Listeria and Listeriosis - Foodborne Illness and Salad Greens

A salad with raw, packaged and pre-washed romaine lettuce - nutritious and convenient, but a potential cause of foodborne illness
A salad with raw, packaged and pre-washed romaine lettuce - nutritious and convenient, but a potential cause of foodborne illness | Source

What are Listeria and Listeriosis?

Periodically we hear on the news that a particular brand of packaged salad greens has been recalled due to the presence of a bacterium that can cause a foodborne illness. Sometimes the bacterium that is discovered is Listeria monocytogenes. Listeria can cause listeriosis, a potentially serious and even deadly disease.

Listeria usually affects specific groups of people, including pregnant women, embryos and fetuses, young babies, elderly people, people with weakened immune systems and people with disorders such as diabetes and liver disease. It occasionally affects people outside of these groups, however. The CDC (Centers for Disease Control and Prevention) classifies Listeria as an important public health problem.

Many people will develop no symptoms from a Listeria infection. In fact, it's thought that up to 10% of us have Listeria living in our intestines without causing any obvious problems. For those who are susceptible to the bacterium, however, the consequences may be severe. Listeriosis may be a mild illness but is more often serious. The death rate may be as high as 20%.

Lettuce is sometimes a source of Listeria.
Lettuce is sometimes a source of Listeria. | Source

Transmission of Listeria

Listeria may be present in the intestine of many mammals, birds, fish and crustaceans. It's passed out of an animal's body in its stool. The bacteria can then be transmitted to us in plant foods contaminated by infected stool, soil or water. Listeria can survive in a variety of environments, which is one reason why it's so troublesome.

Another problem is that Listeria in an animal's body may contaminate food obtained from the animal. The bacteria can be transmitted in meats, fish and dairy products. Cooking these foods properly or pasteurizing dairy products will kill the Listeria bacteria. However, refrigerating the products won't kill the bacteria. In fact, the bacterial culture can actually grow inside the refrigerator. Food contaminated by Listeria looks, smells and tastes normal, which makes it impossible for a consumer to detect the problem.

Cheese made from pasteurized milk is fine, but soft raw milk cheeses should be avoided to reduce the risk of listeriosis.
Cheese made from pasteurized milk is fine, but soft raw milk cheeses should be avoided to reduce the risk of listeriosis. | Source

Symptoms of Listeriosis

Symptoms of a Listeria infection don't appear immediately after a susceptible person eats contaminated food. The bacterium has an incubation period. This period is very variable in length and ranges from as little as three days to as long as seventy. This means that it's sometimes hard for a person to link their symptoms to the ingestion of Listeria.

Gastrointestinal problems - especially vomiting and diarrhea - are common symptoms of a Listeria infection. A fever, muscle aches and fatigue may also occur. People who develop flu-like symptoms from the infection may not realize that they have listeriosis.

If the bacterial infection spreads to the nervous system, more serious conditions may develop. These include a severe headache, a stiff neck, confusion, loss of balance, convulsions and meningitis. Meningitis is inflammation of the meninges, the membranes surrounding the brain. Meningitis may be accompanied by encephalitis, which is inflammation of the brain itself. The two conditions together are known as meningoencephalitis and can be extremely serious.

Five Things to Know about Listeria


Listeriosis is treated with antibiotics. Although antibiotic treatment is often successful, sometimes fatalities occur. The sooner the antibiotic treatment begins, the better the chance of success.

If a person has eaten food known to be contaminated by Listeria but experiences no symptoms, they may not receive any treatment. It's wise to seek advice from a health agency in this situation. This is especially true for someone in a high-risk group.

Listeriosis is considered to be a rare but potentially serious infection. It's one of the more dangerous examples of foodborne illness. Nevertheless, some people experience no symptoms from the infection.

Listeria monocytogenes
Listeria monocytogenes | Source

Behavior of Listeria monocytogenes

Listeria is a rod-shaped bacterium that is capable of movement. The behavior of Listeria in our bodies is complex and isn't completely understood, but researchers know that it has multiple methods to help it invade cells and survive. Although it's a bacterium that we want to avoid, it's actually a fascinating little creature.

