Listeria monocytogenes is found in soil, water, animals, and a wide range of foods, especially ready-to-eat products that aren’t reheated before eating. What makes this bacterium particularly dangerous is its ability to grow at refrigerator temperatures as low as 1°C (about 34°F), meaning cold storage slows it down far less than it does other foodborne pathogens.
Soil, Water, and Decaying Vegetation
The bacterium’s primary natural habitat is soil and decaying plant material. It thrives in moist, organic-rich environments and has been isolated from agricultural land, forests, and urban green spaces worldwide. Soil acts as a long-term reservoir, harboring strains that can persist for extended periods and eventually reach the food chain through crops, grazing animals, or water runoff.
Rivers and streams are another major reservoir. Research in Europe found that strain diversity and detection rates of Listeria monocytogenes spiked during river flooding events, showing how watercourses can rapidly disperse the bacterium across wide areas. Urban waterways are especially prone to contamination because of wastewater effluents, polluted stormwater runoff, and agricultural drainage. From there, the pathogen can reach irrigation systems and, ultimately, the fresh produce you eat.
Livestock and Wildlife
Many healthy farm animals carry Listeria monocytogenes in their intestines and shed it in their feces without showing any signs of illness. Dairy cattle are the most common carriers: one study in Northern Spain found that about 24% of individual dairy cows were actively shedding the bacterium, compared to roughly 8% of beef cattle and just 1.5% of sheep. Interestingly, the same study never detected it in pig herds.
This fecal shedding is one of the key ways the bacterium cycles between animals, pastures, and water sources. Grazing ruminants deposit it onto fields, where it re-enters the soil, gets washed into streams, or contaminates crops grown on fertilized land. Wildlife feeding near agricultural areas contributes to the same cycle.
Raw Milk and Dairy Products
Given how common the bacterium is in dairy cattle, it’s no surprise that raw milk is a significant source. A large global meta-analysis found Listeria monocytogenes in about 3.4% of raw milk samples. Pasteurization is highly effective at killing it, dropping prevalence to around 0.6% in pasteurized milk. That remaining fraction typically reflects contamination that happens after pasteurization, during packaging or handling, rather than survival through the heat treatment itself.
Soft cheeses made from raw or inadequately pasteurized milk (such as brie, camembert, and queso fresco) have been linked to multiple outbreaks over the years. The moist, relatively low-acid environment of these cheeses provides conditions where the bacterium can multiply during aging and storage.
Ready-to-Eat Meats and Deli Products
Deli meats, hot dogs, and smoked fish are among the highest-risk foods for Listeria contamination. The critical point isn’t the initial cooking, which kills the bacterium. The problem is what happens afterward. During slicing, packaging, and handling in processing facilities, cooked products can pick up Listeria from equipment surfaces, particularly slicing machines, conveyor belts, and cooling zones. Because these foods are typically eaten without further heating, any bacteria introduced after cooking can reach the consumer alive.
Proper separation between raw meat areas and post-cooking zones (cutting rooms, packing rooms, delivery corridors) is one of the most important controls in a processing plant. When that separation breaks down, cross-contamination follows.
Fresh Produce and Frozen Foods
Leafy greens, melons, and stone fruits can become contaminated through irrigation water, soil contact, or compost containing the bacterium. Agricultural water picks up Listeria from sewage overflows, storm runoff, and nearby livestock operations. Once contaminated water touches a plant, the bacteria can adhere to surfaces, and in some cases enter the plant tissue itself, making them impossible to wash off completely. A 2011 cantaloupe outbreak in the United States caused 147 illnesses and 33 deaths, one of the deadliest foodborne outbreaks in recent American history.
Frozen foods are not safe from Listeria either. The bacterium survives freezing temperatures readily. A 2025 FDA investigation traced an outbreak of 42 infections, 14 of them fatal, to frozen supplemental shakes distributed to long-term care facilities across 21 states. Freezing does not kill Listeria; it simply pauses its growth until the product thaws.
Food Processing Facilities
Perhaps the most persistent and troublesome reservoirs are inside food processing plants themselves. Listeria monocytogenes forms biofilms, thin layers of bacteria encased in a protective slime, on surfaces throughout facilities. These biofilms have been recovered from floor drains, tables, equipment surfaces, utensils, and conveyor belts. In one documented case, the organism persisted on a meat packing plant’s conveyor belt despite daily cleaning and disinfection.
Some persistent strains develop reduced sensitivity to the disinfectants commonly used in food plants, particularly quaternary ammonium compounds. This means that even facilities with rigorous sanitation programs can harbor the bacterium for months or years in hard-to-reach niches like the undersides of equipment, rubber seals, or deep within drainage systems.
Your Refrigerator at Home
Listeria monocytogenes can grow at temperatures as low as 1°C, which is well below the setting of most home refrigerators. A Dutch consumer survey found that the average home fridge ran at 5.7°C, with the upper shelf averaging 7.7°C. Some refrigerators measured as warm as 17°C. At these temperatures, Listeria can multiply steadily on contaminated foods over days.
Risk modeling has identified three factors that matter most for listeriosis at home: the initial bacterial load on the food, how long you store it, and the temperature of your fridge. Older adults face compounded risk because their refrigerators tend to run about 0.6°C warmer on average, and their immune systems are less equipped to fight the infection. Storing ready-to-eat meats and other high-risk items on the bottom or middle shelf (where temperatures are lowest) and eating them within two to three days of opening meaningfully reduces exposure.
Why It Survives Where Others Don’t
What sets Listeria monocytogenes apart from most foodborne bacteria is its extraordinary tolerance for conditions that would stop other pathogens. It grows across a temperature range of 1 to 45°C. It can handle salt concentrations up to 18%. It tolerates acidic environments down to a pH of about 4.6, and after brief exposure to mildly acidic conditions (pH 5.5), it develops an adaptive response that lets it survive at an even harsher pH of 3.5.
This combination of tolerances explains why Listeria shows up in such a wide variety of foods and environments. Refrigeration, salting, and mild acidification are three of the most common preservation methods in the food industry, and Listeria can withstand all three. It even survives repeated freeze-thaw cycles, losing relatively little of its population after being frozen and thawed 18 times in laboratory testing.