Lecture 4: Microbial Cell walls and Cell Membranes

Question 1

Why do we study the outer layer of the cell surface?

Answer 1

Because it’s where microbes and the immune system interact

Question 2

Differences between Gram +ve and gram -ve bacterial cells

Question 3

What are other names for capsules?

Answer 3

“slime layers” or glycocalyx (‘sugar coat’)

Question 4

What are capsules?

Answer 4

• polysaccharide components outside the cell wall
• usually loose network of polymer fibres extending outward from wall
• usually surrounding pathogenic bacteria

Question 5

How can capsules be identified?Using phase contact microscopy

Question 6

Describe Glycocalyx

Answer 6

• Disordered
• promotes the adhesion of the bacteria to living and inert surfaces and the subsequent formation of adherent, glycocalyx-enclosed populations that are called biofilms.
• Aids in establishing complex consortia of bacteria

Question 7

Describe Capsules

Answer 7

• Organised, tight matrix, not easily removed
• Excludes small particles (e.g. india ink)

Question 8

Describe slime layers

Answer 8

• Diffuse, unorganised, easily removed
• Does not exclude small particles
• Can be difficult to visualise

Question 9

Are capsules required for the growth or reproduction of bacterial cells?

Answer 9

No

Question 10

Describe the role of capsules

Answer 10

• Carbon store
• Protection against desiccation
• May be involved in the capture of nutrients or Acquisition of ions from the environment
• Confer advantages in vivo such as attachment to surfaces (Biofilms; holdfast; to eukaryotic cells)
• Exclude phage, antimicrobials and disinfectants
• Pathogens often capsular and resist phagocytosis (Streptococcus pneumoniae is pathogen when capsulated but easily killed by the host when it isn’t
  )
• Virulence factor

Question 11

What do bacterial adhesins do?

Answer 11

They help bacterial cells to stick to something else

Question 12

What are some examples of bacterial adhesins?

Answer 12

Capsular polysaccharide, extracellular slime, fimbriae, lectins

Question 13

Describe the S layer

Answer 13

• Paracrystalline outer wall layer composed of protein/glycoprotein
• Regularly structured layer external to cell wall
• In some archaea the ONLY cell wall structure
• May protect against ion and pH fluctuations, osmotic stress, predators such as Bdellovibrio
• May protect against host defences
• Sometimes a virulence factor, however theres not much evidence for this

Question 14

In gram negative bacteria, does the S layer interact with the LPS?

Answer 14

Yes

Question 15

What is another name for peptidoglycan?

Answer 15

Murein

Question 16

Describe peptidoglycan (meurin)

Answer 16

• Made up of alternating residues of NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid (lactyl ether of NAG))
• Arranged in dimers which are cross linked by amino acid side chains creating amide bonds
• Consists of chains of linked peptidoglycan subunits joined by cross-links between the peptides
• Is a mesh-like polymer that retain the Gram stain in Gram +ve cells
• Contain D amino acids, like D-alanine (very difficult to break down D amino acids, protect against degradation by proteases)

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Question 17

Describe the synthsis of peptidoglycan

Answer 17

• Chains of linked peptidoglycan subunits joined by cross-links between the peptides
• Often carboxyl group of terminal D-alanine connected to amino group of diamino pimelic acid (DAPA)
• Often carboxyl group of terminal D-alanine connected to amino group of diamino pimelic acid (DAPA)

Question 18

Describe the overall structure of a Gram +ve cell wall

Answer 18

• Up to 90% of the cell wall can be made out of peptidoglycan
• Proteins that go all the way through the wall, so that the wall can interact with the outside world

Question 19

Describe the overall structure of a gram negative cell wall

Answer 19

• There is a lot less peptidoglycan in gram negative cell walls
• The peptidoglycan is between the inner and outer membrane (the periplasmic space)

Question 20

Describe a lysozome

Answer 20

• An antibacterial enzyme
• Degrades the beta 1,4-glycosidic bond in peptidoglycan backbone
• lysozomes are an important host defence against bacteria

Question 21

What happens when a lysozome degrades the peptidoglycan in a bacterial cell wall?

Answer 21

makes cells sensitive to changes in osmotic pressure and more susceptible to attack from the immune system

Question 22

Where are lysozomes found?

Answer 22

in saliva, tears, secreted in airways

Question 23

What antibiotic inhibits peptidoglycan synthesis?

Answer 23

Penicillin

Question 24

What day in WW2 was penicillin important for?

Answer 24

The D-Day landings

Question 25

How does penicillin inhibit peptidoglycan synthesis?

Answer 25

• Penicillin is able to bind to the active sites of certain enzymes in peptidoglycan due to the presence of the 2 D-Ala
• Penicillin inhibits Transpeptidases, preventing cross-linking in peptidoglycan, which ultimately weakens the cell wall, making it more susceptible to osmotic stress

Question 26

What type of bacteria is penicillin used against?

Answer 26

Gram positives

Question 27

What types of bacterial cell is Teichoic acid found in?

Answer 27

Gram +ves

Question 28

what is the role of Teichoic acid in gram positives?

Answer 28

The role is unclear. However, may make the membrane more negatively charged, which would ultimately help with the binding of ions, such as Ca2+ and Mg2+

Question 29

What is the difference between bacterial cell walls and archaeal cell walls?

Answer 29

• Archaeal cell walls don’t have any peptidoglycan or murein
• Some methanogens contain pseudomurein in their cell walls
• Contains N-acetyltalosaminuronic acid instead of N-acetylmuramic acid
• Others contain other polysaccharide or glycoproteins or S-layers (protein or glycoprotein)

Question 30

What are the differences between N-acetyltalosaminuronic acid and N-acetylmuramic acid ?

Answer 30

• Linked by beta 1,3 rather than beta 1,4 links
• Not degraded by lysozyme, not sensitive to penicillin
• No D-amino acids in linker

Question 31

We have sterols in eukaryotes and ……… in bacteria

Answer 31

Hopanoids

Question 32

Describe hopanoids

Answer 32

• Rigid planar molecules while fatty acids are flexible
• Stabilise membrane structure

Question 33

Have hopanoids been found in archaea?

Answer 33

No

Question 34

What is a sterol that is found in eukaryotes?

Answer 34

Cholesterol

Question 35

Why is the outer membrane of gram negative bacteria assymetric?

Answer 35

due to insertion of lipopolysaccharide into external layer of OM

Question 36

Which 2 ways is the outer membrane linked to the bacterial cell?

Answer 36

• Braun’s lipoprotein, which is covalently linked to peptidoglycan and embedded in OM by hydrophobic end
• Adhesion sites (continuum of inner and outer membrane) where the two membranes adhere

Question 37

What allows the transport of substances to the outer membrane and out of the cell?

Answer 37

Adhesion sites

Question 38

How are archaeal membranes different to the memranes of bacteria and eukaryotes?

Answer 38

• Branched chain hydrocarbons attached to glycerol by ether links rather than fatty acids-ester links
• Have isoperene structures, which can be branched and can form rings

Question 39

MAJOR LIPDS OF THE ARCHAEA LAST SLIDE

Question 40

What does the 3 domain system say are the most closely related to each other?

Answer 40

Archaea and eukaryotes

Question 41

Do bacteria have histones?

Answer 41

No

Question 42

Is it true that bacteria and eukaryote cell membranes contain fatty acids, whilst archaeal membranes are based on isoprenes?

Answer 42

Yes