What is the mechanism of action of fluoroquinolones?

• A. Inhibit DNA gyrase only
• B. Inhibit Topoisomerase 4 only
• C. Both A and B
• D. Inhibits Dihydrofolate reductase

Answer: (C)

Fluoroquinolones kill bacteria by blocking two essential DNA-processing enzymes: DNA gyrase and topoisomerase IV. These enzymes manage DNA topology during replication—DNA gyrase introduces negative supercoils to help unwind the double helix, while topoisomerase IV separates replicated chromosomes (decatenation). Fluoroquinolones bind to the enzyme–DNA complex after the DNA strands are cut, preventing the strands from rejoining. This causes double-stranded breaks and shuts down DNA replication, leading to bacterial cell death. Some drugs in this class target both enzymes, which is why the mechanism is best described as inhibition of both. Dihydrofolate reductase is inhibited by another antibiotic, trimethoprim, not fluoroquinolones, so that option is incorrect.