ER - Antibacterials & Drug Efflux II

Question 1

What is the target of Penicillin? (3)

Answer 1

Peptidoglycan Synthesis

• Inhibits cell wall synthesis in bacteria.
• Binds to and blocks transpeptidases that form peptidoglycan cross-links.
• Effective against Gram-positive bacteria, and some Gram-negative bacteria (e.g., Neisseria).

Question 2

How does Penicillin work? (3)

Answer 2

• Weakens cell walls by blocking peptidoglycan cross-linking.
• Causes bacterial cell lysis due to osmotic pressure.
• Targets actively growing bacteria.

Question 3

What regulates Peptidoglycan Synthesis? (3)

Answer 3

Class A Penicillin-Binding Proteins (PBPs)

• Involved in the GTase (glycosyltransferase) and TPase (transpeptidase) reactions.
• These reactions are crucial for forming the peptidoglycan mesh that strengthens bacterial cell walls.

Question 4

What is the significance of the Beta-lactam ring in antibiotics? (3)

Answer 4

• The beta-lactam ring is essential for the antibiotic’s mechanism of action.
• Structurally similar to the D-Ala-D-Ala portion of peptidoglycan precursors.
• This similarity allows it to bind to transpeptidases, inhibiting peptidoglycan cross-linking in bacterial cell walls.

Question 5

When is Penicillin most effective? (2)

Answer 5

• Most effective against actively growing bacterial cells synthesizing new peptidoglycan.
• Less effective against bacteria in the stationary phase or with inactive transpeptidases.

Question 6

What types of bacteria does Penicillin target?
(3)

Answer 6

• Mainly affects Gram-positive bacteria.
• Can also target some Gram-negative bacteria (e.g., Neisseria and Haemophilus).
• Gram-negative bacteria have an outer membrane that limits antibiotic access to the peptidoglycan layer.

Question 7

What is a resistance mechanism to Penicillin? (2)

Answer 7

• Beta-lactamases: Enzymes that break down the beta-lactam ring in Penicillin.
• Altered cell walls that reduce drug susceptibility.

Question 8

What is Vancomycin’s target? (3)

Answer 8

Peptidoglycan Synthesis in Gram-positive bacteria

• Binds to D-Ala-D-Ala residues on peptidoglycan precursors.
• Effective against Gram-positive bacteria but not Gram-negative due to LPS barrier.

Question 9

What causes Vancomycin resistance? (2)

Answer 9

• Replacement of the last D-Ala residue with D-lactate.
• Prevents Vancomycin binding, leading to stable cross-links and cell wall integrity

In the resistant bacteria, stable cross-links are formed.

In the sensitive bacteria, cross-links cannot be formed and the cell wall falls apart.

Question 10

What does Ciprofloxacin target? (3)

Answer 10

DNA Synthesis

• Targets DNA gyrase and topoisomerase IV, essential for DNA replication.
• Stabilizes cleaved DNA complexes, halting replication.

Question 11

How do fluoroquinolones (like Ciprofloxacin) work? (2)

Answer 11

• Inhibit re-ligation of cleaved DNA, causing bactericidal effects.
• Bind to DNA gyrase and topoisomerase IV to stabilize DNA complexes

Question 12

What are some resistance mechanisms against Ciprofloxacin? (2)

Answer 12

1) Target-site modification:

• Mutations in gyrAB and parCE genes reduce the affinity of DNA gyrase and topoisomerase IV to fluoroquinolones (e.g., ciprofloxacin).

2) Upregulation of efflux pumps:

• Overexpression of efflux pumps expels ciprofloxacin from the cell.
• Driven by mutations in regulatory genes of efflux pumps.

Question 13

Fluoroquinolones binding: (2)

Answer 13

• Bind to DNA gyrase: Stabilize the broken DNA complex, halting replication.
• Bind to topoisomerase IV: Stabilize the catenated DNA complex, preventing separation of sister chromosomes after replication.

Question 14

What do Sulfonamides target? (3)

Answer 14

Folic Acid Metabolism

• Act as competitive inhibitors of PABA, blocking folic acid synthesis in bacteria.
• Inhibit a key reaction in the folic acid metabolism cycle necessary for folic acid synthesis in bacteria.

(Without this reaction, bacteria cannot replicate.)

Question 15

What is the bacteriostatic nature and structural action of Sulfonamides? (3)

Answer 15

• Bacteriostatic: Inhibit bacterial growth and multiplication without killing bacteria.
• Humans acquire folate (vitamin B9) through diet, while bacteria must synthesize it.
• Sulfanilamide is structurally similar to PABA, inhibiting tetrahydrofolate biosynthesis in bacteria.

Question 16

What is the resistance mechanism to Sulfonamides? (2)

Answer 16

• acterial resistance to one sulfonamide indicates resistance to all sulfonamides.
• This is due to consistent bacterial sensitivity across the class

Question 17

What is the mechanism of action (1), importance of THF (2), selective binding (1) and combination therapy (1) of Trimethoprim?

Answer 17

Mechanism of Action: Trimethoprim is a reversible inhibitor of dihydrofolate reductase (DHFR), blocking the conversion of dihydrofolic acid (DHF) to tetrahydrofolic acid (THF).

Importance of THF:

• THF is essential for the synthesis of bacterial nucleic acids and proteins.
• Inhibiting THF synthesis causes bactericidal activity and compromises bacterial survival.

Selective Binding: Binds more strongly to bacterial DHFR than to mammalian DHFR, ensuring targeted disruption of bacterial processes.

