Listeria monocytogenes

Introduction to Listeria monocytogenes

Listeria monocytogenes is a pathogenic bacterium belonging to the genus Listeria and the family Listeriaceae. This bacterium is widely distributed in nature and can be found in water, soil, plant surfaces, and the feces of certain animals.

History of the Discovery of Listeria monocytogenes

Listeria monocytogenes was first discovered in 1926 by Edward Murray, a Scottish veterinarian, while studying an unknown disease in rabbits. He initially named the bacterium Bacterium monocytogenes. The term "Bacterium" referred to the genus of bacteria, which in the early 20th century was used as a general term for a large group of bacteria that had not yet been precisely classified. The name "monocytogenes" consists of two parts: "mono," meaning "one," and "cytogenesis," meaning "cell production." This name was chosen because of the bacterium's ability to increase the number of a specific type of white blood cell called monocytes in the bodies of animals. The term "monocytogenesis" specifically refers to this bacterium's characteristic of stimulating the production of monocytes, which is an important indicator of infection in the body.

A year later, British bacteriologist Pirie studied the bacterium more carefully and identified its characteristics. To classify it more precisely, Pirie created a new genus and named it Listeria in honor of Joseph Lister, the British surgeon known as the "father of sterile surgery" for his introduction of antiseptic methods in surgery. The bacterium was then placed in this genus.

Growth Conditions of Listeria monocytogenes

Listeria monocytogenes can survive and grow in a variety of environmental conditions. This bacterium not only survives but also continues to grow under harsh conditions where the growth of other microorganisms is either halted or slowed down.

Listeria is a facultative anaerobic bacterium, meaning it can grow in the presence of oxygen as well as in anaerobic (oxygen-free) conditions. This characteristic allows Listeria to survive and grow in different environments, including vacuum-packed products where oxygen is minimized or entirely removed.

The ideal temperature range for the growth of Listeria is between 30 to 37 degrees Celsius. However, Listeria monocytogenes can grow across a wide range of temperatures, from around -0.4°C (slightly below freezing) to 45°C. At lower temperatures, such as refrigeration temperatures (4°C), its growth slows down, and at freezing temperatures (around -18°C), the bacterium’s growth completely halts. Despite this, the bacterium remains viable and can resume its activity and growth when temperatures rise. Therefore, low temperatures are not an effective way to eliminate Listeria.

This bacterium can grow in environments with high salt concentrations (up to 10%), which allows Listeria monocytogenes to survive and thrive in salty and processed foods.

Neutral environments with a pH around 7, which are neither acidic nor alkaline, are the best for the growth of this bacterium. However, Listeria can also survive and grow in acidic environments (such as vinegar or citrus) with a pH around 4.4, as well as in alkaline environments (such as cooked spinach) with a pH around 9.

Listeria requires moisture for growth and thrives in wet environments, making fresh, moist foods like vegetables and dairy products suitable environments for the growth of this bacterium.

Foods Prone to Listeria monocytogenes Contamination

Listeria monocytogenes, due to its high resistance and ability to grow under various conditions, can be present in many food products. This is particularly concerning for ready-to-eat (RTE) foods, as even if these products are produced under hygienic conditions, they may become re-contaminated due to the widespread presence of Listeria in the environment. Cross-contamination through contact with contaminated surfaces or other materials is a common source of re-infection. Furthermore, since Listeria can multiply gradually at refrigerator temperatures, its presence poses a greater concern for foods with a long shelf life in the fridge.

Food products susceptible to Listeria monocytogenes contamination:

Processed meats like sausages and cold cuts, particularly those sold pre-sliced in supermarkets, are at risk because Listeria can easily spread through cross-contamination from equipment and tools used during processing and slicing, as well as from work surfaces and workers' hands.

Dairy products, especially unpasteurized types like raw milk, can become contaminated due to poor hygiene conditions in the farming environment, including contact with animal feces or contaminated equipment. While pasteurization and sterilization processes eliminate the bacterium, improper execution of these processes or cross-contamination can lead to re-infection. Dairy products made from unpasteurized milk, such as cheese and ice cream, are also at risk. Additionally, Listeria can grow in brine solutions used for preserving cheese.

Fresh vegetables and fruits may become contaminated from contact with soil infected with Listeria. Regular washing is often insufficient to remove all bacteria, particularly Listeria, which can remain in crevices and uneven surfaces of fruits and vegetables, making it difficult to dislodge. Since these products are typically consumed raw and not subjected to heating processes, they pose a higher contamination risk.

