Antimicrobial activity of gatifloxacin tested against Neisseria gonorrhoeae using three methods and a collection of fluoroquinolone-resistant strains.

Gatifloxacin, a new 8-methoxy fluoroquinolone, was tested against 131 strains of Neisseria gonorrhoeae by reference agar dilution, disk diffusion, and Etest (AB BIODISK, Solna, Sweden) methods on supplemented GC agar. Gatifloxacin activity was equal to ciprofloxacin (MIC50, 0.008 microgram/mL) against strains fully susceptible to fluoroquinolones, but was generally four-fold more active (MIC90, 0.064-0.094 microgram/mL) against strains with par C or gyr A mutations and resistance to ciprofloxacin. Etest results were comparable to those generated by the agar dilution test [correlation coefficient (r) = 0.97]. Gatifloxacin zone diameters using 5-microgram disks also correlated well (r = 0.86-0.87) with the agar dilution and Etest MIC results. Breakpoints for laboratory testing of Neisseria gonorrhoeae strains await clinical trial outcome correlations, but susceptibility at < or = 0.125 or < or = 0.25 microgram/mL (> or = 34 mm) seems appropriate. All three tests used in this study seem applicable for laboratory testing of isolates from patients with uncomplicated gonorrhoeae receiving therapy with gatifloxacin.