Bacterial cell walls Flashcards
What is the function of the cell wall?
Strength, mechanical protection, osmotic protection, shape and rigidity
What are bacterial cell walls made of?
Peptidoglycan
What is the monomer of peptidoglycan?
Glycan tetrapeptide
What are the components of a glycan tetrapeptide monomer?
2 sugars: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)
Short chain of AA attached to NAM: L-ala, D-glu, DAP, D-ala
What is interesting about the amino acid chain attached to NAM in peptidoglycan?
It contains D amino acids and DAP, which are unusual amino acids that can’t be used by ribosomes to build proteins
Which two components of glycan tetrapeptide monomers are only found in bacteria and nowhere else?
DAG (diaminopimelic acid) and NAM
What is the polymer structure of peptidoglycan?
Repeating units of NAG then NAM linked by glycosidic bonds. The tetrapeptides are covalently cross linked to each other between the 3rd position on one chain and the 4th position on the other
How are peptidoglycan chains usually cross-linked together?
3rd position on the first chain, the DAP, crossed linked to the 4th position on the second chain, the D-ala
What modifications to the peptidoglycan cross-linking are found in gram-negative bacteria?
None. Its always position 3 to position 4
What 3 modifications to the peptidoglycan cross-linking are found in gram-positive bacteria?
- -NH2 group attached to D-Glu
- DAP replaced with L-Lys
- Peptide crossbridge made of a bunch of Gly between the 3rd position and 4th position
Which species have all 3 modifications to the cross linking of peptidoglycan in their cell walls?
Staphylococcus
Why is targeting peptidoglycan a good way to kill bacteria?
A weakened cell wall makes the cells really vulnerable to osmotic stress. Drugs that target PG will also not hurt our cells because it’s only found in bacteria
What does lysozyme do to peptidoglycan?
Cleaves the glycosidic bond between NAG and NAM
Where is lysozyme found?
Tears and saliva
How do beta-lactam antibiotics work?
They prevent formation of new PG crosslinks, which makes growing cells weaker and vulnerable to osmotic stress
Do peptidoglycan cross-links occur in just one plane?
No, form a 3D lattice
What is the cell wall structure of gram-positive bacteria?
Thick layer of PG, about 20-40 strands. Highly crosslinked in all directions
What do gram-positive bacteria have embedded in their cell walls?
Teichoic acids
What are teichoic acids?
Ribitol phosphate polymers that are covalently attached to NAM
What are the 2 types of teichoic acids?
Wall teichoic acids and lipoteichoic acids
What are wall teichoic acids?
Teichoic acids that are only attached to the cell wall
What are lipoteichoic acids?
Teichoic acids with a lipid tail that is inserted into the membrane
What is the purpose of having the wall teichoic acids in the gram-positive cell wall?
Gives the cell surface a negative charge that will facilitate binding and transport of positively charged ions
What is the purpose of having the lipoteichoic acids in the gram-positive cell wall?
Helps anchor the cell wall to the membrane
What is the cell wall structure of gram-negative cell walls?
Thin layer of PG, only about 3 strands, not as highly cross-linked. PG cell wall sandwiched between the cytoplasmic membrane and the outer membrane
What is the periplasm?
Fluid between the inner and outer membranes of gram negative bacteria
What is in the periplasm?
Soluble proteins and PG
What is the structure of the outer membrane of gram negative bacteria?
Inner leaflet contains normal phospholipids, outer leaflet is mostly LPS and a few phospholipids
Where else is LPS found besides gram negative bacteria?
Nowhere
What are the 3 parts of LPS?
Lipid A, core polysaccharide, O-polysaccharide
Which two parts of LPS are highly conserved among gram-negative bacteria?
Lipid A and the core polysaccharide
What is Lipid A of LPS made of?
Tons of fatty acids and glucosamine sugars that are chemically similar to NAG. Contains no glycerol
What is the core polysaccharide of LPS made of?
Tons of sugars, including some heptoses and an 8C sugar called KDO
What is the O-polysaccharide of LPS made of?
Hydrophilic sugars
Is the O-polysaccharide highly conserved between gram-negative bacteria?
No, varies widely between species and strains
How can the O-polysaccharide be different between gram-negative species?
Sugar composition and length
Which part of LPS is recognized by our immune system?
The O-polysaccharide. Antibodies will be made against that
Why is it a problem if gram-negative bacteria get into our bloodstream?
They can lyse from immune attack and expose the Lipid A of LPS, which causes an immune system freak out and septic shock syndrome
What happens when our immune systems see Lipid A?
Freaks out and elicits a really strong immune response, which causes massive inflammation, high fever, and organ failure
What is another name for the Lipid A of pathogens?
Endotoxin
What are the treatments for septic shock syndrome?
