Listeria monocytogenes Strikes Again

"About 1,600 people get sick each year from listeria infections and about 260 die. Federal officials said they were revamping protocols to prevent listeria infections after several high-profile outbreaks.”

Jonel Aleccia, health writer, Associated Press

“Listeria has two secret weapons for hitting people hard. First, it can replicate even at 40 degrees Fahrenheit, a typical refrigeration temperature that normally stops the growth of other foodborne bacteria…the other secret weapon is its ability to evade antibodies by hopping directly from the inside of one phagocyte to another.”

Frederick Southwick, M.D., professor, member of the Emerging Pathogens Institute, University of Florida.

Listeria monocytogenes (L. monocytogenes) is a highly pathogenic bacterium that causes foodborne infections. Listeriosis, the disease caused by L. monocytogenes, is the third leading cause of death from a foodborne illness in the U.S. (Salmonella and toxoplasmosis rank first and second, respectively.) But, until there is an outbreak, L. monocytogenesis often off the radar screens of the general public. (For two previous posts on Listeria outbreaks caused by contaminated ice cream and Boar’s Head deli meats, respectively, see, “Some Like It Cold: Listeria Outbreak from Contaminated Ice Cream,” Germ Gems, July 13, 2022; “Listeria monocytogenes, ‘The Silent Assassin,’ Speaks Up in Deli Meats,” Germ Gems, August 14, 2024).

Recently, this Gram-positive bacillus reared its ugly head once again and the epidemiologic investigation linked the current Listeria outbreak to FreshRealm chicken fettuccine alfredo meals distributed to Walmart and Kroger. As of June 18, 2025, the Centers for Disease Control and Prevention (CDC) confirmed 17 cases of L. monocytogenes infections across 13 states, with 16 hospitalizations and four deaths including one fetal loss in a pregnant woman.

My goal in writing this week’s Germ Gems post is to re-familiarize my readers with the severity of infections caused by L. monocytogenes.

Listeria monocytogenes (a recap). The pathogen, L. monocytogenes, is a gram-positive bacillus originally isolated from a patient with meningitis in 1919. In 1926,  E.G.D. Murray, a Canadian clinician and microbiologist, and his colleagues first described Bacterium monocytogenes as a zoonotic infection affecting rabbits. In 1940, the genus name was changed to Listeria to honor Joseph Lister, a pioneer of sterile surgery. But it wasn’t until 1981, when an outbreak of listeriosis hit Halifax, Nova Scotia that L. monocytogenes became recognized as a major cause of foodborne illness.

Listeria are hardy germs that spread to and from food, including in food processing facilities where they are hard to fully remove. Some foods are more likely to be contaminated with Listeria than others; it is commonly found in ready-to-eat foods. High-risk foods include deli meats, hot dogs, unpasteurized milk and cheese, soft cheeses, refrigerated pâté or meat spreads, refrigerated smoked seafood, raw sprouts, and raw or undercooked poultry. Pre-cut fruits and vegetables, and soft-serve ice cream can also be a source of Listeria contamination.

While Listeria can be found in a variety of foods, it's crucial to remember that thorough cooking kills the bacteria. For example, deli meats and hot dogs should be reheated to an internal temperature of 165°F (74°C) or until steaming hot to kill Listeria.

Refrigeration (around 40°F or 4°C) stops the multiplication of most foodborne bacteria, including L. monocytogenes, but refrigeration doesn’t kill Listeria. Listeria can survive and even grow, albeit slowly, at low refrigeration temperatures. It can even multiply slowly at temperatures as low as 24°F (-4.4°C). 

Freezing temperatures (0°F [-18C]) will stop Listeria from multiplying, but even these freezing temperatures do not necessarily kill the bacteria. Listeria can survive freezing and thawing and may resume activity when conditions become favorable again.

Symptoms and risk factors. Listeria infections are relatively rare. The infection starts in the gastrointestinal tract and can invade the gut and enter the bloodstream. Symptoms include fever, chills, muscle aches, nausea, and diarrhea. Over time, these symptoms can develop into a more severe form if the bacterium invades the central nervous system, inflaming tissues surrounding the brain and spinal cord (meningitis).

At highest risk of developing severe disease are people with compromised cell-mediated immunity, such as, those at either end of the age spectrum (neonates and elderly adults), organ transplant recipients, cancer patients, and pregnant women as well as their fetuses.

What makes Listeria so pathogenic? The case fatality rate of Listeria infections in 2016-2018 was 18.6 % compared with 0.4% for Salmonella, the leading cause of foodborne disease in the U.S.  

Dr. Frederick Southwick, a professor at the University of Florida College of Medicine, attributes the higher mortality of Listeria, in part, to its ability to evade host defense mechanisms, including antibodies and its ability to grow intracellularly within macrophages. (Liang, J., “Why Listeria, a Less-Common Foodborne Pathogen, Is So Deadly,” Emerging Pathogens Institute University of Florida, December 10, 2024). To stop bacterial growth within macrophages, activated T lymphocytes are required providing cell-mediated immunity.  

Treatment and prevention. Antibiotics that are commonly used to treat listeriosis  include ampicillin (often in combination with gentamicin for invasive disease) and the antibiotic trimethoprim-sulfamethoxazole that readily penetrates the cells where bacteria are multiplying.

Developing vaccines against Listeria is considered a high priority given the severity of Listeria infections, particularly in vulnerable populations, and the increasing problem of antibiotic-resistant Listeria. No vaccine, however, is currently available either commercially or in clinical trials. Scientists are actively working on developing Listeria vaccines and exploring different innovative approaches, such as mRNA vaccines and nanoparticle-based vaccines.  

During outbreaks, shutting down the source of Listeria is key in prevention of listeriosis. This often includes recalls of suspected contaminated foods, such as the ready-to-eat chicken fettuccine alfredo meals implicated in the current outbreak.

Until a vaccine comes along, prevention of listeriosis is dependent upon cooking food properly, refrigeration (keeping temperature below 40F [4C]), and good hygiene. Given the ubiquity of foods that can harbor Listeria (even ice cream), it seems impractical to avoid them all. Nonetheless, if you fall into one of the high risk groups, use caution in your food choices and for all of us, be alert for food recalls issued by public health officials.  It could save your life.