to symptoms such as headache, fever, seizures, and paralysis. 

The standard diagnostic tests for Listeria monocytogenes include:  

• Blood culture: This involves culturing blood samples to identify the presence of Listeria monocytogenes. Blood culture is the most reliable method for diagnosing invasive listeriosis. 
• Cerebrospinal fluid culture is performed on patients with suspected central nervous system listeriosis. The CSF sample is cultured to identify the presence of L. monocytogenes. 
• Stool culture: It can be performed on patients with suspected gastroenteritis caused by L. monocytogenes. The bacteria may be present in the stool of infected individuals. 
• Placental or fetal tissue culture: It is performed on pregnant women with suspected maternal-fetal listeriosis. Listeria monocytogenes in placental or fetal tissue are highly suggestive of maternal-fetal listeriosis. 
• Antibody testing may detect antibodies against Listeria monocytogenes in the patient’s blood or cerebrospinal fluid. However, this method is less reliable for diagnosing acute infection, as antibodies can take several weeks to develop. 
• Fluorescence in situ hybridization (FISH) detects DNA sequences of L. monocytogenes in clinical specimens, such as blood or cerebrospinal fluid. 

• Proper food handling and storage: L. monocytogenes can grow at refrigeration temperatures, so it is important to properly handle and store food to prevent contamination. It includes washing hands and surfaces before and after handling food, separating raw and cooked foods, and storing food at safe temperatures. 
• Listeria monocytogenes can be killed by cooking food to the appropriate temperature. Use a reliable thermometer to ensure that meat, poultry, fish, and eggs are cooked to a safe internal temperature. 
• Avoiding high-risk foods: Certain foods are more likely to be contaminated with L. monocytogenes, such as deli meats, hot dogs, soft cheeses, and smoked seafood. Pregnant women, young children, older adults, and people with debilitated immune systems should avoid these foods. 
• Clean drinking water: Listeria monocytogenes can sometimes be found in water sources, so drinking clean, treated water is essential. 
• L. monocytogenes can be spread from contaminated surfaces or utensils to food. To avoid cross-contamination, use separate cutting boards and utensils for raw meat, poultry, fish, and eggs, and wash hands and surfaces thoroughly after handling these foods. 
• Proper food storage: Refrigerate or freeze perishable foods within an hour of purchase or preparation. Use refrigerated food residues within three to four days and frozen leftovers within two to six months. 
• Avoid unpasteurized products: L. monocytogenes can be found in raw milk and other unpasteurized dairy products, so consuming pasteurized products is risk-free. 

• https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1899877 
• https://en.wikipedia.org/wiki/Listeria_monocytogenes