What Causes Listeria Contamination

Before learning what causes listeria contamination, let us see what is listeria and also look at the features and common symptoms of listeiosis, which is caused by Listeria monocytogenes.

Listeria monocytogenes and Listeriosis

Listeria monocytogenes is the species of pathogenic, Gram-positive and facultative anaerobic bacteria that causes the infection listeriosis. Listeria monocytogenes is a non-spore-forming and rod-shaped bacterium that can survive in the presence or absence of oxygen. Listeriosis is a serious foodborne infection frequently caused by consuming food contaminated with the Listeria monocytogenes. Pregnant women, newborns, older adults, and adults with weakened immune systems are highly vulnerable to listeriosis.

Common Symptoms of Listeriosis

Patients with listeriosis may suffer from fever, headache, stiff neck, confusion, loss of balance, convulsions, muscle aches, and diarrhoea or other gastrointestinal symptoms. These symptoms may vary with the infected person.

Pregnant women characteristically show fever, fatigue and aches. However, infections during pregnancy can lead to birth defects, abortion, child mortality, premature delivery, or life-threatening infection of the newborn.

Older adults and immune compromising people show septicaemia, meningitis, acute febrile gastroenteritis or no symptoms.

What Causes Listeria Contamination

Listeria monocytogenes is regularly found in soil, faeces and water. Thus, raw vegetables and fruits can be contaminated from the soil or from contaminated manure used as fertilizer.  Animals can also carry the bacterium without indicating any illness and can contaminate foods of animal origin, such as meats and dairy products. In addition, cross-contamination is one of the biggest causes of widespread Listeriosis outbreaks, and many of them can originate in food processing plants or restaurant kitchens. It can be spread by contact with an infected product or surface such as hands or utensils during food preparation. Listeria grows rapidly and can thrive in oxygen rich as well as oxygen deficient environments.  Therefore, they can survive on foods that are stored in airtight containers. Listeria can also grow slowly and multiply in some foods in the refrigerator.

Listeria can hide in many foods such as raw and undercooked meats, unpasteurized milk, soft cheeses, and soaked fish, salted fish, ready-to-eat deli meats, processed meat, vacuum packaged meats and hot dogs. Soft Mexican-style cheeses like queso fresco can commonly cause Listeria outbreaks. Other cheeses that are made from unpasteurized milk or that got contaminated during cheese-making may also cause listeriosis.

What Causes Listeria Contamination

Consuing raw and undercooked meat can cause Listeria.

Prevention of Listeria Contamination

It is necessary to follow good hygienic practices and good manufacturing practices during commercial level food handling in order to prevent exposure to listeria.

To prevent cross contamination,

  • Raw meat, egg, seafood, and wastes should be kept away from other foods.
  • Food handlers should clean their hands with warm, soapy water before and after handling food. They should also wash their hands after using the bathroom, changing diapers, or touching animals.
  • Kitchen utensils and surfaces should be cleaned with hot, soapy water before and after they are used to prepare food.
  • Meat, eggs, fruits and vegetables should be washed under running water just before eating, cutting, or cooking.
  • It is so important to sterilise canning jars and canned foods and make sure that foods are prepared based on the good hygienic and good manufacturing practices before long-term storage.
  • Food should be cooked long enough and at a high temperature to kill the harmful bacteria that cause illnesses.
  • Avoid consuming unpasteurized milk or any other product produced from unpasteurized milk
  • Governments should enforce regulations to ensure that contaminated food products are reported, removed and recalled.


Listeriosis is a foodborne disease caused by food or beverages that contain harmful Listeria monocytogenes bacteria. Some people are more likely to develop Listeriosis than others, including infants and children, pregnant women, older adults, and people with weakened immune systems. Common symptoms of Listeriosis include vomiting, diarrhoea, abdominal pain, fever, and chills. Listeriosis can be prevented by properly storing, cooking, cleaning, and handling foods.


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About the Author: Geesha

Geeshani has a BSc (Hons) degree in Food Science and Technology and Master's degree in Food and Nutrition. She is currently a PhD Student at the Massey Institute of Food Science and Technology. Sharing what she learned is a passion of hers and enjoys writing.