week 1 -bacterial cell envelope Flashcards
Gram positive cell envelope
- Cryo-transmission electron micrograph
- PM plasma (cytoplasmic) membrane
- IWZ inner wall zone
o Soluble low density components - OWZ outer wall zone
o Peptidoglycan network
gram negative cell envelope
- Two membranes
- PG thin layer of peptidoglycan
- Outer membrane
o An asymmetric lipid bilayer
o The inner leaflet is composed mainly of phospholipid
o Outermost layer is made of lipopolysaccharide or LPS
o Relatively impermeable
bacterial cytoplasmic membrane
primary roles:
- Separates the cytoplasm from the cell’s environment
- Permeability barrier
- Contains a range of diverse membrane proteins
o Enzymes (I.e. PBPs, peptidoglycan biosynthesis)
o Transporters - Energy conservation and consumption
o Sit of the generation and use of the proton motive force
bacterial cytoplasmic membrane:
phosphatidylethanolamine
primary lipid in bacterial CM
hydrophilic polar head
- phosphate and ethanolamine are charged
fatty acid chains form hydrophobic interior
the bacterial cell wall
- Composed of peptidoglycan
- Forms a sac surrounding the cell
- Gives shape to bacteria
- Resists turgor pressure
o In water environments
o Without good peptidoglycan cell walls would absorb water till bursting - Gram positive (thick) and Gram negative (thinner)
structure of repeating unit of peptidoglycan
Repeating units of two sugars
- N-acetylglucosamine (G)
- N-acetylmuramic acid (M)
Joined by b 1,4 linkage
Tetrapeptide chain (side chain)
- Attached t0 NAM
- Contains D forms of amino acids
- Proteins made on ribosomes L
form)
Crosslinks
- Formed by transpeptidases
- Gram positive may have peptide inter bridge
- Crosslinks between peptide chains give strength
cross linking in the Gram negative cell wall
Cross links between adjacent molecules
* Between amino acids
o Holds everything together
o Maintains rigidity
* Linkage always between 3rd and 4th amino acid
Peptidoglycan – a target for antibacterial
Lysozyme
o First line of defence
o Found in tears, milk, saliva, mucous
o Cleaves the b(1-4) linkage in the peptidoglycan polysaccharide backbone
Peptidoglycan – a target for antibacterial
penicillin
o Antibiotic
Stops crosslinks from being made
Bacteria bursts
Penicillin targets transpeptidases
- Transpeptidases form peptide cross links between adjacent glycan chains
- Essential for structure of peptidoglycan
- Weakened cell wall with fewer cross links
o Osmotic lysis - Selective because peptidoglycan and transpeptidases are unique to bacteria
Teichoic acids
- Up to 60% of the total mass of the cell wall
o Polymers of sugars or sugar derivatives such as glycerol and ribitol
o Negatively charged - Wall teichoic acids
o Covalently linked to the peptidoglycan
Lipoteichoic acids
o Anchored in the cytoplasmic membrane via glycolipids
- Function
o Protecting the Gram positive envelope
o Important in bacteria-host interactions
Adherence and colonisation of host cells surfaces
o Often recognised by host immune system
overall net charge of any bacteria is:
negative
GRAM NEGATIVE CELL WALL
lipopolysaccharide
part lipid part sugar
GRAM NEGATIVE CELL WALL
O-specific polysaccharide
- Highly variable
- Antigenic
- Made up of repeated units
- 3 sugars and a branch
GRAM NEGATIVE CELL WALL
core polysaccharide
- Highly conserved (especially the inner core) likely to be the same/very similar across gram negative bacteria
- Contains KDO and other unusual sugars
GRAM NEGATIVE CELL WALL
Lipid A
- Variation in the length and degree of subsitiution of the acyl chains
Gram negative periplasm
- Subcellular compartment between the cytoplasmic and outer membranes
o Compartment between the two membranes - 10-20% of the volume of the cell
- High protein content
o Proteins for: cell wall synthesis and uptake and transport of nutrients - Oxidising environment – formation of disulphide bonds in proteins
o To give rigidity
o Have to be formed outside cell - Contains chaperones that ensure the correct folding of other proteins
do gram positives had a periplasmic space
- IWZ inner zone wall
o Soluble low density components
o May be a periplasmic space
o Could be site of enzyme reactions
proteins in the outer membrane
- For transport
o Nutrients in
o Waste out - Porins
- Span outer membrane
- Beta barrel
- Solutes diffuse through the water filled channel
- Many antibiotics diffuse though porins
the surface layer OR s layer
- Regularly spaced array of protein subunits
- Self assemble into a structure which coats the entire cell creating a porous lattice
- Covalently attached to the peptidoglycan in Gram positives and to the O-polysaccharide chains of LPS in Gram negatives
Made of protein units shaped so they fit together (self assemble)
Not a membrane it is protective - Porous lattice
capsules and slime
- Extracellular polymeric substances (EPS)
- Often polysaccharides (like O side chains of LPS)
- Capsule
o Discrete layer
o Covalentlu attached to OM lipids or PG - Slime
o Not covalently attached
In conjunction or an alternative to the S-layer
Capsule is attached
Slime is not attached