Listeria Flashcards
How many species are in the genus Listeria?
How many are considered important pathogens?
What is the name of those pathogen(s)?
6
1 Species
L. monocytogenes
L. monocytogenes
Describe the morphology of this bacteria.
Describe how this organism would appear on culture.
Gram-positive rod, small, fat. Coccoid forms occur commonly. Motile at 22 C via peritrichous flagella.
Colonies are small, slow growing (30-40 hr), with a narrow zone of beta hemolysis. The organism is able to multiply at 5C and will eventually survive and outgrow contaminants.
Picture of what a gram stain of Listeria would look like (can easily mistake for a Strep).
For L. monocytogenes describe some of the biochemical reactions you would see.
H2S negative, *catalase pos, rapidly reduces litmus milk, motility at 22C by peritrichous flagella.
Note: Remember all Streps are catalase negative
Describe the habitat for L. monocytogenes.
Ubiquitous in nature in soil, feces, silage and other plant material, milk. The organism multiplies in a pH range of 5.5 to 9.0. Silage that has been properly preserved has a pH of 5.0 or less.
How do humans get infected with L. monocytogenes?
Human infections often result from ingestion of the organism in food.
Raw vegetables, dairy products (ice cream) and meats. In the U.S., the ingestion of raw hot dogs and undercooked chicken account for most of the food-associated disease.
What are some common food handling practices that are done which increase the likelihood of a L. monocytogene infection?
Give an example of how an outbreak of L. monocytogene could happen.
Some commonly used food handling practices such as dipping lettuce in warm water to keep it from browning (increases shelf life) also contribute to increased L. monocytogenes contamination.
You could have Aorganically grown foods being fertilized with livestock manure containing L. monocytogene. Large outbreaks such as the one associated with Mexican cheese in California in the 90’s occur sporadically.
Note: About 1% of humans shed L. monocytogenes in their feces. The role of fecal shedders is thought to be secondary to the ingestion of the organism from natural sources.
What is this?
What organism could you suspect to grow in this environment?
What environmental conditions would there need to be for this organism to thrive?
Corn silage
L. monocytogenes
The organism multiplies in a pH range of 5.5 to 9.0. Silage that has been properly preserved has a pH of 5.0 or less. When the silage is exposed to air or is improperly cared for it can result in increased pH.
True/False: Proper pasteurization will kill even high concentrations of the L. monocytogene organism.
TRUE
What important toxin does L. monocytogenes produce?
How is it different from the Streptolysin O toxin?
Following phagocytosis, this toxin is responsible for destruction of the phagosome and release of Listeria into the cytosol of the macrophage. It has also been demonstrated that this toxin inhibits macrophage-mediated antigen processing. It is cardiotoxic, lethal for mice, cytolytic for many eucaryotic cells including macrophages.
Streptolysin O toxin is leukotoxic, attacks RBC and causes complete hemolysis, toxic to platelets and fibroblasts, very antigenic. (destroys cell membrane)
What is the general pathogenesis of L. monocytogenes.
Most animals and humans are exposed to this organism regularly. See disease in animals often associated with the feeding of poor quality silage or stress such as a sudden change in feed, pregnancy, or intercurrent disease that weakens the immune system. Seen in lymph tissues of normal animals. It is considered to be a facultative intracellular parasite.
What is this picture demonstrating?
L. monocytogenes replicating inside of a macrophage cytosol.
Once inside of an animal/human how does L. monocytogenes replicate?
The organism is ingested primarily by macrophages and is able to escape from the phagosome (Listeriolysin O is essential to this process).
It is surrounded by actin-like filaments from the host cell which aggregate to form a tail-like structure. This structure facilitates the movement of the organism to the cell membrane of the macrophage.
The cell membrane evaginates and the organism is transferred directly to an adjoining macrophage. The organism is not exposed to the humoral immune system and is therefore not susceptible to killing by antibody and complement.
What is happening in this picture?
- ) Phagosome with L. monocytogene bacteria
- ) Phagosome is lysed
- ) Replication in cytosol
- ) Bacteria encapsulated by short actin filaments
- ) Formation of long actin tail
- ) Penetration of neighboring cell (macrophage)
Describe how L. monocytogenes infects cattle and the outcomes of this infection.
What is a halmark sign that a cow might have L. monocytogenes infection?
Usually enters via ingestion, passes through the wall of the ileum, establishes a bacteremia or septicemia, and if the animal is pregnant localizes in the placentomes and eventually causes the death of the fetus and abortion (usually in the 2nd half of pregnancy).
Circling disease.
Describe how circling disease occurs in cattle.
How does L. monocytogenes get to the CNS?
Meningoencephalitis; move in circles with unilateral facial paralysis, fever, blindness, head pressing, and eventually death.
The organism is able to travel to the brain via the olfactory or trigeminal nerves and thus produce a unilateral lesion. It may also spread systemically to the brain.
