What bacterial target do fluoroquinolones inhibit?

• A. DNA gyrase (and topoisomerase IV in some bacteria)
• B. 50S ribosomal subunit
• C. Peptidoglycan cross-linking enzymes
• D. Dihydropteroate synthase

Answer: (A)

Fluoroquinolones block bacterial DNA replication by disabling type II topoisomerases. These enzymes, mainly DNA gyrase and, in some bacteria, topoisomerase IV, regulate DNA supercoiling and untangling during replication and transcription. When fluoroquinolones bind to the enzyme-DNA complex, they prevent re-ligation after DNA cleavage, leading to breaks in DNA and stopping bacterial growth or causing cell death. This mechanism is distinct from other antibiotic targets: inhibitors of the 50S ribosomal subunit disrupt protein synthesis; agents that block peptidoglycan cross-linking affect cell wall synthesis; and drugs like sulfonamides inhibit dihydropteroate synthase in folate production. Thus, the described bacterial target is DNA gyrase and, in some bacteria, topoisomerase IV.