Pathogenesis and Cellular Invasion Mechanisms of Listeria monocytogenes

Infection Mechanisms of Gram-Positive Pathogens

• Gram-positive pathogens utilize surface proteins (‘PTs’) to invade or ‘roade’ host cells.

• Once inside, these pathogens exert control over cellular functions for their own benefit.

Listeriosis and Listeria monocytogenes

• Disease: Listeriosis.

• Causative Agent: The bacterium $Listeria\,monocytogenes$.

• Transmission: Primarily spread through the consumption of contaminated food.

• General Symptoms:

  • Nausea.

  • Diarrhea.

  • Muscle aches.

  • Fever.

• Nervous System Manifestations: If the infection spreads to the nervous system, symptoms include:

  • Headache.

  • Stiff neck.

  • Confusion.

  • Loss of balance.

  • Convulsions.

• Pregnancy Risks: Listeriosis is considered a ‘$10/10$’ issue for pregnant women.

  • The pathogen has the rare ability to cross the placenta, which is not common for most other bacteria.

  • Consequences for the fetus or newborn include miscarriage, stillbirth, premature delivery, and serious infection of the newborn.

Statistics and Infective Parameters

• Infective Dose: Between $100$ and $1000$ organisms are required to establish an infection.

• Incubation Period: Symptoms typically onset $30$ days after exposure.

• Annual Impact (U.S.): Approximately $2,500$ people are infected per year, resulting in $500$ deaths per year.

• Mortality Rates:

  • General death rate falls between $20 - 30\%$.

  • Mortality for patients in the Intensive Care Unit (ICU) reaches $90\%$.

  • Approximately $20 - 25\%$ of infected fetuses die.

Route of Infection in the Host

• The journey of $Listeria\,monocytogenes$ through the human body follows a specific sequence:

  1. Ingestion: Consumption of contaminated food.

  2. Intestine: The bacteria reach the intestinal tract and cross the intestinal barrier.

  3. Lymphatic and Blood Entry: Pathogens enter the lymphatic system and subsequently the bloodstream.

  4. Organ Colonization: They travel to the liver and spleen, which serve as primary sites for replication.

  5. Secondary Bloodstream Entry: After replicating, they get back into the blood.

  6. Systemic Spread: From the blood, they can reach critical targets including the brain, the placenta, and the fetus.

Intracellular Pathogenicity and Internalins

• $Listeria$ is an intracellular pathogen, similar to $Salmonella$.

• To enter host cells, the bacterium uses disease-causing proteins or ‘effectors’ on its surface that mimic particles recognized by the host to be internalized.

• These specific internalization proteins are known as Internalins.

• There are two primary documented types:

  1. Internalin A.

  2. Internalin B.

Internalin A (InlA) and the E-cadherin Receptor

• Internalin A is an $800$ amino acid ($a.a.$) protein found on the listerial cell wall.

• Function: It is responsible for bacterial entry into non-phagocytic cells.

• Human Receptor: The specific receptor for Internalin A is human E-cadherin.

• E-cadherin Details:

  • It is an intracellular junction protein found at adherens junctions.

  • The structure of E-cadherin includes domains labeled $EC1$, $EC2$, $EC3$, $EC4$, and $EC5$.

  • It interacts with tight junctions and desmosomes in the intestinal epithelial cell membrane, connecting with intermediate filaments and actin filaments in the cytoplasm.

• Species Specificity: A critical distinction exists at the molecular level between humans and mice. There is a $1\,a.a.$ difference in the E-cadherin receptor between the two species. Consequently, while $Listeria$ can infect humans via this pathway, it has no effect on mice.

Intestinal Invasion through Cell Extrusion

• The anatomy of the small intestine includes villi (singular: villus).

• New epithelial cells form at the base of these villi and migrate upwards toward the tip.

• At the top of the villus, older cells are removed in a process called ‘cell extrusion.’

• Cell extrusion exposes E-cadherin at the adherens junctions, which are normally tucked away.

• This exposure provides the specific site where $Listeria$ attaches and invades the intestinal lining.

Internalin B (InlB) and the Met Receptor

• Internalin B is a $630\,a.a.$ protein.

• Function: It is able to invade cells without the help of Internalin A.

• Strategy: Unlike Internalin A, it does not rely on cell extrusion sites.

• Target Cells: It can invade various cell types, including hepatocytes (liver cells).

• Host Receptor: The receptor for Internalin B is the Hepatocyte Growth Factor Receptor (HGF-R), also known as Met.

• Mechanism: $Listeria$ hijacks the host's endocytic machinery.

  • Internalin B binds to its receptor (Met).

  • It recruits clathrin to the site of bacterial contact.

  • The bacterium is internalized via clathrin-based endocytosis.

• Timing of Internalization: Video evidence indicates clathrin-based internalization occurs within approximately $72\,sec$. Laboratory tests show that a bead coated with Internalin B alone can be internalized by a host cell in approximately $803\,sec$.

Intracellular Motility and Actin Hijacking

• Once inside the cell, $Listeria$ is initially contained within a vacuole.

• The bacterium becomes motile within the cytoplasm by controlling host cellular functions.

• Actin Tails: The motility is driven by the formation of tails made of actin filaments.

• Effector Protein (ActA): $Listeria\,monocytogenes$ uses an effector on its surface called ActA to control the cell's cytoskeleton.

• Mechanism: ActA recruits host actin to create ‘F-actin’ tails. These tails propel the bacteria through the cell, allowing for further spread. This process has been highlighted in research by Julie Thierot.