Understanding listeria: The refrigerator-resistant bacteria

Listeriosis is a foodborne illness caused by the bacterium Listeria monocytogenes, a hardy gram-positive pathogen commonly found in soil, water, and animal feces.

People typically get listeriosis by eating contaminated food (ready-to-eat meats, soft cheeses, milk, smoked fish, or prepared salads).

Unlike many bacteria, L. monocytogenes can grow in refrigerator temperatures, so it poses a risk even in properly refrigerated foods.

Listeriosis usually causes mild, flu-like symptoms in healthy adults. In severe cases it can invade the bloodstream and brain. Invasive listeriosis often presents as bacteremia (blood infection) or meningitis/meningoencephalitis

In pregnant women it can cause miscarriage or neonatal infections, like other congenital infections. Listeriosis is relatively rare. However, it is a serious disease because of its high fatality rate (up to 20–30%). To better understand this threat, let’s examine how listeriosis affects populations worldwide.

Listeriosis is a global foodborne illness, reported from all continents. However, incidence varies widely.

For example, the EU/EEA reported about 4–5 cases per million people per year in 2019. More than half the cases were reported in Germany, Spain and France. WHO estimates 0.1–10 cases per million people per year depending on region.

Major outbreaks: Large outbreaks occur when a widely distributed food becomes contaminated. Notable recent outbreaks include:

• South Africa 2017–2018: The world’s largest listeriosis outbreak to date involved more than 900 confirmed cases (January 2017–April 2018) and a 27% case-fatality rate. Whole-genome sequencing linked the outbreak strain to ready-to-eat processed meat. This outbreak caused widespread recalls of processed meats in South Africa and neighboring countries.
• Germany 2018–2019: An unusual cluster of 112 invasive cases (mostly elderly) was linked to Listeria-contaminated blood sausage. This was one of the largest reported outbreaks in Europe. Withdrawal of the contaminated sausage ended the outbreak.
• Spain 2019: Spain recorded its largest-ever listeriosis outbreak (over 200 cases, mostly 2019) linked to chilled roasted pork meat.
• United States (selected): In recent years, CDC has tracked many multistate Listeria outbreaks. For example, in 2024 an outbreak from deli-sliced meats infected 61 people (10 deaths) across 19 states.

L. monocytogenes contamination can occur in diverse foods. Between outbreaks, most cases are sporadic and scattered. Listeriosis peaks in summer months in Europe, but the bacterium exists worldwide in food chains.

These cases demonstrate the importance of disease surveillance systems in identifying and controlling listeria outbreaks globally.

Pregnant women, neonates, the elderly and people with compromised immune systems are at highest risk.

Pregnant women: For most healthy adults, a Listeria infection might feel like the flu. But for pregnant women, it can silently cross the placenta and kill the fetus without warning.

One in five cases of listeriosis during pregnancy results in stillbirth or neonatal death—a statistic that has remained stubbornly high despite advances in food safety.

The devastating reality of this statistic was tragically illustrated in South Africa’s 2017 outbreak. By the time the outbreak was over, more than 200 babies were dead. Hospitals across the country reported newborns with sepsis, and miscarriages spiked. The 2017 South Africa outbreak was the deadliest listeriosis outbreak ever recorded.

Neonates (newborns): Infants can get listeriosis from their mothers. Newborns either present very early (days old) with sepsis or pneumonia, or later (1–3 weeks old) with meningitis

Elderly adults (≥65 years): Ageing weakens immunity. Listeriosis incidence is highest in people over 65 – about 4× higher than the general rate. The elderly often develop the invasive form (sepsis, meningitis).

Immunocompromised people: This includes people with cancer (especially on chemotherapy), HIV/AIDS, organ transplants, diabetes, liver disease, alcoholics, or those on steroids/immune-suppressing drugs. In one study, 93% of adults with listeriosis had at least one immunosuppressive condition. Any serious immune deficit greatly raises the chance that Listeria infection will become invasive.

