The quinolones are synthetic antibiotics chemically related to nalidixic acid. Since its introduction, several structural analogues have been synthesized. A fundamental breakthrough was the addition of a fluorine atom. The quinolones interfere with bacterial DNA transcription by inhibiting the enzyme DNA gyrase, that so far has only been found in bacteria. The nature of the activity of the quinolones on DNA gyrase makes it highly unlikely that resistance is carried on plasmids.