Bacterial Pathogens and Diseases 2 (Endotoxins) Flashcards
What is an endotoxin, its structure, function, and contribution to disease?
An endotoxin is a lipopolysaccharide (LPS) molecule found in the outer membrane of Gram-negative bacteria (cell wall), heat stable and a major initiator of the sepsis pathway.
LPS comprises lipid and sugar components, with the lipid component (Lipid A) as an anchor in the bacterial membrane and the sugar component extending outward (Core Polysaccharide + O-Polysaccharide).
Lipid A:
- The active component, not immunogenic thus makes endotoxins effective
- Phosphorylated glucosamines attached to long-chain fatty acids
- Number and type of fatty acid vary by species
- hydrophobic thus interacts with the membrane
Polysaccharide core:
- Ketodeoxyoctanoic acid (KDO) and heptose
- Relatively constant between species
O-side chain:
- Repeat units of tri, tetra or penta-saccharide sugars
- Highly variable between species
- Highly hydrophilic thus interacts with environment
LPS can contribute to disease by stimulating an immune response in the host, which can cause inflammation and tissue damage.
LPS can also activate the host’s coagulation and complement systems, leading to further tissue damage and dysfunction.
Compare endotoxins with exotoxins
Both produced by bacteria, but they differ in structure, function, and mechanism of action.
Endotoxins are part of the bacterial cell wall and are released when the bacteria die, while exotoxins are secreted by bacteria into the surrounding environment.
Endotoxins are generally less potent than exotoxins, but they can still cause harmful effects in the host by activating the immune system and contributing to the pathogenesis of certain diseases.
Exotoxins, on the other hand, are often more potent and can have specific effects on host cells and tissues.
Define what is meant by sepsis
A life-threatening condition of organ dysfunction and failure caused by dysregulated immune response to infection, leading to inflammation and tissue damage.
Primarily driven by the innate immune system response: macrophages, monocytes, granulocytes, natural killer cells & dendritic cells
These cells detect:
- pathogen-associated molecular patterns (PAMPs) such as endotoxin
- damage-associated molecular patterns (DAMPS) from damaged host cells
detection mediated via:
- cell membrane receptors - toll-like receptors (TLR) and C-type lectin receptors
- cytosol receptors - NOD-like receptors, RIG-I-like receptors
Effects:
- production of pro-inflammatory cytokines TNFɑ, IL-1, IL-6
- via inflammasomes to produce IL-1β and IL-18 that cause rapid programmed cell death
Dysregulation:
- Production of reactive oxygen species (ROS) - Hydroxl 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 (general thrombosis) leading to disseminated intravascular coagulation (DIC) with impaired microvasculature function, leading to organ dysfunction
- Mitochondrial damage leads to decreased intracellular ATP and cells enter a state of hibernation - exacerbating organ dysfunction
Resolution:
- Reduce activity of certain immune cells through release of anti-inflammatory cytokines, i.e. IL-10 (suppresses production of IL-6 and γ-interferon and stimulates production of soluble TNF receptor and IL-1 receptor antagonists)
- Autophagy of PAMPs and DAMPs - removal
- Damaged cells undergo apoptosis and engulfment by macrophages
What is the role of endotoxin in sepsis by investigating Meningococcal sepsis?
LPS binds to antigen-presenting cells (APC), i.e. macrophages, causing more cytokine release and priming the APC or dendritic cells to be better at presenting antigen to T cells thus further immune responses (by T-lymphocytes), mediated by exotoxins/super-antigens from Gram-positive bacteria
Macrophages and dendritic cells express abundant TLR, particularly TLR4, which is responsible for the downstream signals to make the cell better at presenting antigens
MD-2 is associated with TLR4 on the cell surface and enables TLR4 to respond to LPS. MD2 has hydrophobic insides, able to detect fatty acid chains, O-side chains are detected by TLR.
Binding of MD2 + LPS complex to TLR (and CD14, a co-receptor associated with TLR4), causing 2 TLRs to dimerize, triggering signals downstream via Myd88-IRAK4-IRAK2/1 (myddosome formation) to TRAF6. Leading to the activation of NF-kB, which translocate to the nucleus and act on pro-cytokine genes, i.e. TNFɑ
What are the effects of pro-inflammatory cytokines
- Increase number, lifespan and activation state of innate immune cells = beneficial to having a robust immune response
- Increase adhesion molecule and chemokine expression by endothelial cells
- Increase production of acute phase proteins in the liver such as complement, fibrinogen and CRP
- Cause fevers as pro-inflammatory cytokines such as TNFɑ & IL-1 are endogenous pyrogens
- 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
- 5+6+7 → formation of a thrombus (immunothrombosis)
- Microbes trapped within this → Attracts and activates further leucocytes
Define what is meant by sepsis
Sepsis is a serious, life-threatening condition caused by a dysregulated response of the body to an infection
This dysregulation can lead to widespread inflammation throughout the body, affecting various organ systems
Sepsis is an extreme reaction to an infection, which occurs when the chemicals the immune system releases to fight infection cause inflammation throughout the body instead
Severe sepsis, like sepic shock is sepsis causing poor organ function or insufficient blood flow to the organs
What is Meningococcal Sepsis
Meningococcal sepsis is caused by the gram-negative bacterium Neisseria meningitidis which produces endotoxin in its outer membrane
Neisseria carries lipoligosaccharides (LOS) has a shorter structure and a different lipid A
Neisseria also produces Blebs, capsule secretion of the membrane which diffuse into the tissue, packed with LOS
Causing excessive immune response, thrombosis, rash, oedema and severe neurological issue such as septic shock
The endotoxin can trigger an excessive immune response in the host, leading to the release of inflammatory cytokines and other mediators that can cause damage to tissues and organs.
In Meningococcal sepsis, this immune response can lead to septic shock