Common symptoms

Listeriosis often starts with non-specific, flu-like or gastrointestinal symptoms.

Many healthy people have only mild illness (especially when it causes gastrointestinal infection). In those cases, onset is sudden (usually 1–2 days after eating) and lasts a few days. Typical symptoms of mild listerial gastroenteritis include:

• High Fever
• Chills
• Muscle or joint aches (body-wide) and general fatigue
• Nausea
• Vomiting
• Watery diarrhea
• Headache

These mild symptoms alone don’t confirm listeriosis, but they often precede the serious forms. In fact, L. monocytogenes was shown to cause outbreaks of febrile gastroenteritis. In such cases, nearly all infected people had fever and many had diarrhea.

Invasive symptoms (sepsis/meningitis)

If listeria spreads into the bloodstream or central nervous system, symptoms become more severe:

• Stiff neck
• Photophobia (light sensitivity)
• Signs of meningitis if the brain/meninges are infected
• Confusion, drowsiness or loss of balance (if encephalitis or brainstem infection)
  Respiratory symptoms (e.g. cough, difficulty breathing) sometimes occur in very ill cases
• Seizures or focal neurological deficits may occur in severe CNS infection
• In pregnant women, invasive infection can lead to
  • Premature delivery
  • Miscarriage
  • Stillbirth
  • Infection of the newborn

In pregnant women, symptoms may remain mild or absent even when the fetus is infected. Often the first sign is fetal distress.

Because early symptoms can be very nonspecific (flu-like), listeriosis is often not suspected until it becomes invasive. Doctors should suspect listeriosis if a high-risk person (pregnant, elderly, immunosuppressed) presents with unexplained fever and muscle aches or any signs of meningitis, especially after eating high-risk foods.

When listeriosis progresses beyond these initial symptoms, serious complications can develop.

Complications

Invasive listeriosis can be life-threatening and may cause lasting damage. Key complications include:

• Meningitis/encephalitis: Listeria commonly causes bacterial meningitis or brainstem encephalitis (“rhombencephalitis”)
• Sepsis and organ failure: If the bacteria invade the bloodstream, infected people can develop septic shock with multi-organ failure (kidneys, liver, lungs, heart)
• Pregnancy/neonatal: Infection during pregnancy frequently leads to fetal or neonatal loss. Miscarriage (spontaneous abortion) and stillbirth are common outcomes. If infants are born alive, they may develop early-onset sepsis (within 1–3 days of birth) or late-onset meningitis (1–3 weeks)

Endocarditis, abscesses, other infections: Rarely, Listeria can infect the heart valves (endocarditis), bones/joints (osteomyelitis), or eyes. Such focal infections can also cause severe damage.

Listeriosis is diagnosed by testing for L. monocytogenes in bodily fluids. Clinicians should consider listeriosis testing when a person at risk (pregnant, elderly, immunosuppressed) has unexplained fever or other compatible symptoms.

For example, CDC advises that any high-risk person who ate a recalled or suspect food and then develops fever and muscle aches (with or without GI symptoms) should seek medical care. In practice, doctors often test patients who have:

• Fever and generalized aches (flu-like illness), particularly if following a known exposure.
• Symptoms of bacterial meningitis (fever plus headache, stiff neck, confusion).
• Pregnant women with fever, especially with any fetal distress or miscarriage.

Conversely, routine screening of asymptomatic exposed persons is not recommended.

Laboratory tests: The standard diagnostic test for listeriosis is culture of L. monocytogenes from a normally sterile site. For invasive disease, this means:

• Blood culture: The most common positive test. In septic cases, Listeria grows in blood cultures.
• CSF culture: If meningitis or encephalitis is suspected, a lumbar puncture is done; culturing cerebrospinal fluid can detect Listeria. (CSF often shows high white cells like other bacterial meningitis.)
• Pregnancy-related: In a pregnant person with listeriosis, Listeria can be cultured from amniotic fluid (via amniocentesis) or from the placenta or fetal tissues after delivery. Neonates with early listeriosis often have positive blood or CSF cultures.

