Listeria & Listeriosis: Symptoms, Testing, and Prevention

We previously examined the causes and impacts of microbiological food contamination, which affects millions of Americans every year. While E. coli and Salmonella are two of the most commonly recognized (and contracted) foodborne illnesses, there is another pathogen that has grown in recognition and transmission since its discovery in 1940: Listeria.

The Centers for Disease Control and Prevention (CDC) estimates that Listeria is the third leading cause of death from food poisoning in the United States. On average, 1,600 people get sick from Listeria each year, and about 260 die. Newborn babies, the elderly, and the immunocompromised are at the highest risk.

What is Listeria?

The genus Listeria includes fifteen different species. The most important of which for the food industry and public health is Listeria monocytogenes, which is transmitted through contaminated food and results in the disease known as listeriosis. While many species of Listeria can be found in foods and in the food processing environment, Listeria monocytogenes is the only species known to cause illness in humans.

Listeria monocytogenes is a gram-positive, rod-shaped bacterium that is remarkably resilient. Its ability to grow at refrigeration temperatures makes it a significant concern for ready-to-eat (RTE) foods.

Common Listeriosis Symptoms

According to the CDC, people who contract listeriosis usually report symptoms 1 to 4 weeks after eating contaminated food. In rare cases, listeriosis symptoms have started as early as the same day of exposure or as late as 70 days after exposure.

Typical Symptoms (Non-Pregnant Individuals):

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

Symptoms in Pregnant Women: Pregnant women typically experience only fever and other flu-like symptoms, such as fatigue and muscle aches. However, infections during pregnancy can lead to severe outcomes for the fetus or newborn, including: Miscarriage, Stillbirth, Premature delivery, or Life-threatening infection of the newborn.

Who is at risk?

While Listeria can affect anyone, certain groups are at significantly greater risk of developing severe illness.

• Pregnant Women: Pregnant women are 10 times more likely to contract listeriosis than the general population. Pregnant Hispanic women, in particular, face an even higher risk, being 24 times more likely to get a Listeria infection.
• Newborns: Infections in newborns can be severe and life-threatening, often resulting from transmission during pregnancy or childbirth.
• Older Adults: More than half of all Listeria infections occur in people over the age of 65. This is because their immune systems are less robust and less able to recognize and rid the body of harmful pathogens as quickly as those under 65. Older adults who take medications to treat chronic conditions such as cancer or diabetes are at even greater risk because these medications can further weaken the immune system.
• People with Weakened Immune Systems: Individuals with medical conditions that impact the immune system, including cancer (especially those undergoing chemotherapy), diabetes, liver or kidney disease, alcoholism, HIV, and AIDS, are also at a higher risk of listeriosis. Treatments and medications for these conditions often include steroids or chemotherapy, both of which decrease the body’s ability to fight illness.

Where does Listeria grow?

Listeria is a particularly resilient bacterium that thrives in moist, humid environments. It also stands apart from other foodborne pathogens because it can grow at refrigerator temperatures (33-40° F / 0.5-4° C) and even survive at temperatures below freezing (32° F / 0° C). This characteristic makes it a significant challenge for food safety, especially for refrigerated ready-to-eat foods.

Refrigerated foods that do not have a secondary barrier, such as low moisture or low pH, have the potential to support the growth of Listeria monocytogenes. Listeria is only killed through proper cooking and pasteurization.

Food Products Commonly Associated with Listeria Growth and Outbreaks:

• Ready-to-Eat (RTE) Deli Meats and Hot Dogs: Even if pre-cooked, cross-contamination can occur after cooking but before packaging.
• Soft Cheeses: Especially those made from unpasteurized milk (e.g., Feta, Brie, Camembert, blue-veined cheeses).
• Unpasteurized Milk and Dairy Products: Raw milk and products made from it.
• Refrigerated Smoked Seafood: Such as smoked salmon.
• Pâtés and Meat Spreads: Refrigerated varieties.
• Raw Sprouts: Such as alfalfa, clover, radish, and mung bean sprouts.
• Fresh Produce: Contamination can occur in the field or during processing. Recent outbreaks have been linked to cantaloupe, leafy greens, and other fruits and vegetables.

