What is Listeria monocytogenes?

Information from National Food Agency, Sweden

Listeria monocytogenes may cause serious illness, particularly in people with weakened immune systems, the elderly and pregnant women. It can grow in food processing facilities and on equipment and contaminate foods produced there. The bacteria can grow at cold temperatures and even without oxygen. In this way, it can cause problems with ready-to-eat foods such as cured and smoked fish.

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What is Listeria monocytogenes?
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What is Listeria monocytogenes?

Listeria monocytogenes is a bacteria that can cause a dangerous infection called listeriosis in humans.

Where is Listeria monocytogenes found?

Listeria monocytogenes is generally present in nature and can be found in a variety of produce. The bacteria easily adapt to new environments and can start growing and they are difficult to eradicate. When present in food processing facilities it can easily contaminate foods manufactured there.

Characteristic for Listeria monocytogenes is that it can grow at refrigeration temperatures and even when deprived of oxygen, for example in vacuum-packaging. Although the bacteria grow more slowly at cold temperatures, the longer foods are kept the greater the risk of the bacteria reaching harmful levels.

Listeria monocytogenes have the capacity to withstand high salt levels and they survive freezing. However, heating to high temperatures kills the bacteria and it is not remarkably acid tolerant (e.g. pickled foods). Foods kept refrigerated for several weeks and then eaten without being heated have the highest risk of containing harmful levels of Listeria monocytogenes. Some examples are vacuum-packed smoked or cured fish, sliced small goods such as hams, pâtés, certain soft cheeses and other cold ready-to-eat foods.


The symptoms of listeriosis vary depending on who gets infected. People outside the risk groups can likely consume contaminated foods without becoming ill. However, for people within the risk groups, listeriosis is a very serious disease with 20-30% mortality. People with weakened immune systems, as well as the elderly, may contract sepsis or meningitis. Other symptoms are gastrointestinal symptoms such as diarrhea and nausea. Pregnant women may experience a flu-like illness or be symptom-free. The infection can spread to and infect the unborn baby. In the worst case, infection can lead to miscarriage or seriously ill babies.

Specific risk groups

The main risk groups for listeriosis are the elderly, persons with weakened immune systems and pregnant women.

Reducing the risk of getting sick

The risk of getting sick can be reduced by avoiding certain ready-to-eat foods and by heating food that may contain the bacteria, until steaming hot. This is particularly important for people with weakened immune systems, pregnant women and older people. Cooking at temperatures of 70 °C or higher kills the listeria bacteria.

To reduce the risk of getting infected by Listeria monocytogenes it is recommended to:

  • Keep cold produce refrigerated, preferably at a temperature of max. 4 °C.
  • Only eat gravlax, smoked fish and sushi if freshly prepared and recently packed. Check the packaging date.
  • Only eat sliced small goods and cold ready-to-eat produce if recently packed. Check the packaging date.
  • Avoid soft cheeses such as brie, gorgonzola, chèvre, vacherol and taleggio etc. if made from unpasteurized milk.
Swedish Food Agency

Source: Swedish Food Agency