lecture 4 Flashcards
microbial cell walls and membranes
layers of gram +ve bacterial cells
capsule, s-layer (slime layers / glycocalyx / sugar coat)
cell wall
periplasmic space
cell/plasma membrane
layers of gram -ve bacterial cells
capsule, s-layer (slime layers / glycocalyx / sugar coat)
outer membrane
periplasmic space with peptidoglycan
cell/plasma membrane
explain the capsule of bacteria
A lot of bacteria pump substances out which sit outside of the cell wall, this is called the capsule/slime layer/glycocalyx.
Some organisms have polysaccharide components outside the cell wall.
Usually, it is a loose network of polymer fibres extending outward from wall. Some are more stretched out.
→ TEM: ultrathin sections of gram negative bacteria (negative staining stains everything BUT cells) to allow us to see the cell’s characteristics
different names for capsules (3)
- Glycocalyx is involved in biofilm formation (forming layers of cells on surfaces) and they aid in establishing complex consortia of bacteria - often different bacteria held together.
- Capsules are called different things because it is different with each organism. Usually, the capsule is the organised, tight matrix, which is not easily removed from the cell. It excludes small particles like india ink so that it doesn’t enter the cell.
- Slime layers are more diffused, unorganised and easily removed. It doesn’t exclude small particles so we can’t negatively stain slime layers, so we can’t visualise it as easily as a capsule.
Normally we can tell an organism is producing a slime layer by looking at its colony on a plate because it becomes quite diffused (still can’t tell if it’s a slime layer or a capsule or a substance that it’s excreting)
role of capsules (6)
- Carbon store (if cell has a lot of energy, stores carbon in capsule)
- Protection against desiccation (drying up)
- May be involved in the capture of nutrients (mainly positive ions -iron manganese zinc- stick to the cell and then can be brought in from the environment)
- Confer advantages in vivo such as attachment to surfaces (biofilms, holdfast, or to eukaryotic cells in wounds)
- Ensure phage, antimicrobial and disinfectants can’t enter
- Pathogens often capsular and resist phagocytosis (streptococcus pneumoniae is a pathogen when capsulated but easily killed by the host when it isn’t encapsulated/ virulence factor) so it makes it more difficult for the immune system to recognise that a bacteria is pathogenic
So capsules are a first line of defense
Help it detect pathogens
explain capsules in pathogens
- most commonly a polysaccharide structures
- can be a glycolipid capsule which is more complicated (extracellular glycolipid of mycobacteria)
- protein capsule to pump protein out to protect the pathogen
- extracellular slime is difficult to define. it’s the main reason why the pathogen is dangerous (slime in lungs damages the bronchi and the villi that leads to the damage that will kill them so the organism isn’t pathogenic itself)
define bacterial adhesins
- capsular polysaccharide, extracellular slime, fimbriae, lectins.
All of them help bacteria adhere to surfaces.
define 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..
• May protect against host defences (sometimes a virulence factor)
define paracrystalline
Paracrystalline means when we look down an electron microscope, we can see the very structured layer on the outside of cells.
We don’t know how the s-layers sit on a gram negative cell
define peptidoglycan / murein
Found in gram +ve cells and has alternating residues of sugar moieties:
- NAG (N-acetylglucosamine)
- NAM (N-acetylmuramic acid (lactyl ether of NAG))
→ these are very similar with a different side chain.
how are the NAG and NAM arranged
They are arranged in dimers which are cross linked by amino acid side chains creating amide bonds between the structures.
It’s a mesh-like polymer that retains the gram stain in gram +ve cells (peptidoglycan is what gets stained during a gram stain). Gram +ve cells have more peptidoglycan than gram -ve cells, which is why the stain washes off gram -ve cells when you add ethanol.
what is the form of the amino acids in peptidoglycan?
All amino acids found in proteins are in the L form, but the non-protein amino acids in the peptidoglycan are in the D form. (the L and D forms are mirror images of each other)
Non-protein amino acids:
- D-glutamic acid
- D-alanine
- Meso-diaminopimelic acid (DAPA)
how do the D-amino acids protect bacteria?
Bacteria use D-amino acids to protect themselves because the D-amino acids cannot be degraded by proteases
And the bacteria can’t breakdown its own cell wall if it is using protease to gather energy from outside.
synthesis of peptidoglycan/murein
• 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)
• Sacs are strong enough to retain shape when isolated yet
are porous, elastic and stretchable
peptidoglycan of gram +ve cell wall
All of the outer layer of Gram positive bacteria
- Thicker than in Gram –ve
- Up to 90% of cell wall, up to 25 sheets of peptidoglycan
- proteins that pass all the way through it
