Listeriosis: Bacterial Infection from Listeria Monocytogenes

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  • 0:05 1985 Outbreak
  • 1:54 Listeria Monocytogenes
  • 3:34 Listeriosis
  • 5:56 Diagnosis and Treatment
  • 6:59 Prevention
  • 8:04 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Women have a lot to worry about during pregnancy. In this lesson we will discuss one more threat to developing babies, the bacteria Listeria monocytogenes, and the foodborne illness it can cause.

1985 Outbreak

In 1985, Los Angeles was not the place to be an expectant mother. Obstetricians started noticing a significant increase in pregnant women exhibiting a vague range of symptoms, including diarrhea, back pain, difficulty breathing and itchy skin. The strange combination of symptoms made making a diagnosis difficult. Eventually, blood cultures starting coming back positive for a dangerous foodborne illness called listeriosis.

The diagnosis was bittersweet for the doctors and patients. Knowing what they were dealing with meant that targeted treatment could begin immediately, but listeriosis can be devastating to fetuses and newborns in spite of treatment. In order to save as many children as possible, focus had to shift to identifying the source of the disease. New mothers from all over the Los Angeles area, both infected and uninfected, were interviewed, comparing all sorts of information about foods they had eaten, restaurants they had visited and places they had gone.

Eventually, a pattern emerged. All of the infected women had recently purchased or eaten a soft cheese produced by one specific company. Investigations into the company showed that they were mixing unpasteurized milk into the cheese, which was contaminating the batches with the bacteria that cause listeriosis. Officials quickly recalled the cheese and closed the factory, ending the outbreak. Unfortunately, the damage had already been done. Eighteen adults and 10 newborns died. Another 20 women suffered miscarriages. In this lesson, we will examine listeriosis and the causative agent: Listeria monocytogenes.

Listeria monocytogenes

Listeria monocytogenes is a Gram-positive, facultative anaerobic, rod-shaped bacteria commonly found in soil, water and animal intestines. 'Gram-positive' refers to the thick layer of peptidoglycan present in the Listeria cell wall, making these cells appear purple in the classic Gram stain. As a facultative anaerobe, Listeria is not only able to survive in the anaerobic intestines but also in the presence of oxygen in the environment. Listeria is also a psychrophile, meaning it can survive and grow in cold temperatures like those found in your refrigerator. We will discuss why this is important for Listeria infection.

Most people acquire Listeria by the fecal-oral route, consuming foods that have been contaminated by infected feces. But since Listeria is normally found in soil and water, anything you eat could potentially contain Listeria. Some of the more common sources include ready-to-eat meats like cold cuts and hotdogs, unpasteurized dairy products and undercooked meat. Recently, more cases have been linked to soft cheeses, like Brie and feta.

The reason that soft cheeses and ready-to-eat meats are common sources goes back to the psychrophilic nature of Listeria. These products are often refrigerated for a long time before they ever get to the supermarket. You are likely to buy them, refrigerate them again and eat them without cooking. During all of this refrigeration time, the bacteria are rapidly growing and dividing. The longer the food is refrigerated, the larger the Listeria population becomes, and the more likely you are to be infected.


But you might be wondering, 'If Listeria is everywhere, and I refrigerate all kinds of food for days and weeks at a time, why have I never gotten sick from it?' The answer lies in your immune system. It is likely that every one of us has been exposed to Listeria, but if you're healthy, your body has eliminated the bacteria before you ever developed any symptoms. Ninety percent of cases occur in the very young, the very old, pregnant women and those with compromised immune function.

For these sensitive groups, what results is listeriosis, a food infection caused by consuming actively growing Listeria monocytogenes. Remember, in a food infection, the disease symptoms are a result of the actively growing bacteria in the patient's body.

In a healthy patient, Listeria can incubate in the intestine for 3-70 days. During this time, the patient may develop diarrhea, fever and abdominal cramps as a result of the bacteria attacking the lining of the large intestine. If the bacteria happen to infect someone in one of the high-susceptibility groups, the disease can get much worse. First, the bacteria invade the lining of the intestine, where they are eventually engulfed by white blood cells. Once inside the white blood cells, the bacteria can invade the bloodstream. By hijacking the white blood cells' own cellular machinery, the Listeria directs the cell to build a small actin 'tail' that the bacteria attaches to its cell surface and uses like a propeller to invade neighboring cells, where it can spread the infection.

Once in the blood, the bacteria can spread to the rest of the body. Patients can expect fever, malaise, jaundice and arthritis. Eventually, the bacteria can cross the blood-brain barrier, causing inflammation of the brain. These patients often experience headaches, neck stiffness, delirium and coma.

In pregnant women, Listeria is particularly dangerous. The parasite infects the uterus, causing a range of very vague, nonspecific flu-like symptoms in the mother that are often overlooked or misdiagnosed. The bacteria are able to cross the placental barrier into the fetus, resulting in mental damage and miscarriage. Many newborns born with Listeria infections don't live long.

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