Bacterial Pathogens and Diseases II (Endotoxins) Flashcards
Describe the membranous layers of bacteria
All bacteria have an inner membrane
Gram +ve have an outer peptidoglycan layer
Whereas Gram -ve have an extra outer membrane
Describe the structure of Gram -ve outer membrane
Outer membrane: bilayered, outer leaflet distinct in that it has lipopolysaccharide (endotoxin)
Where is gram-ve peptidoglycan layer situated?
Within periplasmic space, gram -ve have the peptidoglycan layer
What are the 3 components of lipopolysaccharides (LPS)?
Lipid A component
Polysaccharide Core
O-side Chain
Describe the lipid A component of LPS
Phosphorylated glucosamines attached to long chain fatty acids.
No. and type of fatty acid vary by species.
Hydrophobic
Main toxigenic component of endotoxin
Outline the structure of the polysaccharide core of LPS
Keto Deoxy Octanoic acid (KDO) and heptose
Relatively constant between species.
Hydrophilic
Describe the structure of the O-side chain
Repeat units of tri, tetra or pentasaccharide sugars.
Highly variable between species
Hydrophilic
How do LPS molecules protect against hydrophobic molecules?
LPS molecules non covalently cross bridged by Ca and Magnesium ions – provides a barrier to hydrophobic molecules including bile salts
Outline characteristics of endotoxins
- Endotoxin is lipopolysaccharide (LPS)
- Lipid A active component; not immunogenic
- O antigen is highly immunogenic + immune specific
- Only gram-ve bacteria
- Heat stable
- Not converted to toxoids
- Major initiator of sepsis pathway
What is sepsis?
Life threatening organ dysfunction caused by dysregulated host response to infection by innate immunity
Which cells are active in sepsis?
Sepsis driven by innate immune response:
- macrophages
- monocytes
- NK cells
- Dendritic cells
How does the innate immune response initiate sepsis?
Receptors recognise bacterial components (PAMPs) and/or danger signals (DAMPs) to activate cytokine transcription that will then recruit an inflammatory response
What are PAMPs?
pathogen associated molecular patterns e.g. endotoxin
What are DAMPs?
damage associated molecular patterns from damaged host cells
How is DAMPs/PAMPs detection by innate immune cells mediated?
Cell membrane receptors
- toll-like receptors (TLR)
Cytosol receptors
- NOD-like and RIG-I like receptors
What is the effect of PAMPs/DAMPs recognition by immune cells?
Pro-inflammtory Cytokine production (TNF-a, IL-1, IL-6)
Inflammasomes induce IL-1b and IL-18 production
- causes rapid apoptosis
What are PRRs?
pattern recognition receptors
- recognise PAMPs
e. g. TLRs
Which TLR is commonly paired with sepsis?
TLR4 are the PRRs that pair mostly with sepsis LPS
Outline a similarity between endo- and exotoxins
Both endo- and exotoxins have the ability to produce proinflammatory cytokines either through lymphocytes (exotoxins) or lipopolysaccharides (endotoxins)
Outline how E.coli endotoxin activates TLR4
- Lipid A component of LPS is recognised by MD-2
- MD-2+LPS bind to TLR4 molecule on the cell surface
- Causes dimerization of TLR4 causing intracellular
signalling
How does the TLR4 complex form when endotoxin binds?
MD2 has a coreceptor binding protein CD14 that enables the formation of the complex on TLR4
What is the effect of TLR4 activation?
The binding to TLR4 causes activation of an intracellular cascade that eventually leads to NF-Kꞵ activation
What is the significance of NF-Kꞵ activation?
NF-Kꞵ activation leads to production of proinflammatory cytokines
What is the effect of cytokines on immune cells?
Increase number, lifespan and activation state of innate immune cells
Cause fever
Describe how cytokines effect endothelial cells
Increase adhesion molecules and chemokine expression by endothelial cells
What effect does cytokines have on acute phase proteins?
