Food poisoning by Listeria monocytogenes (Listeriosis)

Listeriosis is a serious and sometimes fatal infection caused by a bacterium called Listeria monocytogenes. This bacterium can contaminate various foods, especially ready-to-eat products that are stored under refrigeration. Listeriosis can affect anyone, but some groups are more vulnerable than others, such as pregnant women, newborns, elderly people, and people with weakened immune systems. Listeriosis can cause different types of illnesses, ranging from mild gastroenteritis to severe invasive infections that can affect the brain, blood, or placenta. Listeriosis is a global public health concern because of its high mortality rate and its potential to cause outbreaks. According to the World Health Organization (WHO), listeriosis affects about 0.1 to 10 people per million population per year worldwide. The Centers for Disease Control and Prevention (CDC) estimates that about 1,600 people get listeriosis each year in the United States, and about 260 die. Listeriosis can be prevented by following good food safety practices, such as avoiding high-risk foods, cooking food thoroughly, washing fruits and vegetables, and keeping food at safe temperatures. Listeriosis can be treated with antibiotics, but early diagnosis and prompt treatment are essential to reduce the risk of complications and death. In this article, we will discuss the source of contamination, the antigenic structure and virulence factors, the epidemiology, the clinical signs and symptoms, the mechanism of pathogenesis, the laboratory diagnosis, the treatment, and the prevention and control of listeriosis.

Listeria monocytogenes is a bacterium that can cause a serious infection called listeriosis in humans and animals. It is widely distributed in the environment and can be found in soil, water, sewage, decaying plant matter, and animals. It can also contaminate food products during harvesting, processing, preparation, packaging, transportation, or storage. Some of the foods that are at high risk of contamination are:

• Ready-to-eat foods such as hotdogs, smoked fish, raw seafood, sausages, pre-prepared or pre-packaged fruits and vegetables, deli meats, and soft cheeses
• Unpasteurized milk and milk products
• Raw or undercooked poultry, meat, and eggs
• Raw pet food

Listeria monocytogenes can survive and even grow under refrigeration and other food preservation measures. It can tolerate high salt concentration, low pH, and a wide range of temperatures. Therefore, proper food storage, handling, and cooking are essential to prevent the growth and spread of this bacterium. Listeria monocytogenes can also be transmitted from mother to child through fluids or placenta during pregnancy or delivery. Pets can also spread the bacteria in the home environment if they eat food contaminated with Listeria monocytogenes.

Listeria monocytogenes has a complex antigenic structure that allows it to evade the host immune system and cause infection. The bacterium has 13 known distinct serotypes based on somatic (O) antigens and 5 serovars based on flagellar (H) antigens. However, only three serotypes (1/2a, 1/2b, and 4b) are responsible for more than 90% of human and animal outbreaks. Serotype 4b is particularly associated with major listeriosis epidemics and severe clinical outcomes.

Listeria monocytogenes also possesses several virulence factors that enable it to survive harsh environmental conditions, adhere to and invade host cells, escape from phagocytic vacuoles, replicate intracellularly, and spread from cell to cell. Some of the most important virulence factors are:

• Sigma factor B (sigB gene): This is a transcriptional regulator that controls the expression of more than 150 genes involved in stress response, cell wall biosynthesis, motility, and virulence. Sigma factor B helps Listeria monocytogenes to adapt to thermal variation, acidic pH, and the presence of antimicrobial agents.
• Listeriolysin O (LLO): This is a pore-forming toxin that lyses the membrane of the phagocytic vacuole and releases the bacterium into the cytosol of the host cell. LLO also modulates the host immune response by inducing apoptosis, cytokine production, and inflammasome activation.
• Phospholipases C (PLC): These are enzymes that degrade phospholipids and facilitate the escape of Listeria monocytogenes from the phagocytic vacuole. PLC also contributes to tissue damage, inflammation, and bacterial dissemination.
• Actin assembly-inducing protein (ActA): This is a surface protein that recruits the host actin cytoskeleton and forms a comet tail that propels the bacterium through the cytoplasm and into neighboring cells. ActA also prevents the recognition of Listeria monocytogenes by autophagy, a cellular defense mechanism.
• Internalins (Inl): These are surface proteins that mediate the attachment and invasion of Listeria monocytogenes into different types of host cells by binding to specific receptors. For example, InlA binds to E-cadherin on epithelial cells, InlB binds to Met on hepatocytes and endothelial cells, InlC inhibits the innate immune response by interfering with interferon signaling, and InlH enhances bacterial survival in macrophages.

