What is Listeria Monocytogenes?
Listeria monocytogenes is a type of bacteria that thrives both inside and outside cells. This bacteria can be identified in a lab because it can survive (and even grow) in cold temperatures, like those of your fridge. Not only that, it also produces an enzyme called catalase and can cause hemolysis, or the breakdown of red blood cells.
In the past, there have been several outbreaks of foodborne illnesses caused by L. monocytogenes. One of the most notable outbreaks happened in 1981 when this bacteria was found in various types of food. Another occurred in 1985 when an outbreak involving 142 cases was linked to a brand of soft cheeses, causing 28 deaths and 20 fetal losses.
L. monocytogenes doesn’t cause the most cases of foodborne illnesses but when it does, it has a high mortality rate due to its unique capabilities. The bacteria can move around inside a cell and it can also survive in cold temperatures—two traits that make it difficult for the food industry to control.
People usually get infected by consuming food contaminated with the bacteria—especially foods like cold deli meats and unpasteurized dairy products. Thankfully, the number of cases involving L. monocytogenes has been going down due to progress made in removing it from food, detections and advances in treatment.
Talking about the infection caused by L. monocytogenes (also known as listeriosis), it can lead to several health problems like sepsis (a life-threatening infection), meningitis (inflammation of the membranes covering the brain and spinal cord), encephalitis (inflammation of the brain), and spontaneous abortion. In a healthy adult, it may cause a fever and gastroenteritis, which is basically an upset stomach that can self-limit over time. Those who are most at risk are pregnant women, infants, individuals with weakened immune systems, and the elderly.
What Causes Listeria Monocytogenes?
Listeria is a type of bacteria that often causes an infection in the brain and spinal cord coverings (a condition called meningitis) in several groups of people. These include babies (neonates), the elderly, and those with weak immune systems. However, when healthy individuals get infected with Listeria, typically, they experience temporary symptoms like a fever and diarrhea since they tend to get a mild form of gut infection.
Under the family of Listeria, we have 10 different types of the bacteria. Among these, L. monocytogenes is the one most commonly found in humans. There are even 13 different strains of L. monocytogenes, depending on variations in their movement (flagellar) and surface markers (antigens). Despite this huge number, only three strains, specifically 1/2a, 1/2b, and 4a, cause disease in humans. Generally, a person needs to ingest a large number of these bacteria to be infected. However, these bacteria can multiply even at cool fridge temperatures, reaching levels that can cause infection, despite usual food storage safety measures. Your immune response also affects how many bacteria are needed to cause infection.
Using medicines like proton pump inhibitors, which suppress stomach acid, actually makes it easier for Listeria monocytogenes to pass through the stomach. This is because a less acidic environment is more favorable for the bacteria, enabling it to enter the cells lining the stomach and intestines (enterocytes) more easily.
Risk Factors and Frequency for Listeria Monocytogenes
Listeriosis is an illness that according to the Center for Disease Control (CDC), affects about 1,600 people each year, and sadly leads to around 260 deaths. It’s more common in certain groups like pregnant women, babies, people with weakened immune systems, and older people (those 65 and over). If a pregnant woman gets the disease, she can even pass it to her unborn baby.
The bacteria causing listeriosis, L. monocytogenes, are found in various places like soil, water, and decaying plants and can also be present in the human digestive tract. Some types of food are more likely to carry these bacteria, including:
- Raw sprouts
- Unpasteurized milk
- Soft cheeses
- Cold deli meats
- Cold hot dogs
- Smoked seafood
Signs and Symptoms of Listeria Monocytogenes
Doctors will often look for signs of Listeria monocytogenes, a type of bacteria, in certain groups of people. These groups include:
- Babies less than 29 days old
- Elderly people
- Those with weaker immune systems
- Pregnant women
Listeria can cause meningitis, which is an infection of the brain and spine. People with Listeria meningitis can have symptoms like fever, stiff neck, headaches, changes in behavior, and other typical signs of meningitis. They might also be confused or disoriented, not knowing their name, where they are, or the date. They could also show positive reactions to certain tests done by doctors, known as Brudzinski’s or Kernig’s signs. It’s also important to ask if patients have been taking medications for stomach acid, as these can increase the risk of catching a Listeria infection.
However, healthy adults and pregnant women with the infection might only feel like they have the flu, with symptoms like fever, diarrhea, headaches, chills, feeling sick, throwing up, and muscle pains. But some people might not show any symptoms at all.
Knowing what a patient has recently eaten can also be helpful in diagnosing them. This could include things like milk, soft cheeses, or cold deli meats and hot dogs as these may contain Listeria.
