Gram Negative Cell Walls

What are the five unique features of the gram negative cell wall?

1. Outer membrane

2. Porin channels

3. Thin layer of peptidoglycan

4. Numerous lipoproteins

5. LPS

What does the outer membrane primarily function as?

A selective permeability barrier

What does the outer membrane allow passage of?

Hydrophilic molecules through porins

What are five additional functions of the outer membrane?

1. Is the negative charge on bacteria

2. Have phage receptors

3. Stabilize mating cells

4. Barrier for periplasmic enzymes

5. Virulence factor

What feature makes gram negatives intrinsically more resistant than gram positives?

Outer membrane

What are two examples of phage receptors?

1. O-antigens

2. OMP-A

What can phages do? How are we combatting antibiotic resistance?

They can attack and lyse bacteria. Phage therapy

What part of the OM gives its virulence? Why?

LPS because it is a potent inducer of cytokines

Where is the outer membrane found? What does it look like?

It is laid over the peptidoglycan and has the typical appearance of a lipid bilayer

What are the four structures that make up the OM?

1. Phospholipids

2. LPS

3. OMP (outer membrane proteins)

4. Lipoprotein (Brauns)

Where are the phospholipids found in the OM?

They form the inner leaflet of the OM (closer to the cytoplasmic membrane)

Where is LPS found?

In the outer leaflet of the OM

Are porins and OMP the same?

No they are different

What anchors the OM to the peptidoglycan?

Lipoprotein (Brauns)

What can LPS also be referred to as?

Endotoxin

What part of LPS contains the endotoxin?

Lipid A

What minor toxic activity can lipid A cause?

Cytokione induction (elicits strong immune response in mammals)

What major toxic activity can lipid A cause?

1. Hypotension

2. Septic shock

What are the two functions of LPS?

1. Confers negative charge and repels hydrophobic molecules

2. Repels fat soluble molecules such as bile that the gal bladder secretes

What are the 3 components of LPS?

1. Lipid A

2. Core polysaccharide

3. O-specific polysaccharide

What part of LPS varies in length and composition?

O-specific polysaccharide

Describe the chemical structure of lipid A

1. Have a hydrophilic head composed of 2 molecules of N-acetylglucosamine phosphates joined by (1,6) glycosidic bonds

2. 6 fatty acids in total are linked to a hydroxyl and amino groups found in the N-acetylglucosamine phosphates

What type of fatty acid is present in lipid A? Are they found in phospholipids?

C14-C-hydroxymyristic acids. No

Where is the polysaccharide joined to lipid A?

Joined to one of the glucosamine molecules

How many residues make up the core oligosaccharide? Is it the closest to the lipid?

10 residues and yes

How many sugars is the core oligosaccharide composed of?

4-5

What are 2 unusual sugars present in the core oligosaccharide?

1. C8 Keto-deoxy-octonic acid (KDO)

2. C7 heptose

Is the composition and arrangement of the core oligosaccharide generally the same in most gram negative bacteria?

Yes

What is KDO commonly used as? Why?

A marker for OM in cellular fractionation experiments because it is uncommon to find it anywhere else than the OM

What is the O-specific polysaccharide? Is it highly variable?

It is a long polysaccharide chain attached to the core oligosaccharide consisting of 4-5 repeating units of sugars. Yes

What does composition and arrangement of the sugars vary among?

Different organisms and even in the same species

What is an example of the O-antigen/O-specific polysaccharide varying among the same species?

Ecoli 0157:H7

What does Ecoli 0157:H7 produce?

Shiga toxin

What does the O and H stand for?

O stands for the O-specific antigen it produces and H is the flagellum antigen it produces

What is Ecoli 0157:H7 associated with? What did it used to be called?

Water contamination and hamburger disease

What is LPS with O-antigen referred to as? Without?

With: Smooth LPS

Without: Rough LPS

Are bacteria more or less virulent without LPS?

Less virulent