Once the bacterium is in our intestine, it enters the cells of the intestinal lining. It then travels to other cells through the membrane that surrounds the cells and the cytosol (fluid) within them. Listeria uses a protein called actin to enable it to move. The bacterium uses molecules within its host cell to assemble its actin "tail". The actin looks like a comet's tail and propels the bacterium forward, as shown in the video below.

Sometimes a bacterium hits the cell membrane as it moves. It creates a protrusion in the membrane and is then engulfed by the membrane of a neighboring cell without ever entering the tissue fluid that surrounds the cells. This enables it to evade the immune system.

Movement of Listeria monocytogenes in a Mammalian Cell

Listeriosis Risk

Listeriosis is most likely to appear - and most likely to have serious effects - in the following groups of people.

Pregnant Women

During pregnancy, a women's immune system is weakened and the immune system of her baby isn't fully developed. If a baby is infected by Listeria during the first three months of its development, there may be a miscarriage. Older babies may be stillborn, be born prematurely or have health problems when they are born. A pregnant woman with listeriosis may only experience flu-like symptoms, but her developing baby may be much more seriously affected.

Elderly People

As we age, our immune system becomes less effective at protecting us from invaders like bacteria and viruses. Therefore elderly people are more susceptible to listeriosis than younger ones.

People With Certain Illnesses

People with AIDS have an impaired immune system that has trouble fighting Listeria. People with other diseases may also have difficulties with Listeria, including those with cancer, liver disease, kidney disease and diabetes.

Organ Transplant Patients

Organ transplant patients receive medications to suppress their immune system so that their body doesn't reject their new organ. These medications can also make the patients susceptible to bacterial infections.

People Taking Corticosteroids

Corticosteroids are useful medications for reducing inflammation. However, they can also hinder the activity of the immune system, making it less able to protect a person from a bacterial attack.

Be careful with raw vegetables like cabbage - they can transmit Listeria.
Be careful with raw vegetables like cabbage - they can transmit Listeria. | Source

How to Prevent Listeriosis

There are many steps that we can take to reduce the chance of developing listeriosis or other foodborne illnesses. Here are the main food safety rules.

  • Wash hands thoroughly before touching food.
  • Clean kitchen surfaces and utensils thoroughly before and after their use.
  • Clean refrigerator shelves frequently. (Pieces of food and drops of liquid that fall in the refrigerator may contain Listeria and contaminate other foods.)
  • Keep raw meats separated from other foods that are often eaten raw, such as fruits and vegetables, both in a shopping basket and in the refrigerator. Place the meats on a lower shelf than other food in the refrigerator.
  • Store leftovers in the refrigerator in leak-proof containers.
  • Refrigerate leftovers promptly and keep the refrigerator at a temperature of 4°C (39°F) or lower.
  • Eat ready-to-eat foods quickly, even when they are stored in a refrigerator.
  • Cook meats thoroughly, using a digital thermometer to check the temperature.
  • Don't drink raw milk and don't eat soft cheeses made from unpasteurized milk, especially if you are in a high risk group. Hard cheeses made from unpasteurized milk are aged for a longer time period and are less likely to contain harmful bacteria.
  • Don't eat raw eggs or raw fish.
  • Wash raw vegetables thoroughly.
  • Avoid pre-mixed raw vegetable mixtures such as coleslaw.
  • Wash raw fruits that you aren't going to peel thoroughly.
  • Use all food by the "Best before" or "Use by" date.
  • Clean reusable grocery bags thoroughly and regularly and use a separate bag for meats (unless the meats are canned).

Food Safety During Pregnancy

Additional Recommendations for High Risk Groups

There are extra food safety recommendations for pregnant women and other people in high-risk groups.

  • Cook ready to eat meats such as luncheon meats, deli meats, cold cuts and hot dogs until they are steaming, since they sometimes contain Listeria. Consider eliminating these meats from your diet.
  • Avoid smoked meats and pâtés (unless the pâtés are canned).
  • Never eat soft cheeses made from unpasteurized milk.

