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