Quinolones exhibit good antibacterial activity against Salmonella spp. isolates and are often the choice of treatment for life-threatening
salmonellosis due to multi-
drug resistant strains. To assess the properties of
quinolones, we performed an in vitro assay to study the antibacterial activities of
quinolones against
recombinant DNA gyrase. We expressed the S. Typhimurium
DNA gyrase A (GyrA) and B (GyrB) subunits in Escherichia coli. GyrA and GyrB were obtained at high purity (>95%) by
nickel-nitrilotriacetic acid agarose resin column chromatography as His-tagged 97-kDa and 89-kDa
proteins, respectively. Both subunits were shown to reconstitute an
ATP-dependent
DNA supercoiling activity.
Drug concentrations that suppressed
DNA supercoiling by 50% (IC50 s) or generated DNA cleavage by 25% (CC25 s) demonstrated that
quinolones highly active against S. Typhimurium
DNA gyrase share a
fluorine atom at C-6. The relationships between the minimum inhibitory concentrations (MICs), IC50 s and CC25 s were assessed by estimating a linear regression between two components. MICs measured against S. Typhimurium NBRC 13245 correlated better with IC50 s (R = 0.9988) than CC25 s (R = 0.9685). These findings suggest that the
DNA supercoiling inhibition assay may be a useful screening test to identify
quinolones with promising activity against S. Typhimurium. The
quinolone structure-activity relationship demonstrated here shows that C-8, the C-7 ring, the C-6
fluorine, and N-1 cyclopropyl substituents are desirable structural features in targeting S. Typhimurium gyrase.