Mechanism of action of quinolones is:

Mechanism of action of Fluoroquinolones The FQs inhibit the enzyme bacterial DNA gyrase (primarily active in gram negative bacteria), which nicks double-stranded DNA, introduces negative supercoils and then reseals the nicked ends. This is necessary to prevent excessive positive supercoiling of the strands when they separate to permit replication or transcription. Reference: Essentials of Medical Pharmacology Eighth Edition KD TRIPATHI page no 760