Listeria monocytogenes is a type of bacteria that causes listeriosis, one of the most dangerous foodborne infections. While relatively rare compared to other food poisoning culprits, it has a fatality rate of about 20% among people who develop invasive infections, and nearly everyone diagnosed with listeriosis ends up hospitalized. What makes this bacterium unusual, and particularly hard to avoid, is its ability to grow at refrigerator temperatures, meaning cold storage alone won’t protect you.
Why Listeria Survives Where Other Bacteria Don’t
Most foodborne bacteria slow down or stop growing in the cold. Listeria monocytogenes does not. It produces special enzymes at low temperatures that actually help it replicate inside your refrigerator. This is why contaminated deli meats, soft cheeses, and smoked seafood are common sources of outbreaks: these are foods that people store cold and eat without reheating.
Listeria is also what microbiologists call a facultative organism, meaning it can survive with or without oxygen. This allows it to thrive in vacuum-sealed packaging, in the gut, and deep inside body tissues. Once it enters your cells, it hijacks your own cellular machinery to move around. A single protein on the bacterium’s surface triggers your cells to build tiny filaments that propel the bacterium forward, pushing it from one cell directly into the next. This cell-to-cell spread lets Listeria avoid your immune system’s defenses in the bloodstream, making the infection especially difficult for your body to fight.
Foods Most Likely to Carry Listeria
Listeria contamination has been found in a wide range of foods, but certain categories pose the highest risk:
- Deli meats, hot dogs, and luncheon meats that aren’t reheated until steaming hot
- Soft cheeses made from unpasteurized milk, including queso fresco, queso blanco, and similar fresh-style cheeses (even pasteurized versions of these cheeses have been linked to outbreaks)
- Unpasteurized milk and any products made from it
- Refrigerated smoked seafood labeled as lox, nova-style, kippered, or cold-smoked
- Refrigerated pâtés and meat spreads
- Raw sprouts, pre-cut fruits, and pre-packaged salads
The common thread is foods eaten without further cooking after they leave the package. Because Listeria multiplies in the fridge, a product that tests clean at the factory can reach dangerous levels by the time you eat it, particularly if it sits for days or weeks.
Two Forms of Illness
Listeria causes two distinct types of illness depending on whether the infection stays in the gut or spreads deeper into the body.
Non-Invasive Gastroenteritis
The milder form acts like typical food poisoning: diarrhea, nausea, fever, and muscle aches. Symptoms usually appear within about 24 hours of eating contaminated food, though the timing can range from 6 hours to 10 days. Most healthy adults who get sick from Listeria experience only this version, and it resolves on its own.
Invasive Listeriosis
The more dangerous form occurs when the bacteria cross from the gut into the bloodstream and, in many cases, the brain. Invasive listeriosis causes bloodstream infections and can lead to meningitis or brain inflammation. The incubation period is much longer: a median of 8 days, with a range anywhere from 1 to 67 days. That long, variable window makes it notoriously hard to trace back to a specific meal. Invasive listeriosis requires hospital treatment with intravenous antibiotics, typically for two to three weeks.
Who Is Most at Risk
Listeriosis is not an equal-opportunity infection. It overwhelmingly affects people with weakened immune systems: older adults, people on immunosuppressive medications, organ transplant recipients, and those with chronic liver or kidney disease. Pregnant women are another major risk group, not because the infection is necessarily severe for the mother, but because of what it does to the pregnancy.
Infection during the first trimester carries an estimated 65% risk of miscarriage. In the second or third trimester, the risk of fetal death drops but remains substantial at roughly 26%. A large study of deliveries following Listeria infection found that 61% resulted in preterm birth, compared to about 8% in the general population. Stillbirth occurred in 13.5% of these cases. Newborns who survive a Listeria infection face up to a 20% fatality rate themselves, and survivors often deal with long-term neurological problems and physical disabilities.
For healthy adults who aren’t pregnant, invasive listeriosis is uncommon. But for anyone in a high-risk group, even small exposures can be dangerous.
Cooking Temperatures That Kill Listeria
Heat reliably destroys Listeria monocytogenes. According to FDA data, cooking food to an internal temperature of 165°F (74°C) kills the bacterium in under one second. At 158°F (70°C), it takes about 2 minutes. Even at 145°F (63°C), holding the food at that temperature for 17 minutes will do the job.
In practical terms, this means following standard safe cooking temperatures for meat and poultry will eliminate Listeria. The specific concern is with foods eaten cold or at room temperature. If you’re in a high-risk group, reheating deli meats and hot dogs until they’re steaming (above 165°F) before eating them is one of the most effective precautions you can take.
Other Ways to Reduce Your Risk
Beyond cooking, a few straightforward habits lower your exposure. Clean your refrigerator regularly, especially any spills from raw meat or deli packages, since Listeria can survive and spread on refrigerator surfaces. Use prepackaged perishable foods within a few days of opening, before bacterial counts have time to climb. Wash raw fruits and vegetables thoroughly, even those with rinds you don’t plan to eat, because cutting through a contaminated surface can push bacteria into the flesh.
If you’re pregnant or immunocompromised, avoiding the high-risk foods listed above is the most straightforward strategy. Listeriosis is rare in the general population, with roughly 1,600 cases per year in the United States. But its severity in vulnerable groups, and the fact that refrigeration doesn’t stop it, makes it a foodborne threat worth taking seriously.