Raw fruits and vegetables - except for sprouts - are still considered to be safe for people in high risk groups, provided they are washed. If you want to be very safe, however, you might want to cook raw produce, because Listeria outbreaks do occur periodically in raw vegetables and occasionally in raw fruits. The risk of infection is low, but it isn't zero.

Cantaloupes are a special problem with respect to foodborne illness. The ridges and indentations of the rind are known to harbor bacteria. These bacteria can enter the flesh of the cantaloupe when the fruit is cut. To reduce this possibility, the rind should be cleaned thoroughly before the fruit is opened. Any scrubbing brushes or other equipment used to clean the cantaloupe should be washed afterwards.

In 2011, there was a deadly listeriosis outbreak in the United States caused by infected cantaloupe.
In 2011, there was a deadly listeriosis outbreak in the United States caused by infected cantaloupe. | Source

In late 2011, a Listeria infection made 147 people ill, killed 33 people and caused one pregnant woman to have a miscarriage. The outbreak was the largest in U.S. history and spread through 28 states. It was caused by contaminated cantaloupes.

Listeriosis and Pregnancy

Bagged or Packaged Salads and Listeria

Packaged, pre-washed salad greens are very convenient and are a quick and easy way to add vegetables to a meal. Nutritionists say that many of us don’t eat enough vegetables. This is especially true for the green kinds. They are full of healthy phytonutrients, vitamins and minerals and should be a major part of our diets. Having pre-washed greens available means there’s very little excuse to avoid eating vegetables - the greens can be pulled out of the package and eaten right away.

Periodically there is an announcement that Listeria or another potentially dangerous bacterium has been detected in salad green packages. A recall generally happens when testing shows that at least one container of a batch of greens is contaminated with bacteria. The whole batch is then recalled for safety reasons, even if no illnesses have been reported.

Vegetables and fruits are very nutritious, but they should be washed properly before use.
Vegetables and fruits are very nutritious, but they should be washed properly before use. | Source

Safety of Pre-Washed Salad Greens

Some people wonder if pre-washed salad greens should be washed again once they get them home, which would reduce their convenience. Food safety experts say that this isn’t a good idea. They say that it’s unlikely that home washing will remove bacteria that industrial washing couldn’t remove. In addition, when they're washed at home the greens may pick up new bacteria from a sink or cutting board. They say we have to accept that there is a small risk of getting a foodborne illness when we eat raw vegetables.

Most of us don't have to worry about the risk of getting sick from eating bagged salads, especially if we are healthy and follow normal food safety rules. People in high risk groups might want to visit their doctor to help them decide whether they should cook all their vegetables, however. Vegetables are still an excellent component of the diet when they're cooked. Although we should definitely keep food safety in mind and take certain precautions, we shouldn't avoid eating healthy foods because of the fear of foodborne illness.

Listeria News

  • The CDC (Centers for Disease Control and Prevention) maintains a webpage about Listeria and listeriosis. The page includes news about the latest outbreaks.
  • The FDA (Food and Drug Administration) maintains a list of food recall press releases.

© 2012 Linda Crampton

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Comments 20 comments

drbj profile image

drbj 4 years ago from south Florida

When I eat at home, Alicia, I usually make a salad to accompany my meal. But now due to your intelligent article on the risk of Listeria in packaged, bagged salad fixins, I'm going to replace those bagged salads with fresh produce that I can personally wash. Thanks for reminding me.

AliciaC profile image

AliciaC 4 years ago from British Columbia, Canada Author

Thank you for the visit and the comment, drbj. Raw produce is so nutritious - and it's delicious, too. It's a shame that we have to be concerned about foodborne illness when we buy it! Luckily the food safety experts say that thorough washing of produce should keep most of us safe.

PegCole17 profile image

PegCole17 4 years ago from Dallas, Texas

I've wondered the same thing about whether to rewash prepackaged salad mixes. Very interesting explanation about the whys and hows of this foodborne illness. Having experienced quite a few brushes with food reactions I was fascinated by your explanation of this.

