Fluoroquinolones: Mechanism of Action (Topoisomerase Inhibitor) and MCQ for NEETPG, GPAT, NORCET

Overview

• Fluoroquinolones are a class of broad-spectrum bactericidal antibiotics.

• Synthetic derivatives of nalidixic acid (a quinolone).

• Common drugs: Ciprofloxacin, Ofloxacin, Levofloxacin, Moxifloxacin, Norfloxacin.

• Participate in GPAT MOCK TEST

Mechanism of Action

Fluoroquinolones inhibit bacterial DNA topoisomerases, which are enzymes essential for DNA replication and transcription.

Targets:

1. DNA Gyrase (Topoisomerase II) – Primarily in Gram-negative bacteria

   • Unwinds supercoiled DNA to allow replication.

   • Inhibition leads to failure in DNA relaxation → halts replication → cell death.

2. Topoisomerase IV – Primarily in Gram-positive bacteria

   • Separates interlinked daughter chromosomes post-DNA replication.

   • Inhibition leads to failure in chromosome separation → cell death.

Both enzymes are essential, but drug affinity varies by bacterial type (Gram-negative vs. Gram-positive).

Bactericidal Action

• Fluoroquinolones act by blocking DNA synthesis, leading to:

  • DNA fragmentation

  • Inhibition of cell division

  • Rapid bacterial cell death

Spectrum of Activity

• Broad-spectrum

  • Gram-negative bacteria: E. coli, Pseudomonas, Salmonella, Shigella

  • Gram-positive bacteria: Staphylococcus aureus, Streptococcus spp.

  • Atypical pathogens: Mycoplasma, Chlamydia, Legionella

• MCQs: Fluoroquinolones – Mechanism of Action

• Get More MCQ: Click Here

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  1. Fluoroquinolones exert their antibacterial action by inhibiting which of the following enzymes?

  A. RNA polymerase
  B. DNA polymerase
  C. DNA gyrase and topoisomerase IV
  D. Peptidyl transferase

  Answer: C. DNA gyrase and topoisomerase IV

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  2. Which enzyme is primarily targeted by fluoroquinolones in Gram-negative bacteria?

  A. Topoisomerase I
  B. Topoisomerase IV
  C. DNA polymerase
  D. DNA gyrase

  Answer: D. DNA gyrase

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  3. In Gram-positive organisms, fluoroquinolones primarily inhibit:

  A. DNA gyrase
  B. RNA polymerase
  C. DNA ligase
  D. Topoisomerase IV

  Answer: D. Topoisomerase IV

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  4. What is the result of inhibiting bacterial topoisomerase by fluoroquinolones?

  A. Enhanced protein synthesis
  B. Blockage of folic acid synthesis
  C. Prevention of DNA replication
  D. Increased bacterial cell wall synthesis

  Answer: C. Prevention of DNA replication

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  5. Fluoroquinolones are classified as:

  A. Bacteriostatic
  B. Fungistatic
  C. Bactericidal
  D. Virucidal

  Answer: C. Bactericidal