Lecture 4

Question 1

From the outside in, what is the outer layer of bacterial cells?

Answer 1

Capsule, S-layer

Gram positive bacteria – Cell Wall
Gram negative bacteria – Outer membrane

Periplasmic space with peptidoglycan in Gram -ve and some gram +ve

Call plasma membrane

Question 2

What are capsules?

Answer 2

Polysaccharide components outside the cell wall

Usually loose network of polymer fibres extending outward from wall

Organised, tight matrix, not easily removed

(called ‘slime layers’ or glycocalyx)

Question 3

What is the role of glycocalyx in the capsule?

Answer 3

Aids in establishing complex consortia of bacteria

Promotes the adhesion of bacteria to living and inert surfaces

Enable bacteria to become harder for phagocytes - increases effective diameter of bacterium

Question 4

What is particular of Biofilm bacteria in the capsule?

Answer 4

Hard to kill

Many persistent infections caused by bacterial biofilms

Question 5

What is the role of capsules?

Answer 5

Carbon store that can be reabsorbed

Protection against desiccation

Capture of nutrients

Question 6

What is the capsule of a pathogen normally made of?

Answer 6

Polysaccharide structures

(Glycolipid/ protein/ extracellular slime)

Question 7

What is the s-layer in a capsule?

Answer 7

Paracrystallline outer wall layer composed of protein

Protect against ion and pH fluctuations, osmotic stress and other harmful bacteria

Support against fluctuations in the environment

(in some archaea the only cell wall structure)

Question 8

What is the peptidoglycan in the capsule?

Answer 8

Mesh-like polymer that retain the Gram stain in Gram +ve cell

Both types of membrane contain it

Alternating residues of NAG and NAM

Arranged in dimers which are cross linked by amino acid side chain creating amine bonds

D-amino acids protect against degradation by proteases

Question 9

What is peptidoglycan synthesis?

Answer 9

Chains of linked peptidoglycan subunits joined by cross-links between peptides

Carboxyl group of terminal D-alanine connected to amino group of diamino pimelic acid

Question 10

What are the structural features of peptidoglycan?

Answer 10

Strong: maintain shape of bacteria
Flexible: divide and fulfil functions

Retain shape when isolated, porous, elastic and stretchable

Question 11

What is the overall structure of a Gram +ve cell wall?

Answer 11

Peptidoglycan is all of the outer layer of Gram +ve bacteria

Thicker than in Gram -ve

Up to 90% of cell wall (25 sheets of peptidoglycan)

Question 12

What is the overall structure of a Gram -ve cell wall?

Answer 12

Little peptidoglycan in cell Gram -ve

10% of cell wall

Between inner and outer membrane

Question 13

What is lysozyme?

Answer 13

‘antibacterial’ enzyme

Degrades beta 1,4 glycosidic bond in peptidoglycan backbone

Important host defence against bacteria

Present in saliva, tears secreted in airways

Question 14

What happens when there is a loss of peptidoglycan in the cell?

Answer 14

Makes cells sensitive to changes in osmotic pressure

Makes them more likely to burst and die

Question 15

What is the effect of penicillin on peptidoglycan synthesis?

Answer 15

Inhibits the synthesis

Halt cell wall synthesis – osmotically sensitive cells lyse

Makes cells liable to lysis - inhibits transpeptidation

Transpeptidation: Initially 2 D-alanine molecules, one gets cleaved during linking with DAPA or L-Lys

Question 16

What are the properties of archaeal cell walls?

Answer 16

No peptidoglycan or murein

Lysozyme cannot break down

Contains N-acetyltalosaminuronic acid

Linked by Beta 1,3

Question 17

What are the basics with membranes?

Answer 17

Phospholipids

Hydrophobic groups inside, hydrophilic groups outwards

Proteins that traverse bilayer have hydrophobic regions

Question 18

What are sterols/ hopanoids?

Answer 18

Rigid planar molecules while fatty acids are flexible

Stabilise membrane structure

Hopanoids: bacteria
Sterols: eukaryotes

Question 19

Why is the outer membrane of Gram -ve asymmetric?

Answer 19

Due to the insertion of lipopolysaccharide into external layer of outer membrane

Question 20

How is the outer membrane bound using Braun’s lipoprotein?

Answer 20

Most abundant protein

Covalently linked to peptidoglycan

Embedded in outer membrane by hydrophobic end

Lipid sticks into membrane anchoring outer membrane to cell

Question 21

What are the 2 ways the outer membrane is linked to the cell?

Answer 21

Braun’s lipoprotein

Adhesion sites

Question 22

How is the outer membrane bound using adhesion sites?

Answer 22

Continuum of inner and outer membrane

2 membranes adhere

Allow transport of substances to outer membrane and out of cell

Visualised using TEM

Question 23

What is the overall structure of archaeal membranes?

Answer 23

Branched chain hydrocarbons attached to glycerol

Ester links (C-O-C)

(shows how 2 domains of life are different)

Question 24

What are the major lipids of archaea?

Answer 24

Single-headed like phospholipids

Makes a lipid bi-layer

Double-headed ester lipids, charged ends making solid membrane layer

Stabilises membrane at extremes of pH/ temperature

Ester linked residues onto glycerol