Lecture 5 LPS structure + functions Flashcards
What is LPS?
common component of the outer membrane of Gram-negative bacteria
(peptidoglycan= major in positive, minor in negative)
Primary function of LPS?
structural component of the outer leaflet of the OM, creates permeability barrier
Neisseria meningitidis: characteristics?
- exclusively human pathogen
- 5-10%= asymptomatic carrier
important cause of meningitis and/or sepsis
•capsular polysaccharides are effective vaccines for serogroups A/C/W/Y but not serogroup B
Characteristic of invasive neisseria meningitidis strains?
Invasive strains are always encapsulated
What are two options when becoming infected with neisseria meningitidis?
- Carriage: growth at nasopharyngeal epithelial surfaces
* Disease: invasion of submucosa, blood circulation
Why is neisseria meningitidis used as a model for LPS biosynthesis?
- Unique among Gram-negative bacteria: viable without LPS
- Endotoxin activity of LPS is a major disease determinant
- LPS structures show extensive antigenic variation
What are the two parts of LPS?
- Lipid A
2. LPS core oligosaccharide (directly attached to lipid A)
Characteristics of Lipid A?
- membrane anchor
- Beta(1’, 6) glucosamine disaccharide
- 1, 4’ bisphosphorylated
- 4 primary acyl chains (C-14)
- 2 secondary acyl chains (C-12 & C-14)
- major determinant of endotoxin activity
Characteristics of 2. LPS core oligosaccharide (directly attached to lipid A)?
- Synthesis at the inner leaflet of the inner membrane
- Consists of an inner- and an outer-core
- Inner-core structure is conserved and consists of heptose units
- Important for the stability of the outer membrane
Many LPS biosynthesis genes display phase variation. Why?
- High frequency (10-3 to 10-4) of variants always present in bacterial population
“pre-adapted” for next step in the infection process
•Immune escape from bactericidal antibodies
•Switch between intracellular vs. extracellular forms
•Mucosal vs. blood growth
•Transmissive vs. invasive forms
•Binding to host cell receptors
•Serum resistance
The outer membrane protein Imp: characteristics?
- partial-loss-of-function mutations in the imp gene increased OM permeability (Imp: increased membrane permeability)
- Imp =essential protein, depletion =s evere membrane defects
- imp gene = present in all Gram-negative bacteria, except those without LPS (e.g. Spirochetes)
Does LPS reach the cell surface in the imp mutant ? How did they find out?
No.
Modification of LPS with sialic acid: can be removed with a certain enzyme.
Imp mutant: sialic acid cannot be removed.
Conclusion: LPS= not surface exposed.
What inserts LPS into the outer membrane?
LptD-LptE complex
Energy provided: LptB molecule. LptD : LPS insertion in outer leaflet.
LPS synthesis pathway?
Cytoplasm, inner leaf IM, flips to outer leaflet of IM, then transported to Lpt complex to outer leaflet of OM.
LPS feedback inhibition: how is it done?
Enzyme in IM which can degrade LpxC (protein in LPS synthesis). Degradation of LpxC is normally inhibited by pbga (enzyme in I membrane).
Aggregation of LPS molecules: pbga = blocked, -> LpxC = inhibited by enzyme in IM. = no LPS biosynthesis.
Lipid A can be modified to change the OM permeability. Modification of Lipid A acylation can be done by different enzymes:
PagL deacylase & Pag Pacylase: acyl composition can be rapidly modified.
lpxE & lpxF: Lipid A dephosphorylation
PLS O-antigen: part of LPS molecule. Characteristics?
• Long chain, consists of repeating sugar units (up to 40) Long highly variable polysaccharide extension of repeat units
• High structural diversity
• Not found in all Gram-negative bacteria, e.g. not in mucosal bacteria
• Important for protection against complement-mediated lysis
• Used for serotyping
- can be important virulence component
Another aspect of LPS: it is only found in the outer leaflet. System is needed to actively maintain this. Two systems are needed to actively maintain this. They are?
PldA: phospholipase which can degraded phospholipids in the outer leaflet of OM
Mla system: can pick phospholipid molecules and transport them back to the IM
What happens upon inactivation of both systems?
If you inactivate both the systems the LPS deficient bacteria becomes much more viable. Shows how important these systems are.
Why is LPS a good target?
• Interfering with LPS biosynthesis or export can reduce OM barrier function, making bacteria more sensitive to existing antibiotics
How is anitibiotic resistance against LPS gained?
• Some antibiotics target LPS: polymyxin/colistin. Resistance mediated by modification of lipid A, adding PEA groups
What kind of receptors recognize common microbial components?
TLR. Adjuvants are often molecules that trigger the TLR receptor.
What does TLR activation result in?
activation of antigen-presenting cells: upregulation of costimulatory molecules, induction of proinflammatory cytokines.
Why is is difficult to work with recognition of LPS in mice models?
the TLR4/MD2 recognition of LPS variants is species-specific. Difficult when e.g. working with mice, when it works in mice but not in humans.
Lipid A mutants occur naturally among meningococcal isolates from patients. Some show reduced cytokine-inducing capacity. why can that be an advantage?
Could be an advantage because the bacteria stay more ‘off the radar’ this way. -> easier spreading
Pathogenic bacteria may express underacylated LPS (lpxL1 mutation) to avoid TLR4 activation
Then, what is the advantage of the WT LPS?
No reduced OM barrier function.
