In-vitro activity of azithromycin, erythromycin, ciprofloxacin and norfloxacin against Neisseria gonorrhoeae, Haemophilus ducreyi, and Chlamydia trachomatis.

The minimum inhibitory concentrations (MICs) of azithromycin, erythromycin, ciprofloxacin and norfloxacin for 300 strains of Neisseria gonorrhoeae, 100 strains of Haemophilus ducreyi and six strains of Chlamydia trachomatis were determined. The two quinolones were more active against gonococcal strains than were the two macrolides. Azithromycin was approximately eight-fold more active against N. gonorrhoeae than was erythromycin (MIC90: 0.25 mg/l azithromycin, 2.0 mg/l erythromycin). The Mtr phenotype of gonococci increased azithromycin MICs approximately four fold. Azithromycin was less active than erythromycin against C. trachomatis. Azithromycin had considerable activity against H. ducreyi and was ten-fold more active than was erythromycin (MIC90: 0.004 mg/l azithromycin, 0.03 mg/l erythromycin). Clinical trials of azithromycin in the treatment of chlamydial infection and genital ulcer disease are indicated.