Listeria monocytogenes is a type of bacteria that can cause a serious foodborne illness called listeriosis. While relatively uncommon, listeriosis can lead to severe health complications, particularly for certain groups of people.
Understanding Listeria
Listeria monocytogenes is classified as a “gram-positive” bacterium, which refers to how it appears under a microscope after a specific staining procedure. This characteristic helps scientists identify the bacterium in laboratory settings. Unlike many other foodborne pathogens, Listeria has a remarkable ability to survive and even grow in cold temperatures, including those found in refrigerators.
The bacterium can also tolerate a wide range of environmental conditions, such as varying pH levels and moderate salt concentrations. This resilience allows Listeria to persist in food production facilities and on various surfaces, making it a persistent concern for food safety. Its ability to grow at temperatures as low as 31 degrees Fahrenheit highlights the importance of proper refrigeration not just for preventing spoilage, but also for slowing bacterial growth.
Sources of Contamination
Listeria monocytogenes is commonly found in environments like soil, water, and decaying vegetation. Contamination often occurs during food harvesting, processing, preparation, or storage. This bacterium can thrive in unsanitary conditions within food production environments. Cross-contamination can also occur when contaminated food comes into contact with other foods or surfaces in the kitchen.
Certain food categories are frequently associated with Listeria contamination:
Unpasteurized milk and dairy products, particularly soft cheeses like feta, brie, and queso fresco.
Deli meats, such as cold cuts and hot dogs.
Smoked seafood.
Raw produce, including sprouts, if grown in contaminated soil or washed with contaminated water.
Listeriosis: Symptoms and Vulnerable Populations
Listeriosis symptoms can vary widely, ranging from mild to very severe, depending on the individual and the form of the illness. In less severe cases, known as non-invasive gastrointestinal listeriosis, symptoms may include fever, muscle aches, nausea, vomiting, and diarrhea, typically lasting one to three days. These symptoms can appear anywhere from a few hours to two or three days after consuming contaminated food, though in some instances, they may take up to two months to manifest.
The more serious form, invasive listeriosis, occurs when the bacteria spread beyond the intestines and can lead to life-threatening conditions. Symptoms of invasive listeriosis may include headache, stiff neck, confusion, loss of balance, and convulsions, often accompanied by fever and muscle aches. This severe form can result in conditions like meningitis or septicemia. Hospital fatality rates for invasive listeriosis can range from 20% to 30%.
Certain populations are particularly susceptible to severe listeriosis. Pregnant women are about 20 times more likely to contract listeriosis than healthy adults. While they may experience only mild, flu-like symptoms such as fever, fatigue, and muscle aches, the infection can have devastating consequences for the fetus, including miscarriage, stillbirth, or premature delivery, or a life-threatening infection of the newborn. Newborns, adults over 65, and individuals with weakened immune systems, such as those with cancer, diabetes, HIV/AIDS, or organ transplants, are also at high risk for serious complications and death. Healthy individuals generally have a low risk of severe illness from Listeria.
Preventing Infection
Preventing Listeria infection involves adhering to several food safety practices at home. Proper cooking temperatures are important; for example, deli meats and hot dogs should be heated to an internal temperature of 165 degrees Fahrenheit. Avoiding cross-contamination is also important, which means separating ready-to-eat foods from raw meats and their juices.
Thorough washing of produce, such as fruits and vegetables, under running water before eating, cutting, or cooking is a simple yet effective step. It is also important to clean and sanitize any surfaces that food may touch, including countertops and refrigerators. Regularly wiping up spills and cleaning the refrigerator can help prevent Listeria growth, as the bacterium can thrive in cool, damp environments.
Adherence to “use-by” dates on perishable foods is also a practical measure, as the longer contaminated refrigerated foods are stored, the more opportunity there is for Listeria to grow. For high-risk groups, specific foods should be avoided:
Unpasteurized milk and products made from it.
Soft cheeses.
Raw sprouts.
Refrigerated smoked seafood that isn’t canned or fully cooked.
Washing hands with soap and warm water for at least 20 seconds before and after handling food is a foundational practice to reduce the risk of transmission.