Listeria monocytogenes is a bacterium found widely in the environment that can cause a severe foodborne illness known as listeriosis. This organism represents a significant public health concern because of its ability to cause serious disease, particularly in susceptible individuals.
The Bacterium and Listeriosis
Listeria monocytogenes is a rod-shaped bacterium that is classified as Gram-positive and facultative anaerobic, meaning it can survive with or without oxygen. A distinguishing characteristic of this bacterium is its ability to grow and multiply at temperatures as low as 0°C (32°F), which includes typical refrigeration temperatures. This particular trait significantly increases its potential to contaminate refrigerated foods and evade common control measures.
The infection caused by Listeria monocytogenes is called listeriosis. This bacterium is considered an intracellular pathogen because it can enter and reproduce inside the host’s cells. Once inside a host cell, it can move rapidly by manipulating the host’s actin filaments, allowing it to spread to neighboring cells and even into the bloodstream or nervous system. This mechanism contributes to the severity of the illness.
Common Sources of Infection
Listeria monocytogenes is commonly found in soil, water, and decaying vegetation, and can also be present in the feces of healthy animals. Humans typically become infected by consuming foods contaminated with the bacteria. This bacterium can also persist in food processing environments, making it difficult to eliminate once established.
Many foods have been linked to listeriosis outbreaks. These include unpasteurized milk and dairy products, such as soft cheeses like Brie, Camembert, feta, and queso fresco. Deli meats, hot dogs, smoked seafood, and raw or processed fruits and vegetables, like cantaloupe, have also been identified as common sources of contamination. Cross-contamination can occur in kitchens if Listeria from raw foods spreads to ready-to-eat foods or surfaces.
Who is Most Vulnerable and Symptoms
Listeriosis can present with a range of symptoms, from mild to severe, depending on the individual and whether the infection remains localized or spreads throughout the body. Less severe forms might cause flu-like symptoms such as fever, muscle aches, nausea, vomiting, and diarrhea, which typically resolve within a few days. However, if the infection spreads beyond the gut, it can lead to more serious and potentially life-threatening conditions.
When Listeria spreads to the nervous system, symptoms can include headache, stiff neck, confusion, loss of balance, and convulsions. Certain populations face a much higher risk of developing severe listeriosis. These groups include pregnant women, older adults (generally 65 years and older), and individuals with weakened immune systems due to conditions like cancer, HIV/AIDS, organ transplants, or certain medications.
Pregnant women are approximately 10 to 20 times more likely to contract listeriosis than other healthy adults. While the pregnant individual may experience only mild, flu-like symptoms, the infection can have devastating consequences for the unborn baby, including miscarriage, stillbirth, premature birth, or severe, life-threatening infection of the newborn. Older adults and immunocompromised individuals are more susceptible because their immune systems are less effective at fighting off infections, allowing the bacteria to grow and spread more easily.
Practical Prevention Strategies
Preventing Listeria infection involves diligent food handling practices and careful dietary choices, especially for those at higher risk. One fundamental step is to maintain proper refrigeration temperatures; while Listeria can grow in cold environments, keeping refrigerators at 40°F (4°C) or below and freezers at 0°F (-18°C) or below helps slow its growth. Using a refrigerator thermometer can ensure these temperatures are consistently met.
Thoroughly cooking raw meat and poultry to their safe internal temperatures helps eliminate the bacteria. It is also important to avoid unpasteurized dairy products entirely. High-risk foods like deli meats and hot dogs should be reheated until steaming hot before consumption. Washing fruits and vegetables thoroughly under running water, even if they will be peeled, helps remove surface contaminants.
Preventing cross-contamination is also important; this includes using separate cutting boards and utensils for raw meats and ready-to-eat foods, and washing hands with soap and warm water before and after handling food. Regularly cleaning and sanitizing kitchen surfaces, especially the refrigerator, and promptly wiping up spills, can further reduce the risk of bacterial spread. At-risk individuals should consider avoiding certain high-risk foods, such as soft cheeses made with unpasteurized milk and refrigerated smoked seafood, unless cooked.
Medical Diagnosis and Treatment
Diagnosing listeriosis typically involves laboratory tests to identify the presence of Listeria monocytogenes. Healthcare providers may collect samples of blood, cerebrospinal fluid (from a spinal tap), or amniotic fluid (in pregnant individuals) for bacterial culture. Isolating the bacteria from these samples confirms the diagnosis.
Once diagnosed, listeriosis is treated with antibiotics. Ampicillin, often in combination with gentamicin, is a common treatment choice. Early diagnosis and prompt antibiotic treatment are important, particularly for high-risk individuals, as this can improve outcomes and reduce the potential for severe complications.