Which statement about the mechanism of action of fluoroquinolones is true?

Which statement about the mechanism of action of fluoroquinolones is true?

• A. They disrupt cell wall synthesis
• B. They inhibit DNA gyrase and Topoisomerase IV
• C. They inhibit RNA polymerase
• D. They inhibit folate synthesis

Answer: (B)

The key idea is that fluoroquinolones kill bacteria by blocking two essential enzymes that manage DNA topology during replication: DNA gyrase and topoisomerase IV. DNA gyrase helps twist and unkink DNA by introducing negative supercoils, which is needed as the chromosome is copied. Topoisomerase IV is important for separating the newly replicated chromosomes so the daughter cells can be divided. When fluoroquinolones bind to these enzyme–DNA complexes, they prevent the enzymes from re-ligating the DNA strands, leading to stalled replication and lethal double‑strand DNA breaks. This action makes them bactericidal and explains why the statement about inhibiting DNA gyrase and topoisomerase IV is true. This mechanism is distinct from agents that disrupt cell wall synthesis, inhibit RNA polymerase, or block folate synthesis, which affect different bacterial processes.

The key idea is that fluoroquinolones kill bacteria by blocking two essential enzymes that manage DNA topology during replication: DNA gyrase and topoisomerase IV. DNA gyrase helps twist and unkink DNA by introducing negative supercoils, which is needed as the chromosome is copied. Topoisomerase IV is important for separating the newly replicated chromosomes so the daughter cells can be divided. When fluoroquinolones bind to these enzyme–DNA complexes, they prevent the enzymes from re-ligating the DNA strands, leading to stalled replication and lethal double‑strand DNA breaks. This action makes them bactericidal and explains why the statement about inhibiting DNA gyrase and topoisomerase IV is true. This mechanism is distinct from agents that disrupt cell wall synthesis, inhibit RNA polymerase, or block folate synthesis, which affect different bacterial processes.