Testing for Listeria Monocytogenes
In order to identify the bacteria L. monocytogenes, doctors need to obtain a culture or sample of bacteria from the blood, fluid surrounding the brain and spinal cord, or fluid from the placenta of a pregnant woman. These samples are then sent to a lab where the bacteria grow on a special gel-like substance called Meuller-Hinton agar. When it grows, the bacteria takes the form of rod-shaped cells and produces colonies that break down red blood cells, a process known as beta-hemolysis.
However, the Centers for Disease Control and Prevention (CDC) have reported that testing a stool sample is not a reliable way to diagnose L. monocytogenes as this method is neither sensitive nor specific. In simpler terms, stool samples might not always accurately identify the presence of L. monocytogenes and might confuse it with other bacteria.
Treatment Options for Listeria Monocytogenes
To avoid getting infected with L. monocytogenes, you should avoid certain foods that are known to carry this bacteria and always wash your hands properly.
When a patient first arrives at the hospital showing signs of possible L. monocytogenes infection, the doctors will first check their overall health and stability and give necessary immediate treatments.
If the doctors suspect meningitis, a type of brain infection, they will typically carry out several tests and use medications. These tests may include blood tests, a lumbar puncture, also known as a spinal tap, to collect a sample of spinal fluid, and computerized tomography (CT) scan of the head, which gives a detailed image of the brain. Antibiotics are typically given to fight the possible bacterial infection.
When it comes to the choice of antibiotics, ampicillin or penicillin G, administered through an IV (a treatment given directly into the bloodstream), are usually used. However, if the patient is allergic to penicillin, trimethoprim-sulfamethoxazole may be used instead. It’s important to know that L. monocytogenes can’t be treated with a group of antibiotics known as cephalosporins.
Additionally, other medications might be given since meningitis can also be caused by other bacteria. For example, ceftriaxone might be given to treat certain bacterial infections, gentamicin could be used for bacterial infections such as Escherichia coli, especially in babies, and vancomycin might be used to treat infections caused by a type of bacteria called Methicillin-resistant Staphylococcus aureus.
For broad treatment of bacterial meningitis in groups vulnerable to L. monocytogenes infections, different combinations of antibiotics may be given depending on the patient’s age and overall health status.
To protect the public from getting infected with L. monocytogenes, the food industry can use approved food additives including bacteriophage (Listeria Phage P100) sprays. A bacteriophage is a virus that can kill bacteria like L. monocytogenes, and this spray could mainly be used on deli meats and cheeses.
What else can Listeria Monocytogenes be?
There are several infectious agents that can cause meningitis, such as the herpes simplex virus, Neisseria meningitidis, and Streptococcus pneumoniae. A lumbar puncture, or spinal tap, can help differentiate between meningitis caused by bacteria and meningitis caused by viruses. Bacterial meningitis is typically marked by a high count of a specific type of white blood cell, called a polymorphonuclear leukocyte.
Besides meningitis, there are other causes of symptoms like a headache and/or fever. These can include:
- Influenza Virus Infection
- Strep Throat (Strep Pharyngitis)
- Migraine Headache
- Tension Headache
- Cluster Headache
- Subarachnoid Hemorrhage
- Acute Middle Ear Infection (Acute Otitis Media)
- Subdural Hematoma
- Epidural Hematoma
- Mononucleosis
- Brain Tumor or Other Intracranial Mass
What to expect with Listeria Monocytogenes
Research shows that L. monocytogenes, a type of bacteria, is the third most common cause of death from foodborne illnesses in the United States, causing roughly 260 deaths each year. If a person is confirmed to be infected with L. monocytogenes, the mortality rate is approximately 15%. However, this rate can be even higher depending on a patient’s overall health and other existing medical conditions.
About a quarter of pregnancy-related cases result in poor outcomes, including the death of the fetus. Recognizing the infection early and starting treatment promptly are key to managing this illness. Common treatments include medications like ampicillin, penicillin G, or trimethoprim-sulfamethoxazole, which have been proven to be very effective against L. monocytogenes.
Preventing Listeria Monocytogenes
People who are at a higher risk of falling ill, such as pregnant women, older adults, and those with weakened immune systems, should be careful about the kinds of food they consume. Certain foods may pose a higher risk to these individuals:
* Raw sprouts
* Milk that hasn’t been pasteurized (heat-treated to kill bacteria)
* Soft cheeses
* Deli meats that are served cold
* Hot dogs that are served cold
* Seafood that’s been smoked
These foods might contain harmful bacteria, so taking precautions about what you eat can help keep you healthy.
