Introduction

Listeria monocytogenes is a pathogenic bacterium that can cause severe diseases, termed listerioses. The bacterial infection is contracted via the ingestion of food contaminated with Listeria. Although cases of listeriosis are relatively low worldwide, they remain a public health concern due to the challenges in controlling and maintaining the pathogen.

Additionally, the bacterium is of high concern for high-risk groups, including the elderly, pregnant women, and those with weakened immune systems.⁵ It is important to understand the characteristics and virulence factors of Listeria monocytogenes for the prevention, treatment, and diagnosis of listeriosis. Hence, this article will explore the characteristics of Listeria monocytogenes and its pathogenic capabilities, which render it a public health concern.

What is Listeria monocytogenes?

Listeria monocytogenes is a gram-positive, rod-shaped bacterium that belongs to the genus Listeria. It is non-spore-forming and has a uniform shape. The bacterium is categorized as an aerobic pathogen, meaning it uses oxygen for metabolic processes. However, it can also grow and survive without oxygen by switching to fermentation, making it a facultative anaerobe.⁶

It can grow in high concentrations of salt (NaCl) and in a wide range of temperatures, from around 1°C to 45°C.⁷ This means it can grow from refrigerator temperatures to high room temperatures. Consequently, the bacterium tends to be found in raw foods, uncooked meats, and unpasteurized dairy items stored within these temperature ranges, as it can multiply under these conditions.

Transmission of Listeria in humans

Listeria monocytogenes transmission can occur in two ways:

1. Faeco-oral: Through the ingestion of food contaminated with the bacteria. Examples include raw foods, uncooked meats, and unpasteurized dairy products.⁹ It is vital to wash your hands after handling raw foods or after going to the toilet
2. Mother to foetus: Through transplacental transmission in the womb or through vaginal transmission during birth⁸

Listeriosis can cause illness in anyone within a population. However, groups at higher risk of listeriosis include those with weakened immune systems, such as pregnant women, neonates (infants), the elderly, or those with compromised immune systems due to underlying health issues, for example, individuals with HIV.

Clinical presentation of listeriosis

Clinical features of listeriosis by Listeria monocytogenes include gastroenteritis, septicaemia, meningitis and spontaneous abortion.

Overall common signs and symptoms include:

• Gastroenteritis: Diarrhoea, fever, stomach cramps and abdominal pain. In neonates nodular skin rash days after birth, vomiting, lethargy⁹
• Meningitis: Fever, stiffness, headaches, confusion and loss of balance¹¹
• Listeriosis in pregnancy: abortion and premature birth⁸ 
• Septicemia: fever, chills, vomiting, muscle and joint pain, diarrhoea¹⁰ 

Virulence factors of Listeria monocytogenes

Listeria monocytogenes most often enters the body through the consumption of foods containing the bacteria. When exposed to gastric acid, proteolytic enzymes, or bile salts, the bacteria use specific genes termed stress response genes to bind to the host cells (the human's own cells).

Virulence factors are molecules, structures, or genes that promote bacterial invasion into host cells, allowing them to spread and cause disease. The growth conditions of Listeria monocytogenes are an important virulence factor, enabling the bacteria to survive at low temperatures outside the host body.^{7,13,14,15,16} 

Diagnosis of listeriosis

After seeking medical advice for the signs and symptoms of listeriosis, laboratory testing will be required. This is important to identify the specific bacteria involved, as the symptoms of listeriosis can overlap with those of various other bacterial diseases. Identifying the specific bacteria allows for the correct treatments to be administered, which is crucial for achieving the best health outcome and clearing the infection.

Laboratory detection

Bacterial culture is a technique used to diagnose a Listeria monocytogenes associated infection.¹ This is done with bacterial culture which grows Listeria monocytogenes from a sample of body tissue or fluid from a patient who has been suspected to harbour a bacterial infection. The sample could include blood, spinal cord (synovial) fluid or placenta.

Molecular methods such as PCR can also be used to detect listeria monocytogenes.¹ DNA is extracted from a patient sample from either blood, placenta or spinal fluid, and amplified using PCR to enable detection and identification of the specific bactrial strain that is present. 

Treatment and prevention

Due to Listeria monocytogenes being a bacterium, infection can be treated with antibiotics.¹⁷ Common antibiotics prescribed include:

• Ampicillin
• Gentamicin
• Trimethoprim-sulfamethoxazole 

Although listeriosis can be treated using antibiotics, increased usage of these drugs increases the risk of antibiotic resistance. Therefore, the best method to avoid infection involves preventative measures. 

Key preventative measures to follow are food safety practices. As the bacteria causes foodborne illness it is important that any bacteria living on our food is destroyed before being ingested. This is why washing, cooking, storing and inspecting our food properly is important to prevent consumption of contaminated food. 

Another practice for prevention is to avoid high-risk foods which could include unpasteurised dairy products and deli/street meats which are more prone to carry bacteria.¹⁸ Spotting symptoms of listeriosis early can help in treating the bacterial infection before it can cause severe disease and prevent fatal health outcomes of both individuals and their unborn children.

Antibiotic resistance

Antibiotic resistance occurs when bacteria develop virulence factors that enable them to overcome the action of antibiotics, rendering these treatments ineffective. Antibiotic-resistant strains of Listeria monocytogenes have been identified, showing resistance to ampicillin, a commonly used antibiotic for listeriosis.³ 

This is a significant issue because if the antibiotic cannot treat the infection, it can lead to severe disease with potentially fatal outcomes or the loss of an infant. This highlights the need for diagnostic testing to identify and prescribe the appropriate antibiotic for the bacteria. Overuse and misuse of antibiotics are the main reasons leading to antibiotic resistance, so antibiotics should only be taken when necessary.

Currently, no listeriosis vaccine is readily available for human use to prevent and target Listeria monocytogenes infections. However, there have been studies on vaccines that target key virulence factors to help prevent the spread of Listeria monocytogenes both intracellularly and extracellularly.⁴

Targeting virulence factors is key to vaccine production, as these are the characteristics that allow the bacteria to survive and multiply. A vaccine could be particularly useful for high-risk groups who do not have strong enough immune systems to fight off infections caused by this foodborne bacteria.

Summary

Listeria monocytogenes is a unique bacterium that can invade a host's cells and use its virulence factors to survive, replicate, and spread within the host. These virulence factors enable it to withstand a wide range of temperatures, allowing it to survive outside the host and contaminate our food. Fortunately, antibiotics have been developed to treat listeriosis caused by Listeria monocytogenes.

However, with the rise of antibiotic resistance, it is important to minimize the use of antibiotics. Therefore, to prevent listeriosis, it is crucial to follow food safety and hygiene procedures, especially for those who are pregnant or have weakened immune systems. Research could lead to a future vaccine being developed to help reduce the use of antibiotics and lessen the burden of disease caused by the foodborne bacterium Listeria monocytogenes.