Listeria monocytogenes, a bacteria.
Listeria monocytogenes are gram positive rods
but sometimes appears a coccobacillus so perhaps not quite as long as a classic rod might be.
They’re facultatively intracellular therefore they must grow
within their target organism or target cell which is many times a macrophage.
When they culture out they have a beta hemolytic pattern so a clear hemolysis
and they are also noted to be catalase positive and again, when seen live under microscope,
they have tumbling motility, they are quite the acrobats on the microscope slide.
Importantly and as a way to differentiate the organisms,
listeria grow best at refrigeration temperatures four to ten degrees Celsius,
in fact to grow best, the culture is actually held at refrigeration temperatures
which is what is called cold-enrichment.
So the listeria are part of our normal gastrointestinal flora
and not just humans but many other animals as well.
They are transmitted or acquired via ingestion of that normal flora
which can be from unpasteurized dairy products or cold deli meats -
remember, cold enriches their growth;
but also via transmission from other body surfaces and especially to babies via transplacental
or vaginal transmission.
How they grow is an intracellur mechanism and they typically target macrophages
and epithelial cells and as part of that, because they grow within the cell,
they’re protected from the humoral or the antibody base response.
In addition then, after the growth pattern and after preserving the health of their target cell,
their carrier if you will, they then cause release of the newly formed listeria bacteria
via the listerialysin O toxin, which lyses the phagosomes
who lyses the cells releasing the bacteria for the purpose of infecting the next cell or causing their disease.
In addition, they can also directly transfer across the cell membranes themselves.
All these are designed to protect the host cell until it’s time for the organism to leave.
So we can see this in schematic form here in this slide the stages in the intracellular life cycle.
In this case, our friendly listeria is infecting an epithelial cell, you can see the photocytes at the surface,
it enters unto the cell on the left side of the screen
and goes through a process of growth creating not just two but many times,
many, many copies of itself as it continues to replicate and then as seen in this slide,
and the cell transfer across the tight junction to the next cell on the line and continue to grow.
However, when they ultimately leave the cell, that toxin is released and it causes death of the cell.
So, while the cell itself is protected initially by the growth of the organism,
it ultimately will fall target to the action of the bacteria itself once the bacteria is done.
Not a very pleasant guest to have.
Disease caused by Listeria monocytogenes typically is gonna be found in immunocompromised patients
either those who have malignancies or an immunodeficiency
or the very young or the very old who have a compromised immune system.
When listeria causes disease in adults, it typically presents as a meningitis and a sepsis,
and again, these are patients who are suffering from a non-intact immune system.
However, pregnant women, when they are infected with listeria, may simply have a mild flu-like illness so malaise,
some fever perhaps nausea which is thankfully not that significant,
however, that does mean that the mother is able to transmit the organism to her baby inside.
Healthy individuals, those of us who have a normal or healthy intact immune system
may experience no more than a self-limited gastroenteritis,
certainly not as severe as other type of food poisoning but enough to be clinically prominent
especially because such cases occur many times in the clustered format,
so you may be aware of listeria clusters related to infected lunch meats
or infected water supply in which time many patients in the cluster are infected
within about 1 or 2 weeks of each other.
Infants, when they are infected by listeria there are two different forms.
The first type of which is called granulomatosis infantiseptica, it’s quite a mouthful
but thankfully does not occur that commonly. In this case, the baby have disseminated abscesses
and granulomas all over and it has a high mortality, a high death rate.
These are typically infants who are infected in-utero by a mother who herself was infected
by that mild listeria type disease.
The more common form of neonatal disease is neonatal listeriosis
and this is called late onset disease.
Now, it’s not necessarily late onset like you might see with other infections
such as group B beta hemolytic streptococci, but it does occur within the first several weeks
and it occurs via exposure to infected secretions at the vaginal exit or the cervical mucosa,
so, babies are born they acquire or inhale typically infected secretions,
the organism incubates and then of the course of the next two to three weeks
it creates infant sepsis and meningitis.
So again, a prominent illness but if recognized and treated in time, not necessarily a fatal one.
Listeria again is cultured in a cold-enriched environment refrigeration temperature.
It is known to cause monocytosis.
Now, how are you gonna remember that?
The key is in the name, listeria monocytogenes.
Think about that and you will remember the fact that patients with listeria disease
will frequently have a predominant presence of monocytes in their peripheral white blood cell count.
Treatment is via penicillins or ampicillin in all cases.
So listeria can cause multiple types of disease, some of them very severe,
most of them typically are just simply gastroenteritis but it comes from many different sites
and it can cause monocytosis and it can come from those lovely dairy meats
which you may have just had for your lunch - sorry.