Listeria monocytogenes
Definitions & Key takeaways
Listeria monocytogenes is a gram-positive, rod-shaped, facultatively anaerobic bacterium, which can cause an infection known as listeriosis. Although rare, listeriosis can be deadly in immunosuppressed people, pregnant women, newborns, and the elderly.
Listeriosis presents with low-grade fever, diarrhea, and vomiting. It can complicate into a high-grade fever, liver abscess and jaundice, and altered mental status in case of disseminated listeriosis. Listeriosis is diagnosed using cultures, and imaging studies like a CT scan may help identify liver abscesses. Treatment is with antibiotics, such as ampicillin and gentamicin combined, or just meropenem in those who are allergic to ampicillin.
Listeria monocytogenes or just L. monocytogenes, is a gram-positive bacteria that causes listeriosis.
Listeria was discovered by a Scottish doctor, J.H.H. Pirie, who named it in honor of British surgeon Joseph Lister,.
It’s also called “monocytogenes” because when it was inoculated in rabbits, their monocyte levels increased. Ok now, L.
monocytogenes is a rod-shaped bacteria with a thick peptidoglycan cell wall. So when gram-stained, it takes in purple dye, making it a gram-positive bacteria.
It is catalase positive and oxidase negative - which means it produces the enzyme catalase, but not oxidase. L.
monocytogenes is a facultative intracellular pathogen, meaning it can live both outside or inside of its host’s cells. It doesn’t form spores and it is facultatively anaerobic, meaning that it can survive in both aerobic and anaerobic environments.Now, when L.
monocytogenes is cultivated on blood agar medium, its colonies cause beta-hemolysis, also called complete hemolysis. That’s because it produces toxins called beta hemolysins, which hydrolyze the hemoglobin within red blood cells to transparent yellow color byproducts.
L. monocytogenes is a motile bacteria, with a very interesting way to move, that depends on both its location, and the temperature.
In an extracellular environment, this bacteria moves by beating its flagella creating a characteristic tumbling motility.
But this is only possible at 37 degrees Celsius and below. That’s because FlaA, the gene that codes for flagellin, which is the structural protein that makes up the flagella becomes downregulated as the temperature rises up to 37 degree Celsius.
So above 37 degrees, there’s reduced production of flagellin proteins, meaning no flagella being made, rendering L. monocytogenes non-motile.In an intracellular environment, L.
monocytogenes moves by an actin-based motility. It start with this bacteria producing a protein called Actin assembly-inducing protein or just ACTA, which recruits small actin filaments at one end.
As more and more actin filaments get recruited and polymerized behind the bacteria’s end pole, that propels the bacteria forward, like a rocket.L.
monocytogenes is a foodborne bacteria, meaning people get infected when they eat contaminated food - especially unpasteurized dairy products and cold deli meats, and this bacteria is known to survive even at very low temperatures in the fridge Once this bacteria gets into the gut, it uses its attachment proteins called internalins to attach to the host’s receptor proteins, such as E cadherin located on goblet cells of the intestinal mucosa.
After attaching to the host’s cells, this bacteria is slowly engulfed by the cell membrane, which invaginates to form a sac on its inner side.
The sac then separates from the actual cell membrane forming what’s referred to a internalization vacuole. Inside the vacuole, Listeria releases listeriolysin O, and bacterial phospholipases, which are enzymes that degrade the vacuolar membrane.
This releases the bacteria in the host’s cell cytoplasm, where it starts using the host’s cell resources and food, and multiply into many identical bacterial cells, by the process of binary fission, which means every bacterial cell splits in two identical copies.
On a side note, if this sounds similar to mitosis… well, it is! But the term binary fission is used to describe division of prokaryotic cells, which don’t have a nucleus, and therefore some steps in replication are different from mitosis.
Inside the cell, L. monocytogenes moves around using the actin-propelled motility, and when it reaches the cell’s membrane, it stretches it towards an adjacent cell.
When it reaches the adjacent cell membrane, that one invaginates as well, so Listeria ends up wrapped in a double-layered sac.
Then, this sac pinches off inside the adjacent cell, as a double membrane vacuole. Inside this vacuole, Listeria releases listeriolysin O and phospholipases to break free into the cytoplasm, repeating the invasion process over and over to invade more adjacent cells and, eventually, get into the bloodstream.
From the bloodstream, the bacteria spreads to other organs, resulting in disseminated listeriosis. This is most commonly seen in people with weaker immune systems, like neonates, the elderly, pregnant individuals, and adults with immunocompromising conditions, like diabetes, malignancy or an HIV infection.
Under these circumstances, Listeria can affect the liver resulting in a liver abscess, or move up to the brain through the blood-brain barrier to cause meningitis, which commonly occurs in neonates, the elderly or adults with diabetes.The worst case scenario, in pregnant individuals, when bacteria crosses the placenta and gets to the fetus.
THis may result in a spontaneous abortion or stillbirth. If the fetus survives to term, it may be born with neonatal listeriosis, which can present with neonatal sepsis and meningitis Now, when listeriosis is limited to the intestinal mucosa, it presents with signs of gastroenteritis such as low grade fever, diarrhea, and vomiting.
With disseminated listeriosis, there’s a high fever, muscle pain and additional symptoms depending on what organs are affected.
With liver abscesses there may be abdominal pain and jaundice, and with meningitis, symptoms include headache, stiff neck and altered mental status.
small. pale nodules referred to as granulomatosis infantiseptica.
Diagnosis is done with cultures from a biological sample, which varies depending on symptoms - so a stool culture is done for gastroenteritis, a blood culture when bacteremia is suspected, cerebrospinal fluid cultures for meningitis, and amniotic fluid cultures are done to diagnose a transplacental infection.
Imaging studies like an ultrasound or a CT scan can be useful to identify liver abscesses. Now, when listeriosis is limited to the gastrointestinal tract, treatment usually comprises of supportive measures like hydration and rest.
Disseminated listeriosis, however, is treated with intravenous ampicillin combined with gentamicin. In people with contraindications to these antibiotics, specifically pregnant individuals, or those who are allergic to ampicillin, meropenem is a good alternative.
Alright, as a quick recap, Listeria monocytogenes is a gram-positive rod-shaped facultatively anaerobic and beta-hemolytic bacteria.
It’s also a catalase positive and oxidase negative , facultatively intracellular pathogen. Listeria has a tumbling motility in the extracellular environment, and actin-based motility intracellularly.
Most commonly, it causes listeriosis in neonates, immunocompromised adults and the elderly. Listeriosis can present with low grade fever, diarrhea and vomiting, which can worsen to high grade fever, jaundice and altered mental status in case of disseminated listeriosis.
Listeriosis is diagnosed using cultures, and imaging studies like a CT scan may help identify liver abscesses. Treatment is done with a combination of ampicillin and gentamicin, or just meropenem in pregnant individuals or those who are
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