Dialysis to get rid of the endotoxin or endotoxin-binding drugs
What are 3 functions of the outer membrane of gram-negative bacteria?
Permeability barrier, reduces drying stress, adherence
How does the outer membrane act as a better permeability barrier than the cytoplasmic membrane?
The hydrophobic portion of LPS has 6 fatty acid tails compared to 2 for phospholipids, which allows them to pack tighter and exclude anything hydrophilic. The hydrophilic portions of LPS also bind a ton of water and exclude anything hydrophobic
How do nutrients and things the cell needs get across the outer membrane if its so inpermeable?
Transport proteins. Lets the cell be really selective since that’s the only way across
Why are gram-negative pathogens more difficult to kill than gram-positive pathogens?
Gram-positive species only have the thick PG layer, which provides physical protection but not much chemical protection. The outer membrane of gram-negative species also provides chemical protection, since it excludes both hydrophobic and hydrophilic antibiotics and lysozyme
How does LPS reduce drying stress?
The polysaccharides will associate with a lot of water, which keeps the surface of the cell hydrated
Considering that the polysaccharides of LPS have a bunch of negatively charged phosphates on them, how do they not repel each other?
They associate with divalent cations that bridge the negative charges so they stop repelling each other
What happens if you treat a gram-negative cell with a chelator?
The cations between the LPS get removed and they start repelling each other, which makes the outer membrane looser and antibiotics can get in
Why can’t we use chelators to treat gram-negative infections?
They would also bind the ions we need
What are two types of outer membrane proteins?
Lipoprotein and porins
What does lipoprotein do?
Anchors the outer membrane to the PG by crosslinks
Where is lipoprotein found?
The inner leaflet of the outer membrane and extends into the cell wall
What is the structure of porins?
Small trimeric protein channels that pass through the outer membrane
What do porins let through the outer membrane?
Small hydrophilic molecules
What are the two types of porins in the outer membrane?
Non-specific and specific
How do non-specific porins avoid letting toxins into the cell?
They’re extremely small, only water and ions can get through. Anything larger than that can’t fit
What do non-specific porins let into the cell?
Water and ions
What do specific porins let into the cell?
Amino acids and sugars
How do specific porins ensure substrate specificity?
They have a binding site in the middle of the channel. Only substrates that can make all the correct interactions can get all the way through
Are specific porins an active transport mechanism since they have binding sites?
No, the binding site is only for specificity. No conformation change happens
What is an adaptation in porins that is seen in pathogens? What’s the downside?
Their porins are often smaller than usual to exclude host toxins. But its harder to get nutrients in so they grow slower
Did acid-fast bacteria evolve from gram-positive or gram-negative bacteria?
Gram-positive
What is the structure of the cell wall of acid fast bacteria?
PG layer on the inside, then a layer of polysaccharides in the middle, then an outer layer of mycolic acids
What do mycolic acids do?
Are extremely hydrophobic and exclude anything hydrophilic. They form a sort of waxy coating on the cell surface
Why are acid fast cell walls even more impermeable than gram-negative outer membranes?
The mycolic acids exclude anything remotely hydrophilic, and the polysaccharide layer underneath excludes anything remotely hydrophobic
How do acid fast bacteria get nutrients in if their cell walls are so impermeable?
Have evolved porins, which is unusual for species that evolved from gram-positive bacteria. But still hard to get nutrients in so they grow really slow
Why don’t acid fast bacteria gram stain?
The mycolic acid layer excludes the dyes, so they end up colourless
How do you stain acid fast bacteria?
Need to use phenols to disrupt the mycolic acids enough for the stain to enter
What type of cell walls do Mycoplasma species have?
No cell walls, lost them over the course of evolution
Did Mycoplasma species evolve from gram-positive or gram-negative bacteria?
Gram positive
How do Mycoplasma species survive osmotic stress if they have no cell walls?
They’re obligate intracellular pathogens, and a eukaryotic cell is an isotonic environment
Do archaeal cell walls have peptidoglycan?
No, they’re much more chemically diverse
What is pseudomurein?
A cell wall polymer found in archaea that is structurally similar to PG, but isn’t PG
How is pseudomurein similar to peptidoglycan?
Uses NAG sugars and the overall structure is very similar
How is pseudomurein different than peptidoglycan?
Uses NAT sugars instead of NAM, and the tetrapeptide attached to it contains only L amino acids. The crosslink is different
Why can’t pseudomurin be destroyed by lysozyme?
Different glycosidic linkage
Why can’t pseudomurin be destroyed by antibiotics that target the cross links?
Different cross links than the ones found in bacteria
What is the surface layer (S-layer)?
A glycoprotein layer that forms a crystal lattice on the archaeal cell surface
