7) Invasion & Intra growth

• Involves dissemination of a pathogen throughout local tissues or the body

• intracellular pathogens achieve invasion by entering the host’s cells and reproducing

• these bacteria are mostly shielded from humoral antibodies

• can exploint nutrients in the host cell

• rare competition from other microbes

—> however, must possess specialized mechanisms to protect them from the harsh effects of the lysosomal enzymes encountered in the cell

Obligate intracellular pathogens:

• can only reproduce inside of host cells

• e.g.

  • Chlamydia spp.

  • Mycobacterium leprae

Facultative Intracellular pathogens:

• can reproduce either inside or outside of the host cells

• e.g.

  • Legionella pneumophila

  • mycobacterium tuberculosis

  • Salmonella spp

Invasion into a host cell occurs by endocytosis -> 2 different mechanisms:

1) “Trigger” mechanism

• bacteria attaching to host cell surface deliver effector proteins into host cell via specialized secretion system

• effectors interact with cellular targets, causing membrane ruffling beneath the invading bacteria

• eventually leads to bacterial engulfment and internalization into a membrane bound vacuole

2) “Zipper” mechanism

• bacterial surface proteins bind receptors on host cell membrane, activating signal cascades that control actin cytoskeleton

• actin-driven pseudopods which fuse in a zippering membrane process progressively entrap bacteria

• bacteria are progressively engulfed until a phagosome is formed

Salmonella pathogenicity Island 1 (SPI1)-encoded T3SS translocates effectors that drive “trigger”-mediated invasion of host cells

• Present on their surface is the outer membrane (OM) protein invasion —> plays crucial role during first phase of infection by faciliating efficient translocation across the intestinal ephithelial barrier (“zipper”-mediated)

• avoidance of the lysosome is central to all intracellular survival strategies

• archieved bei either

  • promoting phagosomal membrane disrupton and escaping the cytosol

  • Preventing fusion of phagosome with lysosome by remodelling it (Salmonella)

    
    

Living within the phagosome, Salmonella:

• prevent fusion of phagosome with lysosome —> remain alive and dividing within the phagosome

Phagosome lysis, Shigella:

• Produce proteins that lyse the phagosome befure it fuses with the lysosome —> allows bacteria to escape into cytoplasm to multiply

a) Listeria invades host cells via a zipper mechanism

b) escapes from phagosome before fusion with lysosome occurs

c) replicated in cell cytosol

d) spreads by actin polymerization -> propels bacteria unidirectional

e) promotes cell-to-cell spreading of Listeria

f) rupture of two-membrane vacuole is mediated by action of LLO and PC-PLC

• obligate intracellular bacteria cannot live outside the host cells

• most are unable to carry out energy metabolism and lack many biosynthetic pathways -> entirely dependent on host cell to supply them with ATP

• Difficult, because obligate intracellular bacteria cannot be grown in artificial media (e.g. agar plates), but require viable eukaryotic host cells -> cell culture, embryonated eggs, susceptible animals

• chlamydia growth cycle involves transformation between distinct forms:

  • elementary body (EB) -> metabolically inactive, extracellular Chlamydia growth form

  • reticulated body (RB) -> non-infectious, can replicate

• Highly infections EB attaches to ephithelial cells and induces ingestion by host

• Once ingested into a phagosome -> fusion of the phagosome with the host lysosome is prevented (ensures EB survival)

• EB reorganises within the phagosome —> metabolically active RB

• Several stimuli (antibiotics, cytokins) can drive chlamydia into a persistent state, lasts until removal of stressor

• If persistence is avoided, or if infection is reactivated from persistence, the RB will ultimately reorganise back into EB (after multiplication) —> released from host cell to infect surrounding epithelial cells

Coxiella:

• “stealth pathogen” -> after inhalation by host, invasion and replication within macrophages without alerting the innate immunse system

• Inside macrophages, bacterium replicates within a compartment that is very similar to a phagolysosome, termed Coxiella-containing vacuole (CCV)

Rickettsiales:

• group of obligated vector borne gram negative bacteria, containing many organisms of clinical and agricultural importance

• each have different mehtod of intracellular survival (??? steht nicht mehr auf der Folie)

• Recognition of pathogens through PAMPs (TLR, NOD, ..)

• Autophagy: degradative pathway by which cytosolic content, organells and patogens are delivered to lysosomes as part of innate immunity