Nalidixic acid is a man-made synthetic antibiotic included in the category called Quinolones. It is a narrow spectrum drug useful against a limited range of microbes and used in only targeted ways.
Its mode of action is disabling the bacterial DNA Gyrase enzyme. This is one of the topoisomerases that is responsible for changing the 3-D structure of DNA during its unpackaging for DNA replication and transcription and repackaging for distribution of the two daughter genomes into the two daughter cells during growth. The part of DNA Gyrase impacted by nalidixic acid is the GyrA subunit, which is responsible for nick-close activity of DNA strands. Without this ability, daughter genomes are intertwined and cannot be separated, meaning genetic damage to separating daughter cells is inevitable.