These virulence factors are regulated by various genetic elements, such as plasmids, transposons, prophages, and pathogenicity islands. The expression of virulence genes is also influenced by environmental cues, such as temperature, pH, osmolarity, oxygen availability, and nutrient availability. Therefore, Listeria monocytogenes can modulate its virulence according to different stages of infection and host niches.

Listeriosis is a rare but serious foodborne disease caused by the bacterium Listeria monocytogenes. It mainly affects people who are at high risk of severe illness, such as pregnant women, newborns, elderly, and immunocompromised individuals. The infection can result in septicemia, meningitis, abortion, stillbirth, and neonatal death.

According to the World Health Organization (WHO), listeriosis has a global incidence of 0.1 to 10 cases per 1 million people per year, depending on the countries and regions of the world. However, the case-fatality rate of listeriosis is very high, ranging from 20% to 30%. In the United States, an estimated 1,600 people get listeriosis each year, and about 260 die. In Europe, about 2,200 cases and 270 deaths were reported in 2017.

Listeriosis outbreaks have been reported in various countries and regions, such as Canada, United States, Europe, Japan, and South Africa. The sources of contamination are usually ready-to-eat foods that are consumed without further cooking or processing, such as deli meats, smoked fish, soft cheeses, raw milk products, and fresh produce. Listeria monocytogenes can survive and grow at low temperatures, such as those found in refrigerators, and can persist in food processing environments for long periods of time.

The incubation period of listeriosis varies from a few days to several weeks, depending on the type and severity of the infection. The dose required for infection is also uncertain, but it may depend on the susceptibility of the host and the virulence of the strain. Listeria monocytogenes can invade various tissues and organs in the human body, such as the intestine, blood, brain, and placenta. The bacteria can also spread from person to person through vertical transmission (from mother to fetus or newborn) or horizontal transmission (through contact with infected fluids or secretions).

The prevention and control of listeriosis require a multidisciplinary approach involving public health authorities, food industry, health care providers, and consumers. Some of the measures include:

• Surveillance and monitoring of human cases and food products for Listeria monocytogenes
• Rapid detection and investigation of outbreaks and identification of sources and vehicles of transmission
• Implementation and enforcement of food safety standards and regulations along the food chain
• Education and training of food handlers and processors on good hygiene practices and risk reduction strategies
• Education and awareness of consumers on safe food handling and storage practices and avoidance of high-risk foods
• Early diagnosis and treatment of listeriosis cases with appropriate antibiotics
• Vaccination of animals against Listeria monocytogenes to reduce the environmental reservoir

Listeriosis can cause different signs and symptoms depending on the person infected and the part of the body affected. The bacteria can cause two types of illness: invasive and intestinal.

Invasive illness

Invasive illness occurs when the bacteria spread beyond the intestines to other parts of the body, such as the blood, brain, or placenta. This type of illness can be very serious and even life-threatening, especially for pregnant women, newborns, older adults, and people with weakened immune systems.

The symptoms of invasive illness usually start within two weeks after eating food contaminated with Listeria, but they can take up to two months to appear. The symptoms may include:

• Fever
• Muscle aches
• Headache
• Stiff neck
• Confusion
• Loss of balance
• Seizures

Pregnant women may have only mild flu-like symptoms, such as fever and muscle aches. However, invasive listeriosis during pregnancy can lead to serious complications, such as miscarriage, stillbirth, premature delivery, or infection of the newborn.

