Bacterial pathogens and disease 2 (exotoxins)

Question 1

What components make up endotoxins?

Answer 1

LIPID A.Phosphorylated glucosamines attached to long chain fatty acids. No. and type of fatty acid vary by species.
→ Hydrophobic POLYSACCHARIDE CORE Ketodeoxyoctanoic acid (KDO) and heptose.
→ Relatively constant between species. HydrophilicO – SIDE CHAIN
→ Repeat units of tri, tetra or pentasaccharide sugars.
→ Highly variable between species Hydrophilic

Question 2

What are the characteristics of an endotoxin?

Answer 2

Endotoxin is lipopolysaccharide (LPS)
→ Lipid A is the active component. – not immunogenic.
→ O antigen is highly immunogenic and immune specific. *Found only in gram negative bacteria.
→ Heat stable
→ Not converted to toxoids.
→ Major initiator of the sepsis pathway.

Question 3

What is sepsis?

Answer 3

Life threatening organ dysfunction caused by a dysregulated host response to infection.

Sepsis primarily driven by the innate immune system response:
→ macrophages,
→ monocytes,
→ granulocytes,
→ natural killer cells
→ dendritic cells.

These cells detect:
→ pathogen associated molecular patterns (PAMP’s) such as endotoxin,
→ damage associated molecular patterns (DAMP’s) from damaged host cells. (induce signals that tell immune system there is damage)

This detection mediated via:
→ cell membrane receptors – toll-like receptors (TLR) and C-type lectin receptors.
→ cytosol receptors - NOD-like receptors, RIG-I-like receptors.

Effect:
→ Production of pro-inflammatory cytokines TNFα, IL-1, IL-6
→ via inflammasomes to produce IL-1β and IL-18 that cause rapid programmed cell death

Question 4

What are the effects of pro-inflammatory cytokines?

Answer 4

1. Increase number, lifespan and activation state of innate immune cells.
2. Increase adhesion molecule and chemokine expression by endothelial cells.
3. Increase acute phase protein such as complement, fibrinogen and CRP.
4. Cause fever.
5. Causes neutrophils to release extra-cellular traps (NETs) made of DNA and antimicrobial proteins that forms a scaffold for platelet activation.
6. Cause release of microparticles by activated platelets 7.Increase tissue factor expression by blood monocytes

→ In sepsis: 5+6+7 → formation of a thrombus (immunothrombosis) – microbes trapped within this → attracts and activate further leucocytes.

Question 5

What happens in the dysregulation of sepsis?

Answer 5

→ Production of reactive oxygen species (ROS) – Hydroxyl and nitric oxide – damages cellular proteins, DNA and lipids and impairs mitochondria.

→ Complement activation (esp. C5a) – increase ROS, granulocyte enzyme release, endothelial permeability and tissue factor expression.

→ Widespread immunothrombosis leading to disseminated intravascular coagulation (DIC) with impaired microvasculature function and organ dysfunction.
→ Mitochondrial damage leads to decreased intracellular ATP and cells enter state of hibernation – exacerbates organ dysfunction.

Question 6

What happens in Sepsis – resolution? ( How can sepsis be controlled?)

Answer 6

→ Active process – not passive.

→ Sepsis can be regulated by production of inflammatory cytokines eg anti-inflammatory IL-10 produced early in process:
1. Suppresses production IL-6 and γ-interferon
2. Stimulates production of soluble TNF receptor and IL-1 receptor antagonist

→ Autophagy of PAMP’s and DAMP’s – removal

Damaged cells – undergo apoptosis and engulfment by macrophages.

Question 7

What is Meningococcal Sepsis?

Answer 7

→ Caused by Neisseria meningitidis
→ Gram negative diplococcus
→ Serotypes A,B,C, Y, W135
→ Serotype A associated with large outbreaks in Sahel region of Africa – Meningitis belt.
→ Serotype B,C and W135 found in UK – declined since introduction of MenC and now MenB vaccine.
→ Can cause disease ranging from meningitis to life threatening meningococcal sepsis.

Question 8

What makes meningococcus so effective in sepsis?

Answer 8

Due to its structure (produces blebs)

Question 9

What pathogen recognition receptor specifically detects the lipopolysaccharides produced by different bacterial species?

Answer 9

TLR4