Listeria (Listeria monocytogenes) pops up in the news every so often with stories of it being found in places like cookie dough or even in refrigerator door linings.  It seems as though Listeria monocytogenes has the potential to grow in the strangest of places. This unique quality is primarily attributed to Listeria’s ability to grow at the near freezing temperatures of refrigerators and freezers.

Usually, food scientists prevent bacterial growth by storing food at colder temperatures or using salt to limit growth.  However, these conditions are no match for Listeria, as it able to thrive under these conditions, unlike other bacteria. Research has also found that Listeria can grow in environments that contain up to 10% salt.

Despite the persistence of Listeria monocytogenes, outbreaks are relatively few and far between with roughly 0.0005% of the United States population getting sick annually. But because Listeria can cause foodborne illness, it is important to be precautious and to get educated.

What is Listeria monocytogenes and where is it found?

Listeria monocytogenes is a type of bacteria commonly found in the environment in multiple places– soil, ground water and on plants.  This bacterium has the ability to grow in farm animals and then make its way into our milk or meat supply. Contaminated milk, however, can be mitigated by pasteurization, which kills the bacteria. It also has the ability to grow on raw vegetables, so it is important to make sure to rinse vegetables with tap water before cooking.

Listeria monocytogenes notoriously grows in food processing facilities that have inadequate cleaning procedures. Pooling drain water or food processing surfaces that are not cleaned regularly can grow the bacteria and then cause its spread around an entire food processing plant.

Who’s at risk for Listeriosis?

When this bacteria is in food, it can make its way to the gastrointestinal tract where it begins infection. Here, Listeria can cause an illness called Listeriosis. 

Most healthy people are not at risk for Listeriosis, even if they eat food that contains the bacteria. However, those with weakened immune systems such as children, those with chronic illness, the elderly, and most often pregnant women, are considered more susceptible to Listeriosis.

Listeriosis is rather mild when incurred in pregnant women, but has detrimental effects on fetal health. Therefore, proper precautions need to be taken during pregnancy in order to reduce the risk of infection. Pregnant women are 20 times more likely to get Listeriosis than non-pregnant women, so while pregnant make sure to only eat or drink foods that say “made with pasteurized milk” and make sure meats are cooked to the correct temperature. Avoiding soft cheeses like Brie and Camembert, deli meats or hot dogs, and uncooked meats or fish are easy ways to ensure that you and your baby’s health are taken care of during pregnancy. Hot dogs can be eaten, as long as they are cooked properly as indicated on the product label.

Other foods commonly associated with Listeria include:

• Refrigerated pâté
• Meat spread from a meat counter
• Smoked seafood found in the refrigerated section

What else can consumers do to reduce the risk of Listeriosis?

• Keep uncooked meats and poultry separate from vegetables, cooked foods, and ready-to-eat foods.
• Clean refrigerator surfaces and set the refrigerator temperature 40°F or less and the freezer 0°F or less to limit growth.
• Properly clean all cooking surfaces and utensils to avoid the spread of Listeria throughout your kitchen.
• Consume perishable and ready-to-eat foods as soon as possible.
• Refrigerate leftover foods within two hours after eating

What are the takeaways?

Now that you know all about Listeria monocytogenes and Listeriosis, there are a few important things to always keep in mind when cooking or cleaning in the kitchen. Make sure to wash your hands, produce, and cooking equipment thoroughly before beginning a meal. Additionally, ensure that all meat and poultry is cooked to the correct temperature by using a food thermometer. Use his handy guide on cooking temperatures from the USDA. Lastly, avoid undercooked meats, poultry or foods made with unpasteurized milk, especially if you belong to any of the most at-risk groups for Listeria monocytogenes (elderly, children, those with chronic illness, and pregnant women).

Sources:

Consumer’s Guide to Food Safety
Listeriosis and Pregnancy: What is Your Risk?
Foodsafety.gov Fact Sheet on Listeriosis
CDC: Frequently Asked Questions about Listeriosis

This blog post was written by Danielle Corrado, food science/policy intern from the University of Massachusetts, Amherst.