Melons like cantaloupes, melons, and watermelons are particularly susceptible to contamination because of their natural surface cracks, which can trap and hold contaminants. Regular washing with water may not fully eliminate the bacteria, as it can hide in these cracks and remain difficult to remove. Since Listeria can survive in refrigerated temperatures and grow slowly, melons stored in the fridge for extended periods, typically consumed raw, have an increased risk of contamination. When cutting these melons, bacteria from the skin may transfer to the edible flesh.

Ready-to-eat salads such as coleslaw and sprouts (e.g., wheat and mung bean sprouts) are at high risk if initially contaminated, due to their high moisture content and storage at low temperatures, which provide ideal conditions for Listeria growth. Since these products are typically consumed cold and are not subjected to any heat treatment to kill the bacterium, the risk of multiplication and contamination is high.

Diseases Caused by Listeria monocytogenes

Listeria monocytogenes is the causative agent of listeriosis, and the primary mode of transmission to humans is through the consumption of contaminated food. This disease is relatively rare, with an incidence rate ranging from 0.1 to 10 cases per million people annually, according to the World Health Organization (WHO). However, despite the low number of cases, listeriosis has become a significant public health concern due to its high mortality rate.

In individuals without underlying health conditions and with a strong immune system, listeriosis usually presents in a mild form. This form, known as non-invasive listeriosis, causes symptoms such as diarrhea, fever, headache, and muscle aches, and in many cases, it improves without the need for specific treatment. However, if more severe symptoms occur, medical intervention and the use of antibiotics may be necessary. This form of listeriosis does not pose a serious public health threat, as most individuals recover without major complications.

On the other hand, individuals with underlying health conditions or weakened immune systems (vulnerable groups) are at higher risk of developing invasive listeriosis. This form is more severe and can lead to serious symptoms such as high fever, muscle pain, bloodstream infection (sepsis), and inflammation of the brain and spinal cord (meningitis). According to WHO's 2018 report, the mortality rate for invasive listeriosis is estimated to be between 20% and 30%.

Vulnerable groups include individuals with specific health conditions or those who are in situations that put them at higher risk. These groups include:

• Pregnant women: According to WHO in 2018, pregnant women are approximately 20 times more likely to contract listeriosis than other healthy adults. The complications of this disease during pregnancy can be severe and may result in miscarriage or stillbirth.
• Newborns: Due to their immature immune systems, newborns are also at higher risk of contracting listeriosis. This disease can lead to complications such as low birth weight, bloodstream infections (sepsis), or inflammation of the brain and spinal cord (meningitis).
• Older adults (over 65 years): As people age, their immune systems naturally weaken, which increases the risk of invasive listeriosis.
• People with HIV: Individuals with HIV and other immune-suppressing conditions are at higher risk due to impaired immune function. According to WHO’s 2018 report, people with HIV are about 300 times more likely to contract listeriosis than healthy individuals.

Generally, the symptoms of listeriosis usually appear a few days to a few weeks after consuming contaminated food. However, in some cases, symptoms may take up to 70 days to appear. This long incubation period can make it more difficult to identify the source of contamination. Additionally, in some instances, the initial mild symptoms may be mistaken for more common illnesses like the flu or a cold. These factors can delay the diagnosis, leading to the disease progressing to more advanced and dangerous stages.

Listeria monocytogenes, in addition to being transmitted through the consumption of contaminated food (the most common route of transmission to humans), can also be transmitted from a pregnant mother to her fetus. This transmission can lead to severe complications, such as miscarriage, premature birth, or serious neonatal infections.

In rare cases, direct contact with infected animals or animal products, especially for those working in the livestock industry, can also lead to transmission of the bacterium to humans. Household pets may, on rare occasions, serve as carriers of the bacterium and, though the risk is limited, there is a possibility of transmission to humans.

Mechanism of Pathogenesis of Listeria monocytogenes

When a person consumes food contaminated with Listeria monocytogenes, the bacterium enters the digestive system through the mouth. Initially, it reaches the stomach, where the acidic environment kills most bacteria. However, due to its ability to withstand harsh conditions, Listeria can pass through the stomach and reach the small intestine.

In the small intestine, Listeria encounters the epithelial cells lining the intestinal walls. These cells act as the first line of defense, preventing harmful agents from entering the body. Listeria, however, employs a specialized mechanism to breach this barrier. It uses proteins that act like "keys" to bind to specific "locks" or receptors on the surface of these epithelial cells, allowing it to enter.

Once inside the epithelial cells, Listeria is initially enclosed in a protective vesicle. These vesicles are typically designed to contain and destroy foreign particles. However, Listeria uses a specific protein to form pores in the vesicle and escape, entering the cytoplasm of the host cell. Inside the cytoplasm, Listeria evades the cell's defense system and begins multiplying using nutrients such as sugars, amino acids, and fats.

