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Flashcards in Bacterial lipopolysaccharides Deck (23):

what is Bacterial lipopolysaccharide (LPS)?

• LPS is a major part of the cell wall - compact ordered arrangement forms a rigid structure, maintains bacterial shape
• Unique, complex glycolipids are integral component of outer membrane
• an endotoxin damaging effect on host when released from cell envelope


whats the Structure of LPS?

Three structurally distinct elements
Core oligosaccharide
Lipid A


what is Lipid A?

• Highly conserved in Gram negative bacteria
• Integral component of membrane, in tightly packed domains
• Acts as anchor for surface LPS structure
• Consists of: phosphorylated N-acetylglucosamine dimers (guN) 6-7 fatty acids attached either to gluN or esterified to other fatty acids


what is the Core oligosaccharide?

• Surface exposed short chain of sugars
• Linked to lipid A by unusual sugar KDO:
• Heptose also unusual
• Inner core highly conserved, outer core more variable


what is the O antigen side chain?

- Provides variability in LPS - major antigenic domain
- Oligosaccharide subunits each of 3-5 sugars
- At least 20 different sugars contributes to variety of antigenic types
- Individual O antigens vary in length
– Up to 40 sugars
– Up to 30 nm in length


describe Assembly of LPS

• Fatty acids and KDO linked to glucosamine disaccharide  lipid A, dissolves in cytoplasmic membrane
• Additional sugars added to form core oligosaccharide
• Sugars of the O side chain linked to carrier undecaprenol phosphate
• Carrier/polysaccharide complex translocated to outer surface of cytoplasmic membrane, joined to lipid
A/core oligosaccharide component
• Complete LPS molecule transferred to outer membrane by ‘flippase’, complex of proteins


what the Functions of LPS

• Maintains OM as permeability barrier - inhibits diffusion of hydrophobic molecules, prevents entry of bile salts, detergents, lipophilic antibiotics
• Interaction with host cells – positive and negative effects on adherence
• Resistance to bactericidal peptides (e.g. defensins)
• Importance indicated by difficulty of isolating mutants entirely defective in LPS


whats the difference between Rough and smooth bacteria?

- ‘Smooth’ bacteria - complete core and O side chain
• ‘Rough’ bacteria - no O side chain, more easily
engulfed and destroyed by phagocytes
• ‘Deep rough’ - loss of parts of core, especially


why is Heptose region of core essential for OM stability?

- cross linkage of LPS
- maintenance of charged environment
- interaction with positive charges on proteins


explain Serum resistance

• Host immune response generates antibodies against O antigen side chains
• Certain O side chains protect bacteria from phagocytosis and bactericidal action of serum
- ‘smooth’ E. coli more resistant in serum assays than ‘rough’
- degree of resistance proportional to LPS content
- certain E. coli serotypes (O7, O8, O18) associated with septicaemia, survive better in serum
- serum resistant strains more likely to cause kidney damage in animal model


how do O chains contribute to serum resistance?

• O chains bind complement poorly, promote degradation of complement components
• Long side chains project O antigen away from bacterial surface
• Antibody reactions occur away from cell surface, less likely to have lytic effect
• O side chains may mask underlying bacterial surface molecules that might activate complement
• Hydrophilic O antigen might act as a water- solubilising carrier for toxic lipid A


what is the Dual Function of LPS in Shigellosis

1. O-antigen
Inhibition of complement activation Reduction to complement-mediated lysis Resistance to phagocytosis
2. Lipid A
Induction of inflammation Disruption of epithelial cell lining


what are the Pathological effects of LPS

• Injection of live/dead Gram negative bacteria into animals causes wide spectrum of pathology
• LPS identified as factor in heat-killed bacteria– purified LPS induces same toxic effects
• i.e. LPS = endotoxin
• Lipid A is the toxic part of LPS..............
• But because lipid A is embedded in the bacterial membrane it causes toxicity only upon cell lysis
• Lipid A released by autolysis, or external lysis by host immune response


whats the prevalence of Bacterial septicaemia

Over 150,000 deaths/year in US


what are the Predisposing risk factors for bacterial sepsis?

– immunocompromised
– extremes of age
– burn injuries
– indwelling urinary or venous catheters


what are the Initial signs of blood-borne infections

– fever
– increased heart and respiration rate
– increased levels of polymorphonuclear


what happens in Bacterial septicaemia?

• Bacteria proliferate in blood–antibiotics effective if given early
• Bacteria lysed by complement and phagocytosis, LPS released into circulation
• Complement further activated, induces production of cytokines
• Cytokines stimulate a massive inflammatory response
• Results in septic shock - collapse of circulatory system, multiple organ failure
• If left too late, antibiotics worsen the situation by lysing bacteria


what are the TLR4 Signals Pro- and Anti- Inflammatory Cytokines

1. Pro-Inflammatory
TNFa, IL-12, IL-6, IL-1b CCL3
2. Anti-Inflammatory IL-10, TGFb, IL1-RA


what happens in the LPS-Induced Cytokine Cascade?

1. Principal cytokine mediators TNFa and IL1-b
2. Activation coagulation system
3. Organ dysfunction
4. Immunotolerance (non-responsive macrophages results in secondary infections)
5. Anti-TNFa antibodies for treatment
6. limited effectiveness because TNFa is only one of
several inflammatory mediators involved


what happens during Inflammation?

High TNFalpha Low IL-10
Activated immune cells Tissue damage


what happens during tolerance?

Endotoxin Tolerance
Low TNFalpha High IL-10
Refractory state Unable to respond to secondary infection


name 4 Other toxic cell wall components

• lipo-oligosaccharides (LOS) of Gram- negative
• toxic lipoproteins of spirochetes
• peptidoglycan fragments and teichoic acids of Gram positive
• “superantigens”


summarise Bacterial lipopolysaccharides

1. LPS consists of two/three regions: lipidA, oligosaccharide core, O-antigen
2. LPS has multiple functions
3. LPS mediates resistance to complement by a
range of mechanisms
4. LPS is an endotoxin and can induce a cytokine cascade leading to severe pathological symptoms