35. Bacterial Pathogens and Diseases II

Question 1

Describe the endotoxin in the gram negative bacteria

Answer 1

• The endotoxin is an integral part of the gram negative bacterial cell wall
• It sits in the outer membrane of the bacteria; it is not secreted or released

Question 2

What are the three main areas of a lipopolysaccharide?

Answer 2

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.
• Hydrophilic

O – SIDE CHAIN

• Repeat units of tri, tetra or pentasaccharide sugars.
• Highly variable between species
• Hydrophilic

Question 3

List some characteristics of endotoxins

Answer 3

• 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 4

What is sepsis?

Answer 4

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

Question 5

Describe the immune response that sepsis is primarily driven with

Answer 5

• Sepsis primarily driven by the innate immune system response. Includes macrophages, monocytes, granulocytes, natural killer cells and dendritic cells.
• These cells detect:
  • pathogen associated molecular patterns (PAMP’s) such as endotoxin,
  • damage associated molecular patterns (DAMP’s) from damaged host cells.

• 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 6

What is the mechanism of sepsis?

Answer 6

The endotoxin from the gram negative bacteria goes and binds to CD14 receptors on macrophages.

The macrophages then pass the endotoxin to TLRs, which then dimerise. This dimerisation activates a cascade event that ends in activation of NF-kβ and the production of TNF-α and other cytokines.

Question 7

What are some effects of pro-inflammatory cytokines?

Answer 7

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

5, 6 and 7 formation of a thrombus (immunothrombosis) microbes trapped within this. This attracts and activate further leucocytes.

Question 8

How can immune function be dysregulated?

Answer 8

• Process described achieves rapid control of localised and minor infections.
• However the process may pass a threshold
→ systemic injury

Question 9

Describe the dysregulation of sepsis

Answer 9

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 10

Describe the process of resolution in sepsis

Answer 10

Active process – not passive.

Anti-inflammatory IL- 10 produced early in process. The helpful effects are:

• Suppresses production IL-6 and γ-interferon
• Stimulates production of soluble TNF receptor and IL-1 receptor antagonist

Autophagy of PAMP’s and DAMP’s ~ causing their removal

Damaged cells – undergo apoptosis and engulfment by macrophages.

Question 11

Describe Meningococcal sepsis

Answer 11

• 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 12

What makes meningococcus so effective in sepsis?

Answer 12

• Has 6 FA chains instead of 5 = highly effective as toxin
• Doesn’t have a long polysaccharide in the end ~ called lipopolysaccharide
• Carbohydrates similar to on humans cells = little immune response

•Blebs – great chunks of outer cell membrane being chucked away – puts the toxin in the environment
- Sheds a lot of polysaccharides constantly in blebs, so produces huge amounts of toxin.