Fluoroquinolones
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The parent drug of the quinolone family was Nalidixic acid, this was found to be very narrow spectrum. It worked well only against gram negative enterobacteriaceae and resistance quickly developed. This limited it's use now for just urinary tract infections and for treatment of Aeromonas salmonicida in fish. Due to the restricted spectrum of the quinolones fluoroquinolones were developed to increase the range. The most commonly used in practice are Enrofloxacin, Marbofloxacin, Danofloxacin, Difloxacin, Ibafloxacin and Orbifloxacin.
Mechanism of Action
Fluoroquinolones work by inhibiting bacterial DNA gyrase or the topoisomerase enzyme and thus inhibit bacterial DNA replication and transcription. The gyrase enzyme is usually the target in gram-negative bacteria, whilst the topoisomerase enzyme is the target in most gram-positive bacteria. This kills the bacteria and so the drug is a bactericidal and concentration dependent.
These drugs also have the ability to cross animal's cell membranes through porins. This means that this group of drugs is capable of acting upon intracellular pathogens.