Identify the antibiotic class that inhibits bacterial DNA gyrase (a type II topoisomerase)

Introduction / Context:
DNA gyrase introduces negative supercoils and relieves torsional stress during replication and transcription in bacteria. Quinolone antibiotics exploit this essential function by stabilizing DNA–enzyme cleavage complexes, blocking replication and causing lethal DNA damage.

Given Data / Assumptions:

• Target enzyme: DNA gyrase (topoisomerase II) in Gram-negative bacteria; topoisomerase IV also targeted in some organisms.
• Drugs to compare: quinolones vs. inhibitors of protein synthesis or cell wall synthesis.
• Outcome: inhibition of DNA replication and bacterial death.

Concept / Approach:
Nalidixic acid is the prototypical quinolone; fluoroquinolones (ciprofloxacin, levofloxacin) are widely used derivatives. They bind to the DNA–gyrase complex, preventing re-ligation of cleaved DNA. Tetracyclines block aminoacyl-tRNA binding to the 30S ribosome; cephalosporins inhibit peptidoglycan cross-linking; macrolides bind 50S to impede translocation—none of which directly inhibit gyrase.

Step-by-Step Solution:
Match mechanism (gyrase inhibition) to drug class (quinolones).Identify nalidixic acid as a quinolone.Exclude protein synthesis and cell wall inhibitors.

Verification / Alternative check:
Genetic mutations in gyrA/gyrB confer quinolone resistance, confirming the target pathway.

Why Other Options Are Wrong:

• Tetracycline: ribosomal inhibitor; no direct effect on gyrase.
• Both (C): incorrect because tetracycline does not inhibit gyrase.
• Cephalosporin: targets penicillin-binding proteins in cell wall synthesis.
• Macrolide: ribosomal 50S inhibitor; not a topoisomerase inhibitor.

Common Pitfalls:
Assuming all bactericidal antibiotics target DNA; mechanisms vary widely across classes.

Final Answer:
Nalidixic acid (a quinolone targeting DNA gyrase)