Microorganisms - Listeria

What is Listeria?

Listeria monocytogenes is a gram-positive, rod-shaped, bacterium that is microscopic in appearance. First observed in scientific literature in the 1920s, it's been widely known among microbiologists since the 1960s. This species of pathogenic bacteria can often be found in moist conditions, including soil, surface water, and decaying vegetation. Salt-tolerant and able to both withstand and grow in temperatures below 1°C, L. monocytogenes is a hardy pathogenic organism. Its flagella - hair-like appendages protruding out of its cell structure - allow it to swim through a variety of hosts, though at a warmer and narrower temperature band.

Vulnerable Populations and Severe Outcomes

Listeria monocytogenes is not a leading cause of foodborne illness, however, it is among the leading causes of death from foodborne illness. The U.S. Centers for Disease Control and Prevention (CDC) estimates that, in the U.S., domestically acquired foodborne L. monocytogenes results in 260 fatalities per year. These numbers are down from figures seen prior to the turn of the century, when the CDC estimated approximately 500 such deaths in the United States annually.

While L. monocytogenes can affect anyone, certain groups are particularly vulnerable to severe illness or death following exposure. These groups include:

• Pregnant Women and Newborns: Pregnant women are at an increased risk of contracting Listeria, which can lead to miscarriage, stillbirth, or severe illness in newborns. Even if the mother experiences only mild symptoms, the infection can have devastating consequences for the fetus or infant.
• Elderly Individuals: As people age, their immune systems weaken, making them more susceptible to severe infections, including Listeria. The high mortality rate among elderly individuals underscores the seriousness of Listeria outbreaks in this population.
• Immunocompromised Individuals: Those with weakened immune systems, such as cancer patients, people living with HIV/AIDS, or individuals on immunosuppressive drugs, are at a heightened risk of severe listeriosis. For these individuals, the disease can quickly escalate to life-threatening conditions, such as meningitis or septicemia.

Outbreak Threats

Listeria can contaminate a broad spectrum of foods, making it a versatile and dangerous pathogen. Unlike some bacteria that thrive only in specific types of food, Listeria monocytogenes can be found in a variety of products, including:

• Ready-to-Eat (RTE) Foods: These foods are particularly vulnerable because they are often consumed without further cooking, which would normally kill bacteria. Examples include deli meats, hot dogs, and soft cheeses like Brie and Camembert.
• Dairy Products: Unpasteurized milk, cheese, and ice cream are at higher risk of contamination. Even pasteurized dairy products can become contaminated after processing if they come into contact with contaminated surfaces or equipment.
• Produce: Fresh fruits and vegetables can harbor Listeria, particularly when they encounter contaminated soil, water, or equipment. This risk increases when produce is pre-cut or processed, as the pathogen can spread during handling.
• Seafood: Smoked fish and other seafood products, especially when consumed raw or undercooked, can be a source of Listeria.

Most experts agree that post-processing contamination from food-contact surfaces represents the clearest and most present danger, and surfaces can harbor the organism for years. Several high-profile L. monocytogenes outbreaks over the past decade have plagued food companies and their consumers, drawing more concern and attention from inspectors, the media, and the general public. Some have dubbed the swelling interest in this pathogen as a "Listeria hysteria."

Advances in Detection and Prevention

One of the most critical advancements in combating Listeria has been the development of rapid detection methods. To assist these vital efforts, food producers count on technology to potentially grow selective organisms like Listeria monocytogenes to detectable levels, while simultaneously preventing less harmful microorganisms from also growing and interfering with accurate testing. Then, food producers must also evaluate its presence and prevalence. Traditional methods of culturing and identifying L. monocytogenes can take days, during which contaminated products might already be in the hands of consumers. The introduction of rapid-test kits, which can provide results within hours, has revolutionized the food industry's ability to respond quickly to potential contamination events.

Some advanced systems can even differentiate between different species of Listeria and identify particularly dangerous strains. This allows for more precise and timely interventions, which help reduce the risk of widespread outbreaks.

In addition to detection, preventive measures have become more sophisticated. Adopting Hazard Analysis and Critical Control Points (HACCP) protocols in food processing facilities has been crucial in identifying and mitigating risks associated with Listeria. Facilities are now equipped with better sanitation technologies, and there is a stronger emphasis on employee training to prevent contamination.

Rapid-test kits for Listeria species as well as the Listeria monocytogenes pathogen have been among the fastest growing tests in the food industry. As global regulatory requirements have changed and become more stringent, the demand for Listeria testing has dramatically increased. In addition, as the food industry has become more international and increasingly competitive, processors want results that can be delivered faster.

Awareness and Responsibility

Certainly, control of Listeria species, including Listeria monocytogenes, is vital. Food processors shouldn't let their guard down to test for this persistent pathogen, methodically sampling end-product and key areas within food production environments as well as other food contact and non-food contact surfaces in plants that could harbor Listeria, is key to preventing contamination. They must also be mindful of the fact that L. monocytogenes can grow in refrigerated temperatures.

While food producers and regulatory bodies play a crucial role in controlling Listeria, consumers also have a part to play. Awareness of the risks associated with certain high-risk foods, such as unpasteurized dairy products and ready-to-eat meats, is essential. Consumers should be educated about safe food handling practices, including proper refrigeration, cooking to appropriate temperatures, and avoiding cross-contamination in the kitchen.

Learn more about pathogen testing.

References

About Listeria Infection, U.S. Centers for Disease Control and Prevention.

Listeria (Listeriosis), U.S. Food and Drug Administration.

Listeria Monocytogenes, National Library of Medicine, National Center for Biotechnology Information.

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