4 - Outer Membrane

Question 1

The gram negative outer membrane (OM)

Answer 1

• 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

Question 2

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

Answer 2

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

Question 3

3 parts of a lipopolysaccharide (LPS)

Answer 3

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

Question 4

Lipid A part of LPS

Answer 4

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

Question 5

Phospholipid structure

Answer 5

fatty acid side chains, glycerol, phosphate

Question 6

Core polysaccharide part of LPS

Answer 6

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

Question 7

Examples of sugars in core polysaccharide

Answer 7

• Heptulose
• Ketodexyoctonate (KDO)
• NAG

Question 8

O antigen or O side chain part of LPS

Answer 8

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

Question 9

What can O-chain length affect

Answer 9

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

Question 10

Importance of LPS

Answer 10

• 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

Question 11

How do LPS contribute to negative charge

Answer 11

Charged sugars and phosphate in core

Question 12

How do LPS stabilise OM

Answer 12

As lipid A is a major constituent of outer leaflet

Question 13

How do LPS help create permeability barrier

Answer 13

• 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

Question 14

How do O antigens elicit host IR

Answer 14

• 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

Question 15

How does the lipid A portion of LPS act as endotoxin

Answer 15

Enters blood stream, triggers septic shock

Question 16

Porins

Answer 16

• 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

Question 17

General porins

Answer 17

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

Question 18

Specific porins

Answer 18

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

Question 19

Energy coupled OM transporters

Answer 19

• 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

Question 20

TonB protein

Answer 20

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

Question 21

Braun’s lipoprotein

Answer 21

• 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

Question 22

Periplasmic space

Answer 22

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

Question 23

Periplasmic enzymes and proteins present in periplasm

Answer 23

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

Question 24

Glucans function

Answer 24

Osmoprotectants

Question 25

Do gram postitive bacteria have a periplasmic space

Answer 25

A narrow periplasmic space between PM and CW

Question 26

Monoderm

Answer 26

One membrane, gram positive

Question 27

Diderm

Answer 27

Two membranes, gram negatives

Question 28

Hypothesis for evolution of monoderms and diderms

Answer 28

- Diderm first: loss of OM? - Monoderm first: Acquisition of OM?