Listeria monocytogenes is a type of bacterium that can contaminate various food products. When people consume food contaminated with this bacterium, they can develop an illness called listeriosis. While relatively rare, listeriosis is a serious foodborne disease that can lead to severe health complications. It is particularly concerning because the bacterium can survive and multiply even at typical refrigeration temperatures, making its control challenging.
Listeria’s Natural Habitats
Listeria monocytogenes is widely present in the natural environment. It can be found in soil, water sources, and decaying vegetation. This bacterium is also commonly present in the digestive tracts of animals.
The widespread presence of Listeria means it can easily enter agricultural environments. For instance, vegetables can become contaminated through contact with contaminated soil or water used for irrigation. Animals can shed the bacteria in their feces, contributing to its distribution and potentially contaminating crops. This hardiness allows Listeria to persist in diverse conditions.
How Listeria Enters Food
Listeria can enter food through primary and secondary contamination. Primary contamination occurs when raw ingredients are initially contaminated from their natural environment. Examples include unpasteurized milk or produce that has come into contact with contaminated soil or water. Raw meats and poultry can also carry Listeria.
Secondary contamination often happens within food processing facilities through cross-contamination from contaminated equipment, surfaces, or workers. Listeria is particularly adept at forming biofilms, which are protective communities of microbes encased in a self-produced matrix, making them difficult to remove from food contact surfaces and equipment. Biofilms allow Listeria to persist in a facility for extended periods, acting as a continuous source of contamination.
Post-processing contamination is another significant pathway, where food becomes contaminated after steps like cooking or pasteurization, typically during slicing, packaging, or handling. This can happen in deli counters where slicing equipment or shared utensils are not adequately sanitized.
Foods Most Susceptible to Contamination
Certain foods are particularly vulnerable to Listeria contamination, often due to their characteristics and how they are processed and consumed. Ready-to-eat (RTE) foods are a primary concern because they are consumed without further cooking or heating, which would otherwise eliminate the bacteria. These foods include items like deli meats, cold cuts, and hot dogs, which can become contaminated after cooking, such as during slicing or packaging.
Soft cheeses, especially those made from unpasteurized milk, are also frequently linked to Listeria outbreaks. The bacteria can survive the cheese-making process if milk is not pasteurized, and the cheese’s moisture and pH can support bacterial growth. Other high-risk foods include smoked fish and refrigerated seafood that are not cooked after smoking. Pre-cut fruits and vegetables, such as pre-packed melon slices or salads, can also be susceptible if contaminated during processing or handling. The ability of Listeria to multiply at refrigeration temperatures makes these foods a concern, as the bacteria can grow to dangerous levels over their shelf life.
Controlling Listeria Contamination
Controlling Listeria contamination requires comprehensive measures throughout the food supply chain. In the food industry, good manufacturing practices (GMPs) are crucial, including stringent sanitation programs and regular cleaning of equipment and surfaces. Environmental monitoring helps identify and address potential contamination sources before outbreaks.
Temperature control is also important, as maintaining refrigerators at 40°F (4°C) or below can slow Listeria growth, though it does not eliminate the bacteria. Food businesses implement strategies like separating raw and ready-to-eat food areas to prevent cross-contamination and training employees on proper hygiene. The use of seamless surfaces that are easy to clean helps prevent biofilm formation.
Consumers also play a role in preventing Listeria infection through safe food handling practices at home. Washing hands thoroughly before and after handling food. Preventing cross-contamination by keeping uncooked meats separate from ready-to-eat foods and using separate cutting boards. Promptly refrigerating perishable foods and consuming ready-to-eat items quickly.