No antibody test or rapid at-home test exists for listeriosis. PCR-based methods are used in some research/lab settings, but traditional culture remains the diagnostic gold standard.

Note: testing of stool is generally not done for diagnosis, even though the bacteria infects the gut.

Once L. monocytogenes is cultured, it must be identified by laboratory methods (it is a characteristic Gram-positive rod, catalase-positive, motile at 25°C). Laboratory confirmation is needed to guide therapy. Meanwhile, if listeriosis is strongly suspected, clinicians often start treatment promptly rather than waiting for culture results.

Listeriosis is treated with antibiotics; there is no vaccine or specific antiviral. Prompt initiation of effective antibiotics is crucial to improve outcomes. The standard therapy for invasive listeriosis is:

• Ampicillin (or penicillin) + gentamicin: Ampicillin (or high-dose penicillin G or amoxicillin) is the drug of choice. An aminoglycoside such as gentamicin is usually added (at least for the first week)
• Alternative (if penicillin-allergic): Trimethoprim-sulfamethoxazole (cotrimoxazole) is an effective alternative. For mild illness, high-dose oral cotrimoxazole may be used, but invasive cases require IV therapy.

In severe cases, intensive care (IV fluids, pressors for blood pressure, respiratory support) is needed.

Pregnant people with confirmed infection should be hospitalized and treated; fetal monitoring is indicated. In known pregnancy listeriosis, termination of pregnancy may be considered in extreme cases to save the mother (per obstetric guidelines).

Effective infection prevention strategies for listeriosis focus on food safety since no vaccine is currently available.

There is currently no licensed vaccine for human listeriosis. Vaccine research (including experimental attenuated strains and mRNA vaccines) is underway in animal models, but none is available in clinical use.

Here are some infection control measures needed to prevent or minimize the risk of listeria infections:

• Industrial/retail controls: Factory hygiene and tracing are critical to prevent outbreaks. The challenge in controlling monocytogenes is considerable. The bacterium is ubiquitous, and resistant to common preservative methods, such as the use of salt, smoke or acidic condition in the food. Its ability to survive and grow at refrigeration temperatures (around 5 °C) make prevention harder. Food processors and restaurants should follow strict sanitation to prevent Listeria contamination. Public health agencies monitor outbreaks and recall contaminated products. Consumers can subscribe to local food recall alerts.
• Food safety at home: Wash hands, utensils, and cutting boards thoroughly after handling raw foods. Use ready-to-eat foods quickly (reheat deli meats until steaming hot).

Foods with a short shelf-life (e.g. soft cheeses, smoked fish) should be eaten promptly. Listeria can grow slowly even in the fridge, so the shorter the storage time the better.

• Avoiding high-risk foods: Immunocompromised people, the elderly, and especially pregnant women should avoid foods known to harbor Listeria. These include unpasteurized dairy products like (raw) milk and cheeses, deli meats and hot dogs, smoked seafood, and refrigerated pâtés or meat spreads. If consuming deli meats, heating them until steaming kills Listeria.

• Awareness for high-risk groups: Healthcare providers should inform at-risk populations about listeriosis.

For example, pregnant women are advised to avoid high-risk foods and to report fever to their doctor. Simple preventive tips (from WHO/CDC) emphasize thorough cooking and avoiding risky refrigerated foods.

Listeria monocytogenes poses a unique threat. It survives preservation methods like refrigeration where other bacteria perish.

Pregnant women, newborns, the elderly, and immunocompromised individuals face the highest risk. They might develop severe, potentially fatal listeriosis. While initial symptoms often mimic the flu, invasive infection can lead to sepsis, meningitis, and pregnancy complications. Mortality rates can reach 20-30%.

Diagnosis requires laboratory cultures from blood or cerebrospinal fluid, with prompt antibiotic treatment critical for survival. Prevention relies entirely on food safety practices as no vaccine exists, making awareness of high-risk foods crucial for vulnerable populations.

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