Dr. Zhanel - Cell Wall Active Ab

Question 1

Anything that is a inhibits the cell wall = ?

Answer 1

bactericidal

Question 2

List agents that are cell wall active agents

Answer 2

penicillins
cephalosporins
glycopeptides/lipopeptides
beta-lactam like agents

Question 3

Which agents are beta-lactams?

Answer 3

penicillins

cephalosporins

Question 4

The following belong to which class of medications:

amoxicillin, amoxicillin/clav, cloxacillin, piperacillin/tazo

Answer 4

penicillins

Question 5

The following belong to which class of medications:

cefazolin, cefuroxime, ceftriaxone

Answer 5

cephalosporins

Question 6

The following belong to which class of medications:

vancomycin/daptomycin/telavancin, dalbavancin, oritavancin, bacitracin

Answer 6

glycopeptides/lipopeptides

Question 7

The following belong to which class of medications:

carbapenems-imipenem or meropenem and monobactams-aztreonam

Answer 7

beta-lactam like agents

Question 8

Describe a gram negative cell wall

Answer 8

• less peptidoglycan
• two membrane layers
• have porins present in the outer membrane layer

Question 9

Describe a gram positive cell wall

Answer 9

• thick peptidoglycan
• one membrane layer
• have surface proteins present

Question 10

Describe the makeup of peptidoglycan

Answer 10

strands of NAM-NAG linkages are produced and then cross-linked to other strands of NAM-NAG linkages for strength

Question 11

Describe the cross-link of peptidoglycan

Answer 11

• tetra peptide chains (amino acids) come from either strands of NAM-NAG linkages
• the tetra peptide chains are linked together to by a peptide inter bridge (gram positive cells)

Question 12

What is the peptide inter bridge made of?

Answer 12

5 Gly

Question 13

Describe how cycloserine prevents peptidoglycan synthesis and therefore prevent cell wall formation

Answer 13

• Cycloserine is bactericidal
• It prevents the D-Ala and D-Ala from joining
• Cycloserine looks like D-Ala so when the enzyme that joins the D-Ala’s together (D-Ala Lygase) comes to join, it binds to cyclosporine because it looks like D-Ala
• This prevents cross-linking, therefore prevents cell wall formation and kills the bacteria

Question 14

Describe how bacitracin prevents cell wall formation

Answer 14

• Bacitracin binds to bactoprenol lipid carrier and it prevents it from dephosphorylating
• Therefore, it prevents NAM-NAG formation from starting all over again
• Prevents cell wall synthesis
• Leads to death of cell very rapidly

**Bacitracin blocks the dephosphorylation of bactoprenol pyrophosphate

Question 15

Describe how vancomycin prevents cell wall formation

Answer 15

-Vancomycin binds to D-Ala D-Ala so we do not get cross-linking and the cell ends up rapidly dying because the cell wall isn’t made

Question 16

How do beta-lactams (cephalosporins and penicillins) prevent cell wall formation?

Answer 16

• beta-lactam ring looks like D-Ala
• When PBP (penicillin binding protein) comes to bind to D-Ala, it will bind to the drug (beta-lactam) instead, therefore there is no cross-linking
• This will prevent cell wall formation

Question 17

How can bacteria form resistance against beta-lactams?

Answer 17

1) Change the amino acids so you’re not looking for D-Ala and it will not bind to the drug (essentially you are changing the target). This is not very common, it would be very hard to completely change the target and cell wall composition.
2) Mutate PBP so it doesn’t recognize the penicillin (make it more specific to only recognize D-Ala). This is done very commonly, it is easy to tweak the specificity.
3) Create an enzyme that breaks down the B-lactam (Beta-lactamases). This prevents the drug from binding to the PBP’s. This is very specific. MIC skyrockets.
4) Pump the drug out or prevent it from getting into the cell wall. MIC goes from 4 - 8 ???
5) Cell could form a capsule. Cells would then live together (co-operative living similar to Hutterite colony). Cells share nutrients, information and genetic information. Very difficult to kill if part of a capsule.

Question 18

What does PBP bind?

Answer 18

It performs cross-linking of D-Ala to DAP.

Question 19

How do penicillins work as cell wall inhibitors?

Answer 19

Penicillins bind to PBP and inhibits it therefore there is no cross-linking and cell wall is not formed

Question 20

How do Beta-lactamases work?

Answer 20

Beta-lactamses hydrolyze the Beta-lactam molecule and it can’t attach to PBP. Cell wall synthesis goes on and Beta-lactam (penicillin or cephalosporin) becomes ineffective.

*Beta-lactamses are a form of resistance.

Question 21

Beta-lactams are _____

Answer 21

bactericidal

Question 22

What is transglycosidation

Answer 22

when NAM and NAG are linked together

Question 23

What is transpeptidation

Answer 23

cross-linking done by PBP

Question 24

Give a summary of peptidoglycan synthesis

Answer 24

1) UDP derivatives of NAM and NAG are synthesized in the cytoplasm
2) Amino acids are sequentially added to UDP-NAM to form the pentane-tide chain (the two terminal D-Ala’s are added as a dipeptide). ATP energy is used to make the peptide bonds, but tRNA and ribosomes are not involved.
3) NAM-pentapeptide is transferred from UDP to a bactoprenol phosphate carrier at the cell membrane surface
4) UDP-NAG added NAG to the NAM-pentapeptide to from the peptidoglycan repeat unit. If a pentaglycine interbridge (in gram-positives) is required, the glycines are added using special glycol-tRNA molecules, not ribosomes.
5) NAM-NAG peptidoglycan repeat unit is transported across the cell membrane to its outer surface by the bactoprenol pyrophosate carrier
6) The newly made peptidoglycan unit is attached to the growing end of a peptidoglycan chain to lengthen it by one repeat unit
7) The bactoprenol carrier returns to the inside of the cell membrane. A phosphate is released during this process to give bactoprenol phosphate, which can now accept a new NAM-pentapeptide
8) Peptide cross-links between peptidoglycan chains are formed by transpeptidation.

Question 25

Beta-lactams interacting with PBP’s resulting in inhibition of peptidoglycan synthesis which activates _____ which break down existing peptidoglycan and lead to cell death

Answer 25

autolysins

Question 26

How do glycopeptides work?

Answer 26

They bind to the terminal D-Ala D-Ala residues thus preventing cross-linking of the peptidoglycan