Listeria monocytogenes is a type of bacteria found widely in nature that can cause a serious illness called listeriosis. This microorganism is particularly notable for its ability to survive and even grow in conditions that many other bacteria cannot tolerate, including refrigerated temperatures. Understanding how this bacteria originates and subsequently enters the food supply is important for public health. This article explores Listeria’s natural presence, its pathways into food, the types of foods it commonly affects, and how people can become infected.
Where Listeria Naturally Lives
Listeria monocytogenes is common in the environment. It is frequently detected in soil, water, and decaying vegetation, and in the digestive tracts of many wild and domestic animals, even if they show no signs of illness. Its ability to survive in diverse conditions, including varying pH levels and salt concentrations, contributes to its widespread nature.
Agricultural fields can become contaminated through soil or the use of manure as fertilizer, allowing Listeria to enter the food chain. Its presence in these natural settings provides a constant reservoir from which contamination can occur, highlighting the challenges in controlling its spread.
How It Contaminates Food
Contamination of food by Listeria monocytogenes can occur at multiple points, from raw materials to processing environments. Raw agricultural products like unpasteurized milk, raw meats, and vegetables can carry the bacteria directly from their natural habitats. For example, vegetables may become contaminated if grown in soil containing Listeria or irrigated with contaminated water.
Cross-contamination within food processing facilities is a significant pathway. Once Listeria enters a facility, it can be difficult to remove due to its ability to form biofilms on equipment, surfaces, and drains. These biofilms allow the bacteria to persist, transferring to food products that contact contaminated surfaces. Workers can also inadvertently spread the bacteria if proper hygiene protocols are not followed.
A unique characteristic of Listeria is its capacity to grow at refrigeration temperatures, a trait known as psychrotolerance. Unlike most other foodborne pathogens that are inhibited by cold, Listeria can multiply at temperatures as low as -0.4°C to 0°C. This makes refrigerated foods a particular concern, as the bacteria can increase to unsafe levels over the product’s shelf life.
Foods Commonly Affected
Listeria monocytogenes frequently contaminates ready-to-eat (RTE) foods, which are products consumed without further cooking. These foods are susceptible due to the absence of a final “kill step,” like high-heat cooking, that would eliminate the bacteria. Additionally, many RTE foods have extended shelf lives under refrigeration, providing ample time for Listeria to multiply to higher concentrations.
Examples of foods often associated with Listeria contamination include soft cheeses, especially those made from unpasteurized milk. Deli meats, cold cuts, and hot dogs are also frequent sources, often contaminated after cooking during slicing or packaging. Cold-smoked seafood, such as smoked salmon, poses a risk because it is typically not cooked after smoking.
Other susceptible items are raw sprouts, unpasteurized milk and dairy products, and certain pre-cut fruits and vegetables. Refrigerated pâtés and meat spreads have also been linked to outbreaks.
How People Get Infected
Humans primarily become infected with Listeria monocytogenes by consuming contaminated food. Once ingested, the bacteria can cause listeriosis, an illness ranging from mild to severe. The incubation period, the time between consumption and symptom onset, can vary significantly, from as short as one day to as long as 70 days.
While foodborne transmission is the main route, Listeria can also be passed from a pregnant woman to her unborn baby during pregnancy or at birth. This transmission route is critical for at-risk populations, as consequences for newborns can be severe. Preventing ingestion of the bacteria through contaminated food sources remains the primary focus.