Intestinal illness

Intestinal illness occurs when the bacteria cause inflammation of the gut. This type of illness is less common and less severe than invasive illness. It is often not diagnosed because laboratories do not routinely test stool samples for Listeria.

The symptoms of intestinal illness usually start within 24 hours after eating food contaminated with Listeria and last for one to three days. The symptoms may include:

• Diarrhea
• Vomiting

Some people with intestinal illness may develop invasive illness later on. Therefore, anyone who has symptoms of listeriosis should seek medical attention as soon as possible. Listeriosis can be treated with antibiotics if diagnosed early.

Listeria monocytogenes is a facultative intracellular bacterium that can invade and replicate within various types of host cells, such as epithelial cells, macrophages, hepatocytes, and placental trophoblasts. The mechanism of pathogenesis of L. monocytogenes involves several steps:

• Entry into host cells: L. monocytogenes can attach to the surface of host cells by using surface proteins called internalins (Inl), which bind to specific receptors on the host cell membrane. For example, InlA binds to E-cadherin on epithelial cells, and InlB binds to Met receptor on hepatocytes. After attachment, L. monocytogenes induces its own internalization by triggering actin rearrangement and membrane ruffling at the site of entry.
• Escape from phagosomes: Once inside the host cell, L. monocytogenes is enclosed in a membrane-bound vacuole called a phagosome. However, L. monocytogenes can escape from the phagosome by using a pore-forming toxin called listeriolysin O (LLO) and two phospholipases (PlcA and PlcB), which degrade the phagosomal membrane and allow the bacterium to access the cytosol.
• Replication in the cytosol: L. monocytogenes can multiply rapidly in the cytosol by using host nutrients and avoiding immune recognition. L. monocytogenes also expresses a protein called ActA, which recruits host actin and forms a tail-like structure at one pole of the bacterium. This actin tail propels the bacterium through the cytosol and enables it to reach the cell membrane.
• Cell-to-cell spread: L. monocytogenes can spread from one infected cell to another without exposing itself to the extracellular environment. This is achieved by forming membrane protrusions at the cell surface that are driven by actin polymerization. These protrusions are then engulfed by neighboring cells, creating a double-membrane vacuole that contains the bacterium. L. monocytogenes can then escape from this vacuole by using LLO and phospholipases and repeat the cycle of intracellular infection.

The mechanism of pathogenesis of L. monocytogenes allows it to evade host immune defenses, disseminate through the bloodstream and lymphatic system, and reach various tissues and organs, such as the liver, spleen, brain, and placenta. This can result in severe clinical manifestations of listeriosis, such as sepsis, meningitis, encephalitis, and fetal infection.

Listeriosis is usually diagnosed when a bacterial culture grows Listeria monocytogenes from a body tissue or fluid, such as blood, cerebrospinal fluid (CSF), or the placenta. However, culture methods may take several days and have low sensitivity. Therefore, other methods such as microscopy, serology, and molecular techniques may be used to complement or replace culture.

Microscopy

In Gram stain smears of CSF sediments or other specimens, L. monocytogenes may be seen as gram-positive coccobacilli that may be mistaken for pneumococci or corynebacteria. In wet mount preparations of nutrient broth cultures, L. monocytogenes exhibits characteristic end-over-end tumbling motility when incubated at room temperature for 1 to 2 hours. This motility can also be observed on semisolid media such as motility agar or methylcellulose.

Serology

Serological tests can detect antibodies against L. monocytogenes in the serum of infected patients. The most commonly used tests are the Widal test, which detects agglutination reactions with O and H antigens of L. monocytogenes, and the enzyme immunoassay (EIA), which detects antibodies against listeriolysin O (LLO), a major virulence factor of L. monocytogenes. However, these tests have limitations such as cross-reactivity with other bacteria, low specificity and sensitivity, and lack of commercial availability.