Combination Therapy: Often combined with sulfamethoxazole for enhanced antibacterial effect.

Question 18

What is the mechanism of action (3), effects (1) and resistance (3) of Tetracycline?

Answer 18

Mechanism of Action:

• Targets protein synthesis as a 30S inhibitor.
• Tetracycline binds reversibly to the 30S ribosomal subunit, blocking the binding of aminoacyl-tRNA to the acceptor site on the mRNA-ribosome complex.

Effects: Exhibits both bacteriostatic (inhibiting growth) and bactericidal (killing bacteria) effects.

Resistance Mechanisms:

• Modification of Binding Site: Alters the target of tetracycline.
• Changes in Active Transport: Involves efflux pumps that expel the drug from bacterial cells and porin regulation that affects drug accumulation.

Question 19

What is the mechanism of action (3), Bacteriostatic vs. Bactericidal (1) and resistance (2) of Erythromycin?

Answer 19

Mechanism of Action:

• Binds to the peptide exit tunnel in the 50S ribosomal subunit (near the peptidyltransferase center).
• Prevents peptide chain elongation during protein synthesis.
• Inhibits the formation of the 50S ribosomal subunit.

Bacteriostatic vs. Bactericidal:

• Primarily bacteriostatic (inhibits growth), but can be bactericidal to some bacteria at high concentrations.

Resistance Mechanism:

• The binding site overlaps with those for clindamycin and streptogramins.
• Ribosomal methylation reduces binding of all three antibiotics, similarly impacting their efficacy.

Question 20

What are the mechanisms of action for Chloramphenicol and Linezolid?

Answer 20

Mechanism of Action: Both antibiotics interfere with peptide bond formation during translation elongation.

Chloramphenicol:

• Prevents full A-tRNA accommodation, blocking the peptidyltransferase activity essential for peptide bond formation.

Linezolid:

• Also prevents full A-tRNA accommodation, but operates through a different mechanism than chloramphenicol.

Question 21

What is the mechanism of action (1), binding specificity (2) and effects on translation (2) of Aminoglycosides?

Answer 21

Mechanism of Action:

• Inhibit protein synthesis by binding with high affinity to the A-site on the 16S ribosomal RNA of the 30S ribosome.

Binding Specificity:

• Different aminoglycosides have varied specificity for different regions on the A-site.
• All aminoglycosides alter the conformation of the A-site.

Effects on Translation:

• Promote mistranslation by inducing codon misreading during the delivery of aminoacyl-tRNA.
• This leads to error-prone protein synthesis, resulting in faulty proteins.

Question 22

What is the classification (2), mechanism of action (3), consequences of disruption (1) and safety (1) of Polymyxin B?

Answer 22

Classification:

• Last-resort antibiotics; part of a group of basic peptides.
• Effective against most gram-negative bacteria.

Mechanism of Action:

• Act as cationic detergents.
• Bind with high affinity to negatively charged lipopolysaccharides (LPS) in the outer membrane of gram-negative bacteria.
• Disrupt the integrity of the outer membrane, allowing interaction with the inner cytoplasmic membrane.

Consequences of Disruption:

• Increase membrane permeability, leading to bacterial cell death.

Safety and Usage:
* Due to significant toxicity when administered systemically, polymyxins have historically been restricted to topical use.

Question 23

What is the bactericidal activity of Polymyxin B?

Answer 23

Polymyxins are bactericidal, exhibiting a time-dependent rate of killing at concentrations above their Minimal Bactericidal Concentration (MBC).

Question 24

What are the resistance mechanisms to Polymyxin B in gram-positive (1) and gram-negative (3) bacteria?

Answer 24

1) Gram-Positive Bacteria Resistance:

• Gram-positive bacteria (e.g., Proteus & Neisseria) are intrinsically resistant.

2) Gram-Negative Bacteria Resistance:

• Modified LPS Structure: Involves cationic substitution of negatively charged phosphates, which reduces binding of polymyxins.
• Capsule Formation: Bacteria can form capsules that protect the outer membrane from polymyxin action.
• Over-Expression of OprH: In Pseudomonas aeruginosa, increased levels of the outer membrane protein OprH enhance membrane stability by interacting with LPS.

Question 25

What is Zosurabalpin (2) and its mechanism of action (2) and binding mechanism (1)?

Answer 25

Zosurabalpin

• Is a novel class of tethered macrocyclic peptide (MCP) antibiotics.
• It specifically targets the lipopolysaccharide (LPS) transporter.

Mechanism of Action:

• Blocks the transport of bacterial lipopolysaccharide from the inner membrane to the outer membrane.
• Inhibits the LptB₂FGC complex, which is essential for LPS transport.

Binding Mechanism:

• Recognizes a composite binding site formed by both the Lpt transporter and its LPS substrate.

Question 27

What is Halicin (1) and its mechanism of action (3) its resistance (1) and spectrum of activity (2)?

Answer 27

Halicin

• Is an antibiotic discovered using deep learning (AI) on known drug-like compounds to identify new antibiotics.

Mechanism of Action:

• Unusual action involves sequestration of iron inside bacterial cells.
• Interferes with the bacteria’s ability to regulate pH balance across the cell membrane.
• Disrupts electrochemical gradients necessary for energy storage and transfer.

Resistance and Activity:

• Its different mode of action allows Halicin to remain effective against bacterial strains resistant to many commonly used antibiotics.

Spectrum of Activity:

• Exhibits broad-spectrum antibiotic activity.
• Concerns arise regarding its application in humans.