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MICR3330 > 4 - Outer Membrane > Flashcards

4 - Outer Membrane Flashcards

1
Q

The gram negative outer membrane (OM)

A
  • Is an asymmetric and chemically unique lipid bilayer membrane containing lipopolysaccharides (LPS)
  • Contains unique transport proteins called ‘porins’
  • Braun’s lipoprotein joins the OM to the cell wall
  • Space between OM and PM is the periplasm
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2
Q

OM is an asymmetric and chemically unique lipid bilayer membrane containing lipopolysaccharide (LPS)

A
  • The outer leaflet replaces phospholipid with LPS
  • The inner leaflet is phospholipid typical of plasma membranes
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3
Q

3 parts of a lipopolysaccharide (LPS)

A
  • Lipid A
  • Core polysaccaride
  • O antigen (O side chain)
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4
Q

Lipid A part of LPS

A
  • Fatty acid side chains attached to a glucosamine phosphate disaccharide
  • Embedded in the outer leaflet of the OM
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5
Q

Phospholipid structure

A

fatty acid side chains, glycerol, phosphate

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6
Q

Core polysaccharide part of LPS

A
  • Joined to lipid A
  • Up to ten sugars (6-7 carbons)
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7
Q

Examples of sugars in core polysaccharide

A
  • Heptulose
  • Ketodexyoctonate (KDO)
  • NAG
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8
Q

O antigen or O side chain part of LPS

A
  • A polysaccharide chain extending out from the core polysaccharide
  • Up to 40 repeat units of 3, 4 or 5
    sugars
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9
Q

What can O-chain length affect

A

Colony appearance (e.g. rough = no chains, smooth = two or more reapeat units of o-chain)

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10
Q

Importance of LPS

A
  • Contributes negative charge to bacteria
  • Helps to stabilise OM
  • Helps create permeability barrier
  • O antigen elicits host immune response
  • Lipid A can act as an endotoxin
  • Can mediate adhesion to cells
  • Composition of O antigen is useful in identification
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11
Q

How do LPS contribute to negative charge

A

Charged sugars and phosphate in core

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12
Q

How do LPS stabilise OM

A

As lipid A is a major constituent of outer leaflet

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13
Q

How do LPS help create permeability barrier

A
  • Due to shape and interactions between neighbouring LPS molecules which are tightly packed
  • Restricts entry of small molecules (e.g. antibiotics)
  • Long side chains protect against complement and phagocytosis
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14
Q

How do O antigens elicit host IR

A
  • bactericidal antibodies may bind the strain specific form of LPS (host defence)
  • Many pathogens can rapidly change antigenic nature of O chains and evade IR
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15
Q

How does the lipid A portion of LPS act as endotoxin

A

Enters blood stream, triggers septic shock

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16
Q

Porins

A
  • Comprise 3 transmembrane protein subunits (trimer) forming small pores in OM
  • Allow transport of hydrophilic compounds that are otherwise excluded by the hydrophobic lipid bilayer
17
Q

General porins

A
  • Form passive channels across the OM
  • Channels are hydrophilic, water filled, non specific
  • Small molecules diffuse through
18
Q

Specific porins

A
  • Exhibit solute specificity
  • e.g. Maltoporins transport maltose and glucose by facilitated diffusion
19
Q

Energy coupled OM transporters

A
  • Large molecules are bound to specific OM receptors and transported across
  • The receptor-solute complex in OM interacts with TonB protein which spans periplasm
  • An ABC transporter in PM transfers solute to cytoplasm
20
Q

TonB protein

A

Transfers proton motive force energy for solute transport from PM across periplasm, to OM receptor

21
Q

Braun’s lipoprotein

A
  • Has hydrophobic end embedded in OM
  • Forms strut-like connections
    between OM and the cell wall
  • the link between the OM and PG
    maintains structural and functional integrity of the cell envelope
  • Maintains the correct distance between the PG and OM
22
Q

Periplasmic space

A
  • Compartment enclosed by OM & PM
  • Composed of gel like “periplasm” encompassing the cell wall
  • Contains periplasmic enzymes and other proteins
23
Q

Periplasmic enzymes and proteins present in periplasm

A
  • Binding proteins
  • Degradation & hydrolytic enzymes
  • Detoxification enzymes
  • Peptidoglycan synthesis enzymes
  • Chemoreceptor proteins
  • Glucans
24
Q

Glucans function

A

Osmoprotectants

25
Q

Do gram postitive bacteria have a periplasmic space

A

A narrow periplasmic space between PM and CW

26
Q

Monoderm

A

One membrane, gram positive

27
Q

Diderm

A

Two membranes, gram negatives

28
Q

Hypothesis for evolution of monoderms and diderms

A
  • Diderm first: loss of OM?
  • Monoderm first: Acquisition of OM?

MICR3330 (29 decks)

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