Molecular techniques

Molecular techniques can identify L. monocytogenes DNA or RNA in clinical samples using polymerase chain reaction (PCR) or nucleic acid hybridization methods. These techniques are rapid, sensitive, and specific and can also provide information on the strain type and virulence profile of L. monocytogenes. The most commonly used target for PCR is the 16S ribosomal RNA gene sequence of Listeria, which can be amplified from blood, CSF, or amniotic fluid samples. Other targets include genes encoding virulence factors such as LLO, internalin A (InlA), actin assembly-inducing protein (ActA), and sigma factor B (sigB).

Listeriosis is a serious infection that requires prompt medical attention. Depending on the severity of the symptoms and the risk factors of the patient, different treatment options may be available.

• For mild cases of intestinal illness, antibiotics are usually not needed. The patient should drink plenty of fluids to prevent dehydration and rest until the symptoms subside.
• For invasive cases of listeriosis, such as meningitis, septicemia, or infection during pregnancy, antibiotics are essential to clear the infection and prevent complications. The most commonly used antibiotics are ampicillin, gentamicin, and penicillin G. These antibiotics can be given intravenously for at least 6 weeks.
• For pregnant women with listeriosis, antibiotics can also reduce the risk of transmitting the infection to the fetus or newborn. However, some cases may still result in miscarriage, stillbirth, or neonatal death.
• For patients with other underlying conditions that weaken their immune system, such as HIV/AIDS, cancer, or organ transplantation, antibiotics may need to be combined with other supportive therapies to improve their outcome.

The choice and duration of antibiotic therapy may vary depending on the individual situation and the susceptibility of the bacteria. Therefore, it is important to consult a doctor for proper diagnosis and treatment of listeriosis. A gastroenterologist (a specialist in digestive disorders) may be involved in the care of patients with listeriosis.

Listeriosis is a serious, but preventable and treatable disease caused by the bacterium Listeria monocytogenes. This bacterium can be found in soil, water, vegetation, and the feces of some animals and can contaminate foods. Listeriosis can affect anyone, but some people are more vulnerable than others, such as pregnant women, the elderly, and people with weakened immune systems. Therefore, it is important to take measures to prevent and control listeriosis by following these recommendations:

• Choose safer foods: Avoid eating foods that are more likely to be contaminated with Listeria, such as unpasteurized milk and dairy products, soft cheeses (such as queso fresco and brie), deli meats and hot dogs, smoked fish, raw sprouts, and cut melon left out for more than two hours. Instead, choose pasteurized milk and dairy products, hard cheeses (such as cheddar and parmesan), cooked meats and fish, cooked sprouts, and fresh or frozen fruits and vegetables. If you are pregnant, elderly, or immunocompromised, you should also avoid eating raw enoki mushrooms or ask that restaurants cook them thoroughly.
• Cook food thoroughly: Listeria can survive and multiply at low temperatures usually found in refrigerators. Therefore, you should cook food to a high enough temperature to kill the bacteria. Use a food thermometer to check that meat, poultry, seafood, and eggs are cooked to the safe minimum internal temperature. Reheat leftovers and ready-to-eat foods until steaming hot before eating.
• Keep food safe: Listeria can spread from contaminated food to surfaces and other foods. To prevent cross-contamination, you should wash your hands with soap and water before and after handling food; wash fruits and vegetables under running water before eating; separate raw meat, poultry, seafood, and eggs from other foods in your shopping cart, refrigerator, and during preparation; use separate cutting boards, knives, and utensils for raw and cooked foods; and clean and sanitize your kitchen surfaces and utensils regularly.
• Store food properly: Listeria can grow on foods kept in the refrigerator for a long time. To prevent this, you should store food in sealed containers or wrap them tightly; use precooked and ready-to-eat foods as soon as possible; follow the expiration dates on food labels; refrigerate leftovers within two hours of cooking or one hour if the temperature is above 90°F; and keep your refrigerator at 40°F or lower and your freezer at 0°F or lower.

By following these prevention and control measures, you can reduce your risk of listeriosis and protect your health and the health of your loved ones. If you suspect that you have listeriosis or have been exposed to contaminated food, you should seek medical attention immediately. Listeriosis can be diagnosed by blood tests or other laboratory methods and treated with antibiotics.

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