Lecture 4: Microbial Cell walls and Cell Membranes Flashcards
Tuesday 8th October
Why do we study the outer layer of the cell surface?
Because it’s where microbes and the immune system interact
Differences between Gram +ve and gram -ve bacterial cells
What are the 2 types of glycocalyx?
Slime layer + capsule
What are capsules?
- polysaccharide components outside the cell wall
- organised, tightly-bound network of polymer fibres extending outward from wall
- usually surrounding pathogenic bacteria
How can capsules be identified?
Using phase contact microscopy
Describe Glycocalyx
- Network of sugars and proteins that 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
Describe Slime layers
Describe slime layers
- Diffuse, unorganised, easily removed
- Does not exclude small particles
- Can be difficult to visualise
Are capsules required for the growth or reproduction of bacterial cells?
No
Describe the role of capsules
- 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
What do bacterial adhesins do?
They help bacterial cells to stick to something else
What are some examples of bacterial adhesins?
Capsular polysaccharide, extracellular slime, fimbriae, lectins
Describe the S layer
- 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
In gram negative bacteria, does the S layer interact with the LPS?
Yes
What is another name for peptidoglycan?
Murein
Describe peptidoglycan (meurin)
- 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)
-
Describe the synthsis of peptidoglycan
- 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)
Describe the overall structure of a Gram +ve cell wall
- 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
Describe the overall structure of a gram negative cell wall
- There is a lot less peptidoglycan in gram negative cell walls
- The peptidoglycan is between the inner and outer membrane (the periplasmic space)
Describe a lysozome
- An antibacterial enzyme
- Degrades the beta 1,4-glycosidic bond in peptidoglycan backbone
- lysozomes are an important host defence against bacteria
What happens when a lysozome degrades the peptidoglycan in a bacterial cell wall?
makes cells sensitive to changes in osmotic pressure and more susceptible to attack from the immune system
Where are lysozomes found?
in saliva, tears, secreted in airways
What antibiotic inhibits peptidoglycan synthesis?
Penicillin
What day in WW2 was penicillin important for?
The D-Day landings
How does penicillin inhibit peptidoglycan synthesis?
- 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
What type of bacteria is penicillin used against?
Gram positives
What types of bacterial cell is Teichoic acid found in?
Gram +ves
what is the role of Teichoic acid in gram positives?
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+
What is the difference between bacterial cell walls and archaeal cell walls?
- 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)
What are the differences between N-acetyltalosaminuronic acid and N-acetylmuramic acid ?
- 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
We have sterols in eukaryotes and ……… in bacteria
Hopanoids
Describe hopanoids
- Rigid planar molecules while fatty acids are flexible
- Stabilise membrane structure
Have hopanoids been found in archaea?
No
What is a sterol that is found in eukaryotes?
Cholesterol
Why is the outer membrane of gram negative bacteria assymetric?
due to insertion of lipopolysaccharide into external layer of OM
Which 2 ways is the outer membrane linked to the bacterial cell?
- 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
What allows the transport of substances to the outer membrane and out of the cell?
Adhesion sites
How are archaeal membranes different to the memranes of bacteria and eukaryotes?
- 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
MAJOR LIPDS OF THE ARCHAEA LAST SLIDE
What does the 3 domain system say are the most closely related to each other?
Archaea and eukaryotes
Do bacteria have histones?
No
Is it true that bacteria and eukaryote cell membranes contain fatty acids, whilst archaeal membranes are based on isoprenes?
Yes
