Listeria gets into food through a chain that starts in soil and water, moves through processing facilities, and often reaches the final product long after cooking or preparation is complete. Unlike most foodborne bacteria, Listeria monocytogenes grows at refrigerator temperatures, which means contaminated food can become more dangerous the longer it sits in your fridge, not less.
Listeria Starts in Soil and Water
Listeria monocytogenes is a free-living soil microbe found across a wide range of environments. It tolerates high salt concentrations, temperature extremes, and can persist in the environment for extended periods. It shows up regularly in surface waters used for agriculture, with canals and creeks more frequently contaminated than rivers or ponds. Seasonal factors like water type, pH, and weather all affect how much Listeria is present at any given time.
Rain plays a particularly important role. Research in New York State found that upstream dairy farm density was only associated with Listeria detection in waterways following rain events, suggesting that runoff washes the bacteria from soil, manure, and animal environments into the water supply. From there, irrigation carries it directly onto crops. A multivariate analysis of produce farms in New York found that irrigation within three days of sample collection was associated with a 39-fold increase in the odds of isolating Listeria from the surrounding soil and surfaces. That single factor, irrigation timing, was the strongest predictor of contamination in produce growing environments.
Processing Facilities Are a Major Source
Once Listeria enters a food processing plant, it can become nearly impossible to fully eliminate. The bacterium attaches to surfaces like stainless steel and forms biofilms: thin, sticky layers of cells encased in a protective matrix. These biofilms are far more resistant to cleaning chemicals and sanitizers than free-floating bacteria. The protective layer surrounding the cells absorbs and neutralizes sanitizers before they can reach the living bacteria underneath. Even dead cells within the biofilm can shield surviving ones from further treatment.
The biggest problem is where these biofilms form. They accumulate in spots that are hard to reach during routine cleaning: welding joints, corners, pipe connections, and dead-end sections of tubing systems. These hidden niches allow Listeria to survive for months or even years inside a facility, repeatedly contaminating batches of food that pass through. A product can be fully cooked or otherwise treated to kill bacteria, then pick up Listeria simply by touching a contaminated conveyor belt, slicing blade, or packaging surface on its way out the door.
This is exactly why so many Listeria outbreaks involve ready-to-eat products. The food won’t be cooked again before someone eats it, so any contamination that happens after the kill step goes straight to the consumer.
Deli Counters and Retail Equipment
Retail delis are one of the most common points where Listeria reaches people. Mechanical slicers are a well-documented source of cross-contamination: one contaminated product run through a slicer can transfer Listeria to every product sliced afterward. The CDC has identified deli-sliced meats as a recurring vehicle in outbreaks, including a 2025 outbreak linked specifically to meats sliced at delis.
FDA food safety codes require that slicers be fully cleaned and sanitized at least every four hours and disassembled before cleaning. In practice, compliance is inconsistent. A CDC study of nearly 300 randomly selected delis found that roughly half of deli managers reported cleaning slicers less often than that minimum. Worker-level data told a similar story: in only about 46% of delis were all slicers being fully cleaned at the required frequency. Chain-owned delis, those with written cleaning policies, and shops with more food safety training performed better, but the gap remained significant across the industry.
Listeria spreads easily among deli equipment, surfaces, hands, and food. Products sold at deli counters, especially those sliced or prepared on-site, carry higher risk than their pre-packaged equivalents for this reason.
Why Refrigeration Doesn’t Help
Most bacteria slow down or stop growing in the cold. Listeria doesn’t. It can grow at temperatures as low as -1.5°C (about 29°F), which means standard refrigeration at 4°C (40°F) slows it but doesn’t stop it. Its optimal growth range is 30 to 37°C (86 to 99°F), but even in a properly functioning refrigerator, Listeria multiplies slowly over days and weeks.
This is what makes Listeria fundamentally different from salmonella or E. coli in terms of food storage. A contaminated deli meat or soft cheese sitting in your fridge for two weeks will have a higher Listeria count at the end than at the beginning. Foods with long refrigerated shelf lives, like hot dogs, smoked fish, deli meats, and soft cheeses, give the bacteria more time to reach dangerous levels.
The Foods Most Often Involved
Recent CDC outbreak data from 2024 and 2025 illustrates the pattern clearly. Outbreaks have been linked to prepared pasta meals, ready-to-eat foods from a single manufacturer, supplement shakes, deli-sliced meats, ready-to-eat meat and poultry products, and queso fresco and cotija cheese. The common thread is that these are all foods eaten without further cooking.
Deli meats and prepared foods are vulnerable because they’re handled extensively after any heat treatment. Soft cheeses, particularly those made from unpasteurized milk, can harbor Listeria introduced during the cheese-making process. Fresh produce picks up the bacteria from contaminated irrigation water or soil, and because salads, pre-cut fruit, and sprouts are eaten raw, there’s no cooking step to kill what’s there.
How Regulations Address the Problem
The U.S. applies what amounts to a zero-tolerance policy for Listeria in ready-to-eat foods. The FDA’s guidance defines an effective control as one that reduces Listeria to fewer than 0.04 colony-forming units per gram of food, essentially undetectable. If Listeria is found in a ready-to-eat product, the FDA recommends the manufacturer reprocess it with a validated kill step, divert it away from human consumption, or destroy the lot entirely. The USDA enforces similar requirements specifically for ready-to-eat meat and poultry.
These strict standards exist because even small amounts of Listeria in food can be dangerous for pregnant women, older adults, and people with weakened immune systems. The fatality rate for listeriosis is far higher than for most other foodborne infections, which is why regulators treat any detectable presence in ready-to-eat food as unacceptable.
Reducing Your Risk at Home
The contamination chain from soil to your plate involves many steps you can’t control, but a few practical habits lower your exposure. Eat deli meats and hot dogs within three to five days of opening or purchasing them at the counter, since Listeria levels rise with storage time. Heating deli meats until steaming (165°F) kills the bacteria. Keep your refrigerator at 40°F or below, and clean up spills from deli products or raw meat promptly, since Listeria can spread to other foods through shared surfaces and drips.
For produce, washing under running water helps remove surface contamination but won’t eliminate bacteria that have been internalized through root systems or cuts in the skin. Pre-cut and pre-washed salad mixes carry slightly more risk than whole, uncut produce because processing increases surface area and handling opportunities. Cooking any vegetable eliminates the risk entirely.