Increase acute phase protein such as complement , fibrinogen and CRP
Outline the effects of cytokines that lead to a thrombus formation
- Causes neutrophils to release extracellular traps (NETs) made of DNA and antimicrobial proteins that form a scaffold for platelet activation.
- Cause release of microparticles by activated platelets
- Increase tissue factor expression by blood monocytes
What is the purpose of thrmobus formation by cytokines?
Formation of a thrombus (immunothrombosis) traps microbes within this
Attracts and activates further leukocytes.
How do we define all the cytokine actions occurring at once?
All these effects together are called septicemia or septic shock
Why is there such a drastic immune response to LPS?
Very potent immune response even in presence of small amounts of LPS as gram -ve bacteria in the bloodstream is life threatening and can be lethal
What are the common symptoms cuased by LPS?
LPS causes the following symptoms:
- Iron deficiency
- Fever
- Hypoglycaemia
- DIC Thrombosis
- Hypotension
- Shock
What effect does LPS have on macrophage activity?
LPS activate macrophages to produce TNF (IL-1):
- causes a fever
- Decreases Fe; liver tries to collate Fe so bacteria can’t grow
- Causes endothelial cells to express receptors
How does LPS effect mast cells?
LPS causes mast cells to degranulate producing mediators
How do LPS cause liver damage?
Endotoxins also lead to liver damage due to release of acute phase proteins reducing glucose levels
What is the effect of LPS Platelet activation?
Also activates platelets causing:
Increased vascular permeability; neutrophil recruited to damage site are able to get into tissue; can cause low BP ⇒ shock
Disseminated intravascular coagulation (DIC) ⇒ thrombosis
How does LPS lead to thrombosis?
Complement pathway can also be activated as well as clotting mechanisms ⇒ hypotension and thrombosis
Outline the signs and symptoms of sepsis
- Increased vascular permeability
- Hypotension leading to hypovolemic shock
- Fever
- Disseminated intravascular coagulation (DIC)
- Multiple organ failure
What is bacteraemia?
Bacteria present in blood but have short lifespan due to phagocytosis
What is septicaemia?
Long lasting harmful bacteria in blood unable to be phagocytosed by macrophages
Septic shock mortality: 50-85%
What is the aim of sepsis dysregulation?
Achieves rapid control of localised and minor infections
Why does sepsis dysregulation not always occur?
However dyregulation may pass threshold → systemic injury
Tissue factor; initiation of thrombin formation from prothrombin + clotting
Tissue factor pathway; extrinsic
How does sepsis damage mitochondria?
Production of reactive oxygen species (ROS) – Hydroxyl and nitric oxide – damages cellular proteins, DNA and lipids and impairs mitochondria
What are the consequences of complement pathway activation in sepsis?
Complement activation (esp. C5a) – increase ROS, granulocyte enzyme release, endothelial permeability and tissue factor expression
How does sepsis lead to DIC thrombosis?
Widespread immune thrombosis leading to disseminated intravascular coagulation (DIC) with impaired microvascular function and organ dysfunction
What is the consequence of mitochondrial damage in sepsis?
Mitochondrial damage leads to decreased intracellular ATP and cells enter a state of hibernation – exacerbates organ dysfunction
How is sepsis mediated?
Sepsis is actively negatively regulated by anti-inflammatory cytokines e.g. IL-10
What are the regulatory effects of IL-10 for sepsis?
suppresses proinflammatory cytokines; IL-6 and ɣ-interferon
Produces receptors for proinflammatory cytokines (TNF and IL-1) to bind and downregulate them - antagonis
What causes meningococcal sepsis?
Caused by Neisseria meningitidis
Gram negative diplococcus
What are the different types of meningococcal sepsis?
Serotypes A,B,C, Y, W135 - varying O side chains
What is the effect of meningococcal sepsis?
Can cause disease ranging from meningitis to life threatening meningococcal sepsis.
> meningococcus is very effective in sepsis