Testing for Listeria

Detecting Listeria is critical for food safety and preventing outbreaks. Laboratories utilize various methods, ranging from traditional culture-based techniques to rapid molecular diagnostics.

Traditional Laboratory Methods

The standard laboratory method for determining if Listeria species are present in a food sample includes:

1. Enrichment Step: Food samples are incubated in a selective broth to encourage the growth of Listeria while suppressing other bacteria.
2. ELISA (Enzyme-Linked Immunosorbent Assay): If a sample is found to be positive using the ELISA method, which screens for the presence of Listeria antigens.
3. Selective Agar Plating: The enrichment sample is streaked on selective agar plates to isolate individual colonies.
4. Biochemical Testing: Suspect colonies are subjected to biochemical tests to confirm the species. The biochemical profile can be used to determine which species are present in the sample.

This process is labor-intensive and requires an experienced analyst who can interpret the samples to get accurate results, which can take up to one week.

Advanced and Rapid Technologies

New technologies have significantly reduced the time and complexity of Listeria detection, allowing for quicker intervention and enhanced food safety.

• PCR (Polymerase Chain Reaction) Systems: Systems like the BAX® System (or similar PCR-based platforms) can detect the presence of Listeria in less than 3 days. PCR technology amplifies specific DNA sequences of the bacteria.
• Advantage: While traditional ELISA methods often only screen for the genus Listeria, PCR systems can screen for both the genus Listeria and specifically for Listeria monocytogenes.
• Application: In most situations, just confirming that Listeria monocytogenes is present in the environment or in a food sample is sufficient to warrant intervention through cleaning, sterilization, and destruction of the food product.
• Whole Genome Sequencing (WGS) and Ribotyping: For outbreak investigations, it is often useful to determine the genetic fingerprint of the Listeria strain.
• Ribotyping (e.g., Qualicon RiboPrinter™): Can rapidly determine a genetic fingerprint for a number of bacterial species, including Listeria, to aid in investigating the source of a specific Listeria strain and prevent large outbreaks.
• Whole Genome Sequencing (WGS): This cutting-edge technology provides the most detailed genetic information, allowing scientists to precisely track the source of an outbreak, identify related strains, and understand the pathogen's characteristics with unprecedented accuracy. WGS has become the gold standard for foodborne pathogen surveillance and outbreak investigation by agencies like the CDC and FDA.

Food Safety Testing: Our lab services include Listeria tests. View our food safety tests to see Medallion Labs' capabilities.

Prevention Strategies for Food Manufacturers

Preventing Listeria contamination is paramount for food manufacturers. A robust food safety program is essential.

• Good Manufacturing Practices (GMPs): Strict adherence to hygiene, sanitation, and operational controls in processing environments.
• HACCP (Hazard Analysis and Critical Control Points): Implementing a comprehensive HACCP plan to identify and control Listeria hazards at critical points in the production process.
• Environmental Monitoring Programs (EMP): Regular testing of food contact and non-food contact surfaces in the processing environment to detect Listeria before it contaminates products. This is crucial for identifying harborage sites.
• Sanitation Standard Operating Procedures (SSOPs): Effective cleaning and sanitization protocols, especially for equipment that is difficult to clean or prone to harboring bacteria.
• Raw Material Control: Sourcing ingredients from approved suppliers with strong food safety programs.
• Temperature Control: Maintaining proper refrigeration temperatures throughout the cold chain, as Listeria can grow at low temperatures.
• Employee Training: Ensuring all employees understand the risks of Listeria and proper hygiene practices.
• Product Formulation: For some products, adjusting pH or water activity can inhibit Listeria growth.

Listeria is a dangerous pathogen that can cause serious health risks if left unchecked. Luckily, the risk can be reduced significantly with proper food handling, packaging, and production. If you’re concerned about the presence of Listeria in a food sample, or curious about the testing process, connect with Medallion Labs to speak with our team of expert analysts.