E4: Antibiotics; Cell wall inhibitors

Question 1

Inhibitors of Cell wall synthesis fall under what categories?

Answer 1

B-lactams:

• Penicillins
• Cephalosporins

Misc/Other:

• Vancomycin
• Carbapenems
• Aztreonam
• B-lactamase inhibitors

Question 2

Structurally, how do Gram negative & Gram positive bacteria differ?

Answer 2

Gram+ bacteria (A):

• •Thick cell wall (peptidoglycan layer)
• •No outer membrane

Gram- bacteria (B):

• Thin cell wall (peptidoglycan layer)
• Has an outer membrane that contains lipopolysaccharide (LPS) and Porin protein channels

Question 3

Where are Penicillin-Binding Proteins (PBPs) & B-lactamases found?

Answer 3

Penicillin-binding proteins (PBP) are found in the cytoplasmic membrane

β-Lactamases are located in the outer portion of the cell wall

Question 4

How does Gram-staining work?

Answer 4

Stains: Crystal Violet & Safranin

Crystal violet (+ Iodine (KI)): Purple

• Crystal violet & Iodine (KI) interact & form large complexes within the inner and outer layers of the cell
• Traps the purple crystal violet color in the cell

Wash (Decoloriser):

• Sometimes a 1:1 mix of acetone and ethanol
• Must be timed carefully

Safranin: Pink

•Counterstain

Results:

Gram-positive bacteria: Stain purple

•Thick layer of peptidoglycan retains the Crystal Violet-Iodine complexes

Gram-negative bacteria: Stain Pink

• Thinner layer peptidoglycan layer & outer membrane, so they do not retain during wash
• Subsequently counter-stained pink by the safranin.

Question 5

Explain how Cell wall synthesis occurs.

Drug SoA?

Answer 5

Sites of action of bacterial cell wall synthesis inhibitors.

The bacterial cell wall consists primarily of peptidoglycan, a polymer constructed from repeating disaccharide units of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc).

Numbers indicate the steps involved in the synthesis of the cell wall of Staphylococcus aureus.

Steps:

1. Bactoprenol pyrophosphate (bactoprenol-P-P) is dephosphorylated to regenerate the carrier molecule, bactoprenol phosphate (bactoprenol-P).
2. Uridine diphosphate-MurNAc (UDP-MurNAc) is added.
3. UDP-GlcNAc added
4. Glycine pentapeptide (5-Gly-tRNA) added
5. The disaccharide peptide is transferred to the peptidoglycan growth point.
6. The cross-linking of peptidoglycan strands is catalyzed by transpeptidase, a type of penicillin-binding protein.

• In this reaction a glycine of one strand forms a peptide bond with the penultimate d-alanine of an adjacent strand, and the terminal d-alanine is released.
• Bacitracin blocks step 1
• β-lactams & vancomycin block step 6 by different mechanisms
• Fosfomycin inhibits [enolpyruvyl transferase] the enzyme that catalyzes the condensation of UDP-GlcNAc with phosphoenolpyruvate to synthesize UDP-MurNAc.

Question 6

Structurally speaking, how do Penicillins & Cephalosporins differ?

Answer 6

Pencillins:

• B-lactam Ring (BLR)
• Thiazolidine Ring (TR)

Cephalosporins:

• B-lactam Ring (BLR)
• Dihydrothiazine Ring (DR)

Question 7

Describe how B-Lactamases affect B-lactams

What are some B-lactamase inhibitors?

Answer 7

[β-Lactamases]:

Hydrolyze the BLR, converting;

• Penicillins to penicilloic acids (inactive)
  
  • May react with body proteins to form antigens
• Cephalosporins to cephalosporanic acids (inactive)

β-Lactamases] inhibitors:

• Clavulanic Acid
• Sulbactam
• Tazobactam

Question 8

What are the (3) mechanisms of Bacterial resistance to B-lactams?

Answer 8

1. B-lactamase inactivation of B-lactams
2. Reduced affinity of PBP’s for Penicillins
3. Reduced drug entry through outer membrane Porins

Question 9

How do CSF levels differ after administration of Penicillin G differ between healthy patients and those with Meningitis?

Answer 9

• Normal meninges: concentration in CSF remain low
• Meningitis (inflamed): concentration in CSF is much higher
• Why? Inflammation increases meningeal permeability to penicillin G

Question 10

How do Benzathine & Procaine Penicillin G differ in durations of actions?

What about Procaine with Probenecid?

Answer 10

Benzathine penicillin G: Produces low plasma concentrations of the drug for several weeks

Procaine penicillin G: Produces higher plasma concentrations for about 24 hours

Probenecid: Inhibits renal excretion of penicillin G

• Prolongs its half-life & increases its plasma concentrations

Benzathine and procaine derivatives of penicillin G are

long acting and administered IM

Question 11

How are Cephalosporins similar to Penicillins?

How do they differ?

Answer 11

Similar to penicillins in;

• Beta-lactam core
• MOA
• Routes of elimination
• Adverse effects

Difference:

Cephalosporins are more resistant to B-lactamases than Penicillins

Exhibit 2-10% cross-allergenicity with penicillins

Question 12

What are the trends in cephalosporin generations?

Answer 12

As Cephalosporin generation # increases, the drugs become:

• less active on Gram positive bacteria
• more active on Gram negative bacteria
• More resistant to gram negative bacterial beta-lactamases

Question 13

Name a Narrow spectrum Penicillin

Answer 13

Penicillin G

Question 14

Name a penicillinase Resistance Penicillin

Answer 14

Nafcillin

Question 15

Name an exteneded spectrum Penicillin

Answer 15

Amoxicillin

Question 16

Name a 1st-gen Cephalosporin

Answer 16

Cefazolin

Question 17

Name a 2nd-gen Cephalosporin

Answer 17

Cefprozil

Question 18

Name a 3rd-gen Cephalosporin

Answer 18

Ceftriaxone (Rocephin)

Question 19

Name a 4th-gen Cephalosporin

Answer 19

Cefepime (Maxipime)

Question 20

Name a B-lactamase Inhibitor

Answer 20

Clavulanate (In augmentin)

Question 21

Name a Monobactam

Answer 21

Aztreonam (Azactam)

Question 22

Name a carbapenem

Answer 22

Imipenem (In primaxin)