Question

The fluoroquinolones act by:

• A. \( \text{Inhibiting folic acid synthesis, reducing nucleotide production and DNA synthesis} \)
• B. Inhibiting DNA gyrase and topoisomerase IV, causing supercoiling and fragmentation of bacterial DNA
• C. \( \text{Disrupting peptidoglycan cross-linking, weakening the bacterial cell wall} \)
• D. \( \text{Inhibiting ribosomal subunits, leading to the cessation of protein synthesis} \)

Hint:

Fluoroquinolones, such as ciprofloxacin and levofloxacin, are commonly used to treat bacterial infections. Their unique target, DNA gyrase, makes them effective against both Gram-positive and Gram-negative bacteria.

Answer 1

The Correct Option is B

Step 1: Mechanism of fluoroquinolones. Fluoroquinolones are a class of broad-spectrum antibiotics that target bacterial enzymes involved in DNA replication. Specifically, they inhibit DNA gyrase and topoisomerase IV, which are essential for relieving supercoiling tension during DNA replication and transcription. 

Step 2: Effects of inhibition. By inhibiting these enzymes, fluoroquinolones prevent bacterial DNA from properly unwinding and replicating. This results in supercoiling, DNA fragmentation, and ultimately bacterial cell death. 

Step 3: Comparison with other options. - Option \( (A) \): Describes the mechanism of sulfonamides and trimethoprim. 
- Option \( (C) \): Refers to the action of beta-lactam antibiotics (e.g., penicillins). 
- Option \( (D) \): Refers to antibiotics like tetracyclines and aminoglycosides. 

Conclusion: The correct answer is \( (B) \).

Answer 2

The question asks how fluoroquinolone antibiotics kill bacteria. Let's match each proposed mechanism to the drug class it actually belongs to.

1. Inhibiting folic acid synthesis: This is the mechanism of sulfonamides and trimethoprim, which block enzymes in the folate pathway that bacteria need to make nucleotides for DNA. Fluoroquinolones do not touch this pathway.
2. Inhibiting DNA gyrase and topoisomerase IV: Bacterial DNA has to be constantly uncoiled and recoiled during replication and transcription, and DNA gyrase and topoisomerase IV are the enzymes that manage this. Fluoroquinolones bind these enzymes and lock them onto the DNA, causing the DNA strands to become overwound and break apart, which is lethal to the bacterial cell.
3. Disrupting peptidoglycan cross-linking: This is how beta-lactam antibiotics like penicillins work, weakening the bacterial cell wall until the cell bursts. Fluoroquinolones act inside the cell on DNA, not on the outer cell wall.
4. Inhibiting ribosomal subunits: This describes protein synthesis inhibitors such as tetracyclines and aminoglycosides, which jam the ribosome rather than acting on DNA-handling enzymes.

Only the DNA gyrase and topoisomerase IV mechanism belongs to fluoroquinolones, since their whole class is defined by targeting these two DNA-supercoiling enzymes rather than the cell wall, ribosome, or folate pathway.

So the correct answer is Inhibiting DNA gyrase and topoisomerase IV, causing supercoiling and fragmentation of bacterial DNA.