AliciaC profile image

AliciaC 4 years ago from British Columbia, Canada Author

Hi, Peg. I guess I've been lucky - the last episode of foodborne illness that I experienced (as far as I'm aware of) was in my childhood. That was a horrible event that affected my whole family! Since then I haven't had any problems. Thanks for the comment.

teaches12345 profile image

teaches12345 4 years ago

Excellent advice and one that will may save some lives. I must check on the refrigerator this week, I do clean it regularly, but now you have me thinking. Voted up!

AliciaC profile image

AliciaC 4 years ago from British Columbia, Canada Author

Thank you for both the comment and the vote, teaches. I appreciate them both. I try to remember the food safety rules too. I'd hate it if my family got a foodborne illness.

Peggy W profile image

Peggy W 4 years ago from Houston, Texas

Hi Alicia,

Excellent article. I was fascinated by that video showing how listeria can go from cell to cell and reproduce avoiding the immune system. Scary! Your food safety tips are well advised. I would never have thought to cook deli meats! Since I am not in a high risk group I doubt that I will start doing that. But good to know, for those who have to take extra precautions. Voted up, useful, interesting and will share. Thanks!

ethel smith profile image

ethel smith 4 years ago from Kingston-Upon-Hull

Very useful. I had heard of Listeria but in a vague sort of way. You have filled in all the gaps thoroughly. Thanks Peggy

AliciaC profile image

AliciaC 4 years ago from British Columbia, Canada Author

Hi, Peggy. Thanks for the comment, the votes and the share! Yes, I find Listeria and its behavior fascinating too - although I wouldn't be happy if I had listeriosis! Listeria is one of the most dangerous bacteria that cause foodborne illnesses because of its "sneaky" and complex behavior.

AliciaC profile image

AliciaC 4 years ago from British Columbia, Canada Author

Thank you very much for the visit and the comment, Ethel.

prasetio30 profile image

prasetio30 4 years ago from malang-indonesia

Very interesting hub and again....I learn many things from you, Alicia. Thanks for share with us. Thumbs up for you and pressing the buttons here.


AliciaC profile image

AliciaC 4 years ago from British Columbia, Canada Author

Thank you very much, Prasetio! I appreciate your visit and the votes.

ologsinquito profile image

ologsinquito 2 years ago from USA

I was very careful when pregnant to avoid certain high-risk foods, such as soft cheese. This is a good warning, because we usually don't hear about lysteria.

AliciaC profile image

AliciaC 2 years ago from British Columbia, Canada Author

Hi, ologsinquito. Yes, it is very important for pregnant women to be aware of the potential dangers of Listeria. Thanks for the comment.

rebeccamealey profile image

rebeccamealey 2 years ago from Northeastern Georgia, USA

Good stuff to know. I don't think I have ever heard of These illnesses. I will remember NOT to rewash my greens, I suppose. Thanks!

AliciaC profile image

AliciaC 2 years ago from British Columbia, Canada Author

Thanks for the visit, Rebecca. I wash most of my greens, but not the prepackaged salad greens that have already been washed. So far I've had no problems!

AudreyHowitt profile image

AudreyHowitt 2 years ago from California

Oh so good to know!!

AliciaC profile image

AliciaC 2 years ago from British Columbia, Canada Author

Thanks, Audrey! I appreciate your comment.

PegCole17 profile image

PegCole17 20 months ago from Dallas, Texas

This illness seems to come and go on the news as cases are reported. With the long incubation period, it would be difficult to trace back the origin of how a person contracted it. I know that it scared me off from eating cantaloupes last summer. I'm wondering if we're seeing more of this because we import veggies from countries whose fertilization and sanitation practices may be in question.

AliciaC profile image

AliciaC 20 months ago from British Columbia, Canada Author

Hi, Peg. Thanks for the interesting comment. Listeria is certainly worrying, whatever the cause!

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    Linda Crampton (AliciaC)1,242 Followers
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    Linda Crampton is a teacher with an honours degree in biology. She enjoys writing about human biology and the science of health and disease.

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