Bacterial Pathogens and Diseases II (Endotoxins) Flashcards
what are endotoxins a part of?
the bacterial cell wall
what do all bacteria have?
an inner membrane
what do gram positive bacteria have?
an outer peptidoglycan layer
what do gram negative bacteria have?
extra membrane on the outside – they have an inner and outer membrane, and in between this (the periplasmic space) you have the peptidoglycan layer
what is a big component of gram negative bacteria?
lipopolysaccharide – the endotoxin
-causes symptoms
describe the polysaccharide structure:
split into 3 parts:
LIPID A COMPONENT
- Main toxigenic component of the endotoxin
- Toxic lipid chain that anchors into the membrane
- Hydrophobic
- No. and type of fatty acid vary by species
CORE POLYSACCHARIDE
- Relatively constant between gram negative species
- unique sugars
- Hydrophilic
O – SIDE CHAIN
- repeats units of sugars tri, tetra or pentasaccharide sugars in different combinations
- highly variable between species AND between strains of the same species
- hydrophilic
variability in lipids are responsible for what?
the differences in virulence that the lipid A component projects
how do strains differ in terms of structure, and how will this affect pathogenicity?
bacteria can acquire genes that vary their genetic makeup, for example certain coli strains can make diff combinations of polysaccharides on the hydrophilic o-side chain so they differ antigenically, so you might have an antibody against 1 side chain that doesn’t recognise another
eg. difference in o side chain that means complement can no longer bind and assemble on the surface
what are LPS molecules non covalently cross bridged by?
Ca and Magnesium ions
-provides a barrier to molecules including bile salts
what exactly is the “endotoxin”?
the lipopolysaccharide (LPS)
- Found only in gram negative bacteria.
- Heat stable
- Not converted to toxoids.
- Major initiator of the sepsis pathway.
can you make an immune response against all the parts of the LPS?
No,
the lipid A is the active component and not immunogenic, ie. you cannot make an immune response against it or a vaccine
but, the o antigen is highly immunogenic and immune specific.
Sepsis – What is it?
Life threatening organ dysfunction caused by a dysregulated host response to infection.
what is sepsis primarily driven by?
the innate immune system response:
- macrophages
- monocytes
- granulocytes
- natural killer cells
- dendritic cells
in sepsis, the cells of the innate immune system detect what?
PAMPs and DAMPs
what are DAMPs?
damage associated molecular patterns from damaged host cells
Eg if a bacteria attaches to surface of an epithelial cell and releases toxins that damage the endothelial cell – the components of the damaged cells will be recognised by dendritic cells or macrophages etc.
what are PAMPs?
pathogen associated molecular patterns eg. endotoxin
how are PAMPs and DAMPs detected?
cell membrane receptors (TLR’s, C-type lectin receptors)
cytosol receptors (NOD-like receptors, RIG-I-like receptors)
what is the effect of PAMPs and DAMPs being detected by cell membrane and cytosol receptors?
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
what are the effects of pro-inflammatory cytokines?
Increase number, lifespan and activation state of innate immune cells.
Increase adhesion molecule and chemokine expression by endothelial cells
Increase acute phase protein such as complement , fibrinogen and CRP
Cause fever.
Causes neutrophils to release extra-cellular traps (NETs) made of DNA and antimicrobial proteins that forms a scaffold for platelet activation
Cause release of microparticles by activated platelets
Increase tissue factor expression by blood monocytes
consequences of sepsis?
increased vascular permeability
hypotension leading to hypovolaemic shock
DIC (disseminated intravascular coagulation)
multiple organ failure
bacteria vs septicaemia?
Bacteraemia – you can find bacteria in the blood, but these are phagocytosed and killed quickly. But, fi those bacteria cannot be cleared, you get symptoms of immune response – septic shock and septicaemia.
Sepsis – dysregulation
Production of ROS 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 DIC with impaired microvasculature function and organ dysfunction.
Mitochondrial damage leads to decreased intracellular ATP and cells enter state of hibernation – exacerbates organ dysfunction
resolution of sepsis?
Autophagy of PAMP’s and DAMP’s – removal
Anti-inflammatory IL-10 produced early in process
Damaged cells – undergo apoptosis and engulfment by macrophages.
Meningococcal Sepsis - what causes it?
Neisseria meningitidis, a gram negative diplococcus
- has a potent LPS that drives sepsis
Neisseria meningitidis - how do the strains differ and what is the consequence?
the strains all have different antigenic natures – the LPS o-side chain all vary from each other. This means if you have antibodies against strain A, you will NOT have protective immunity against strain C because you aren’t recognising the same structure
What makes meningococcus so effective in sepsis?
meningococcus LPS is different to the normal standard LPS. Lacks o-antigen on the surface, and has only got short o-antigen rather than a long one – called an oligosaccharide rather than a polysaccharide, but it has the same very potent immunostimulating effect.
Lipid blobs of the membrane hat it keeps releasing, full of LPS. Once inside the body it starts producing lots of these blebs, causing an overwhelming endotoxin response – not all gram negatives to do this, but nisserium does.
clinical presentation of meningococcus septacaemia
Tumbler test (for someone with purpurik rash, gram negative septacaemia)
- 30% of people with septacaemia get pin prick rashes on their skin which get wider and larger and start to become purpurik and then they coallesce together
- large haemorrhages in the skin which wont move due to the thrombotic effect, and all the platelets having been used up
- oedema due to the inflammatory process.
