Bactericidal Assays for Fluoroquinolones

Fluoroquinolone antibacterials rapidly kill susceptible bacteria at clinically relevant drug concentrations; all evidence indicates that these drugs exert their lethality on Escherichia coli by inhibiting the normal function of the DNA gyrase (a bacterial topoisomerase II) in this organism. Mutations around a limited region of the DNA gyrase A subunit (residues Ala67-Gln106) have been shown to confer resistance to these agents in E. coli (1 ). This has also been documented in many other organisms, indicating that DNA gyrase is a universal target for these drugs. This relationship has been somewhat complicated recently by the observation that in Staphylococcus aureus , mutations in another type II enzyme, topoisomerase IV, also confer resistance to fluoroquinolones (2 ).