In the summer of 2024, a Listeria outbreak linked to a plant producing Boar’s Head deli meat products caused 59 hospitalizations and 10 deaths. The disease, caused by Listeria monocytogenes bacteria, is the third leading cause of death from a foodborne illness in the United States. Salmonella and toxoplasmosis rank first and second, respectively.
Infections are rare but serious, with symptoms including fever, chills, muscle aches, nausea, and diarrhea. With time, they can develop into a more severe case as they infect the central nervous system and inflame the tissues surrounding the brain and spinal cord. Over the course of several weeks, the bacteria may eventually enter the brainstem—a condition that is as deadly as it sounds.
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.
“This is why there’s a problem with deli meats, which are refrigerated for prolonged periods,” said Frederick Southwick, M.D., a University of Florida Emerging Pathogens Institute member and a UF College of Medicine professor. “Even if there’s only a minor contaminant when it was manufactured, it will continue to grow. Then, when you eat it, you get a large inoculum.”
Listeria infections begin in the gastrointestinal cells. But the bacteria’s other secret weapon is its ability to evade antibodies by hopping directly from the inside of one phagocyte to another. These cells are supposed to protect the body by ingesting debris, dying cells and pathogens like Listeria. Most other types of bacteria would then be trapped in a compartment called a phagosome and killed.
But Listeria evolved to survive inside phagocytes by making a hole in the phagosome wall and escaping before it can be killed. The bacteria then induce the host cell to contract, getting pushed out toward the cell membrane until they pop directly into the adjacent cell.
As with other diseases, those with compromised immune systems are more likely to develop critical infections. This includes pregnant people since their bodies have reduced immune function to prevent the risk of a fetus being rejected. But unlike other foodborne illnesses, a Listeria infection in a pregnant person could cause an embryo to die.
While Listeria is most famously associated with deli meats, other foods, including unpasteurized dairy products, spinach and melons, have also been known to carry the bacteria.
“If the processing is improper or there’s contamination, pretty much anything could carry it, unfortunately,” Southwick said. “So, it’s really got a wide spectrum of food that can be contaminated.”
To avoid catching Listeria, Southwick advises his pregnant or otherwise immunocompromised patients to avoid unpasteurized dairy products and heat processed foods to at least 165 degrees to kill the bacteria. He also warns against keeping any deli meats for longer than a week, so that Listeria can’t multiply too much. While freezing can help, it is not foolproof, given that Listeria can continue to grow slowly even at 24 degrees Fahrenheit.
Southwick recommends a thorough wash for foods that aren’t usually cooked but may still carry Listeria, like sprouts and melons. Pre-cut fruit can be especially riskysince it can be hard to ensure that whoever processed the produce cleaned it before cutting it up.
“I think these forms of hygiene are very important, not just for listeria, but for all foodborne illnesses,” Southwick said.
In the United States, undercooked or contaminated poultry is responsible for around one million illnesses each year. For anyone wishing to avoid that fate, Southwick recommends checking that chicken is thoroughly cooked and sanitizing any surface in contact with raw chicken to prevent cross-contamination.
The size and scope of the modern-day food chain also make it challenging to contain outbreaks of foodborne illness. Contamination at just one plant can quickly be distributed across the country.
“The key is to shut down that source. Otherwise, it’ll continue to spread,” Southwick said. “The CDC is very important for identifying immediately when there’s an outbreak. It’s really important to get those investigators on track right away.”
Before devoting many years to Listeria research, Southwick studied how white blood cells move. “What they do is they sort of crawl and pull themselves along,” he explained.
By contracting protein strands called actin filaments, the white blood cells push themselves forward. These proteins are calcium sensitive — when calcium levels go down, the actin can grow. When the calcium goes up, it shuts it off.
Southwick had been studying this mechanism for about a decade when the University of California, Berkeley Professor Daniel Portnoy, Ph.D., showed him an electron micrograph of the bacterium.
“And there was this cloud around it, and I recognized the cloud immediately as actin filaments,” Southwick said.
Together they studied cells that had been infected with Listeria, using a fluorescent stain that specifically marks actin filaments. Southwick and Portnoy saw a tail resembling a rocket ship on the bacteria. The actin had assembled at the back of the bacteria and formed filaments, forcing Listeria to move through the cytoplasm of the cells.
Southwick compares the movement to that of a ratchet. As the bacteria moves forward, the actin forms new filaments that prevent it from moving backward.
“The ability of listeria to move in this way was really impressive,” Southwick said. “We saw these bright fluorescent tails forming as the bacteria is moving cell to cell. It was almost like the Fourth of July.”
Their discovery shed light on a key mechanic of Listeria infections — how the bacteria can move deeply into host cells.
Written by: Jiayu Liang