After replication, Listeria uses another protein to form actin filaments, which act like tails, pushing against the cell membrane. This pressure causes the cell membrane to bulge and form a bridge-like protrusion between two cells. This mechanism allows Listeria to move from one cell to another without being exposed to the immune system, rapidly spreading within the intestinal tissue.

Once Listeria has multiplied sufficiently within the intestinal epithelial cells, it uses proteins to penetrate deeper layers of the intestinal tissue, including the lymphatic and connective tissues (which consist of a network of blood vessels and lymph nodes). These tissues are responsible for collecting excess fluid from the tissues and transporting immune cells. After entering the lymphatic system, Listeria can spread into the bloodstream. The bloodstream serves as a highway, transporting the bacterium to various organs such as the liver, brain, and, in pregnant women, the placenta.

When Listeria enters the bloodstream, immune cells attempt to capture and destroy the bacterium. Immune cells engulf Listeria and activate special enzymes to break it down. However, Listeria produces proteins that help it evade this process and enter the interior of the immune cells, where it remains hidden from immune attacks.

Once Listeria reaches vital organs like the liver, brain, and placenta (in pregnant women), it can cause severe infections. In the liver, these infections may lead to abscesses and severe inflammation. In the brain, Listeria can cause meningitis, a dangerous infection of the brain membranes, which leads to symptoms such as fever, seizures, and even death. In pregnant women, Listeria can reach the fetus, causing serious complications such as miscarriage or preterm birth.

Measures to Prevent Infection with Listeria Monocytogenes

To prevent contamination caused by Listeria monocytogenes, maintaining hygiene and safety at all stages of the food chain, from production to consumption, is essential. This bacterium is usually destroyed through processes such as pasteurization and thorough cooking, but if hygienic standards are not observed during production, processing, and storage, the risk of its growth and multiplication still exists. Proper management and safe storage of food, especially regarding products susceptible to contamination, plays a key role in preventing the transmission of this bacterium and protecting the health of consumers.

Below are some important points regarding the consumption of these food products:

1. Keeping Clean: This means that surfaces, utensils, and hands should be regularly washed and kept clean to prevent food contamination. This is particularly important after handling raw food.

2. Keeping Raw and Cooked Foods Separate: Raw foods such as meat and vegetables should be stored separately from cooked or ready-to-eat foods to prevent bacteria from transferring to cooked foods.
3. Cooking Food Thoroughly: Foods, especially meats, poultry, and seafood, should be thoroughly cooked. This ensures that bacteria are effectively eliminated.
4. Storing Food at Safe Temperatures: Safe temperatures refer to storing food in the refrigerator at about 4°C and freezing food at around -18°C. This slows or halts bacterial growth.
5. Using Safe Water and Ingredients: Food and water should be sourced from safe and hygienic supplies to prevent contamination with bacteria.
6. Avoid Consuming Unpasteurized Milk and Dairy Products: These products may contain Listeria because they are unpasteurized, and even if these products are kept in the refrigerator, the risk of bacterial growth remains.
7. Heating Processed Meat Products Before Consumption: Processed meat products such as sausages and cold cuts, which are bought in bulk or whose packaging has been opened and stored in the refrigerator, should be heated before consumption until steam comes out. This kills any potential bacteria.
8. Avoid Consuming Prepackaged Salads: Due to the raw nature of prepackaged salads and the potential for cross-contamination in the refrigerator, these products can become contaminated with Listeria. Since there is no possibility of heating and killing the bacteria in these products, it is better to avoid consuming them.
9. Washing Fruits and Vegetables: For washing fruits and vegetables, first remove any damaged or soft spots, and then wash hands for 20 seconds with warm water and soap to eliminate contaminants and bacteria. Ensure that kitchen surfaces, sinks, and tools such as cleaning brushes are clean to prevent cross-contamination. Fruits and vegetables should be washed under running water, and for more thorough bacterial removal when washing firm fruits like melons and apples, a brush should be used. For softer fruits like peaches, they should be gently washed by hand. Although washing with running water is generally sufficient according to health organizations like the FDA, for added security, a solution of water and vinegar or lemon water can be used. To do so, mix one cup of white vinegar or half a cup of lemon juice with three cups of water, and soak the fruits and vegetables in it for 10 to 15 minutes. After this step, rinse the fruits and vegetables under running water to remove any remaining vinegar or lemon juice. Finally, dry the fruits and vegetables with a clean towel to remove excess moisture and prevent the creation of an environment suitable for bacterial growth. Then, store the fruits and vegetables in clean containers or bags in the refrigerator.

Following these tips is essential for maintaining health and should be followed by everyone. However, sensitive groups should observe these precautions more carefully as they are at greater risk.