Listeria in Food: Why This Pathogen Is Different and More Dangerous

Most foodborne bacteria slow down in a cold refrigerator. They don’t stop entirely, but 38°F drops them into near-dormancy. You have a few days of safety before populations grow to concerning levels.

Listeria monocytogenes doesn’t follow those rules.

This pathogen keeps multiplying at refrigerator temperatures — slowly, but consistently. A contaminated package of deli meat that looks and smells fine on day 4 can have a meaningfully larger Listeria population than it did on day 1. The bacteria is patient in ways that other foodborne pathogens aren’t.

That one difference makes Listeria the most lethal common foodborne pathogen in the US.

The Numbers Behind the Risk

Listeria infects about 1,600 people per year in the United States. Compare that to Salmonella’s 1.35 million annual infections — Listeria is rare. But rarity doesn’t mean low risk. Of those 1,600 cases, about 260 people die. That’s a fatality rate around 16-20%, compared to under 0.1% for most other foodborne bacteria.

The demographics tell you who faces the danger. About 90% of serious Listeria cases occur in four groups: pregnant women and their newborns, adults over 65, people with weakened immune systems (cancer patients, organ transplant recipients, HIV-positive individuals), and people on immunosuppressive medications. For a healthy 30-year-old, Listeria is still a risk, but serious illness is uncommon. For a pregnant woman or someone on chemotherapy, it’s a different calculation entirely.

What Makes Listeria Different

Three characteristics separate Listeria from most foodborne pathogens.

It grows at refrigerator temperatures. The temperature danger zone for most bacteria tops out around 40°F on the low end — refrigerator temperature stops most bacterial growth. Listeria grows between 32°F and 113°F. At 37°F (your refrigerator’s typical temperature), growth is slow but real. A contaminated product stored for 5-7 days gives Listeria time to multiply to infectious levels even under refrigeration. This is why refrigerator storage times matter more for Listeria than for most other pathogens.

It forms biofilms. In food processing environments, Listeria can form structured communities on surfaces — biofilms — that are much harder to remove than free-floating bacteria. These biofilms can persist on equipment even after standard cleaning, allowing contamination to continue for years without obvious signs. The 2011 cantaloupe outbreak, which killed 33 people and remains the deadliest US foodborne illness outbreak in the modern era, was traced to Listeria biofilms in a packing facility’s equipment.

Its incubation period is 1 to 4 weeks. Most foodborne illness appears within hours to a few days. Listeria’s long incubation window means that by the time someone gets sick, they’ve typically eaten dozens of different foods since the contamination event. Identifying the source is genuinely difficult, and patients often can’t connect their illness to a specific meal. This is one reason Listeria outbreaks can continue for weeks before the contaminated food source is identified and recalled.

The High-Risk Foods

Listeria contamination is most dangerous in ready-to-eat foods — products consumed without a subsequent cooking step that would kill the bacteria.

Deli meats and hot dogs are the highest-risk category. They’re processed and then packaged in environments where Listeria can contaminate the finished product after the heat treatment step. They’re stored in the refrigerator where Listeria continues growing, and they’re often eaten cold or at room temperature. If you’re in a high-risk group, the CDC and FDA both recommend heating deli meat and hot dogs to 165°F (steaming hot) before eating.

Soft cheeses, especially those made with unpasteurized milk, have been linked to multiple outbreaks. Queso fresco, queso blanco, brie, camembert, and feta carry higher risk. The soft texture, near-neutral pH, and high moisture content are good conditions for Listeria. Hard cheeses (cheddar, parmesan, Swiss) are lower risk because their lower moisture content inhibits growth. Pasteurized cheeses are lower risk than unpasteurized, but Listeria contamination can occur after pasteurization during the aging and packaging process.

Raw sprouts are grown in warm, humid conditions that favor Listeria (and Salmonella and E. coli). The FDA advises high-risk groups to avoid raw sprouts entirely.

Cold smoked seafood, such as lox or smoked salmon, is refrigerated and not cooked. Listeria can grow in these products during refrigerated storage. Canned or shelf-stable smoked seafood is lower risk because of the heat processing step.

Cantaloupe and other melons carry surface contamination risk. Cutting into the melon transfers surface bacteria into the flesh, and the neutral pH of cantaloupe supports growth. The 2011 Jensen Farms outbreak demonstrated this risk at scale.

The Pregnancy Connection

The link between Listeria and pregnancy complications is worth understanding in detail, because the mechanism is not obvious.

During pregnancy, the immune system downregulates specific cell-mediated immunity responses — particularly T-cell activity. This is a normal adaptation that prevents the immune system from attacking the developing fetus, which is genetically distinct from the mother. But the same T-cell responses are critical for fighting Listeria. Pregnant women are approximately 10 times more likely to get listeriosis than healthy nonpregnant adults.

More importantly, Listeria can cross the placenta. The bacteria can infect the fetus even when the mother’s symptoms are mild — flu-like fatigue, muscle aches, maybe a low fever. By the time a pregnant woman might suspect something is wrong, the fetus may already be infected. This leads to miscarriage, stillbirth, premature delivery, or a newborn with neonatal listeriosis (which carries a 20-30% mortality rate even with treatment).

This risk explains why the dietary restrictions for pregnant women are stricter than general population guidelines. The consequences of a single Listeria exposure are not a sick day. They’re potentially catastrophic for the pregnancy.

What Refrigeration Does and Doesn’t Do

Refrigeration slows Listeria growth substantially. The difference between 40°F and 70°F is dramatic — growth that takes a week in the refrigerator might take hours at room temperature.

But slow growth is not no growth. Expiration dates on ready-to-eat products are calibrated in part around Listeria growth rates — a product with a 14-day use-by date was tested to ensure that Listeria populations remain below dangerous levels throughout that window under proper refrigeration. Using a product past its date, or storing it in a warmer-than-expected refrigerator, erodes that safety margin.

The expiration dates article explains how use-by dates differ from best-by dates and which ones are actual safety cutoffs versus quality indicators.

Safe Handling for High-Risk Groups

For pregnant women, elderly adults, and immunocompromised individuals, these practices reduce Listeria risk meaningfully.

• Heat deli meats and hot dogs to 165°F (steaming throughout) before eating
• Avoid soft cheeses made with unpasteurized milk
• Don’t eat refrigerated pate or meat spreads (shelf-stable canned versions are lower risk)
• Avoid cold smoked seafood unless cooked; canned or shelf-stable forms are acceptable
• Don’t eat raw sprouts
• Clean the refrigerator regularly, especially the vegetable drawers and deli drawer
• Store refrigerated ready-to-eat foods toward the lower end of their recommended storage windows

For healthy adults without risk factors, these precautions are still reasonable but the risk calculus is different. Listeria is present in the food supply in low levels, and a healthy immune system handles low-dose exposures effectively. The gut microbiome article explains how a robust gut flora contributes to this defense.

The core takeaway is that Listeria is unusual enough that standard refrigerator logic — “it’s fine if it’s cold” — doesn’t fully apply. Cold slows it. Time and temperature together determine the risk.