Bacterial lipopolysaccharides Flashcards
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
O-antigen
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
heptose
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
- O-antigen
Inhibition of complement activation Reduction to complement-mediated lysis Resistance to phagocytosis - 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
