In vitro activity of norfloxacin against clinical isolates of Chlamydia trachomatis

In recent years, Chlamydia trachomatis has been shown to be an important etiologic pathogen in several ocular syndromes in both adults and neonates, as well as in a number of genital tract infections. Antimicrobial chemotherapy of chlamydial ocular infections is complicated by the frequency of intercurrent infection with other pathogens, which often necessitates treatment with combinations of different agents. It would be desirable to replace such combination therapy with a single broad-spectrum agent active against most potential pathogens. Norfloxacin is a new broad-spectrum fluoroquinolone with bactericidal activity against gram-positive and gram-negative aerobic organisms. The activity of norfloxacin against a number of clinical isolates of C. trachomatis was tested in vitro. McCoy cell monolayers in microtiter plates were inoculated with these isolates, and various concentrations of norfloxacin were added to the culture supernate. After three days of incubation, cells were stained with iodine, and inclusions were counted by microscopic scanning of the microtiter plate. The cells were also examined by immunofluorescence. Norfloxacin clearly inhibited the replication of all clinical isolates of C. trachomatis tested (MIC 20 μg/ml). The MIC₉₀ and MIC₅₀ values were about 15 and 10 μg/ml, respectively. The in vitro activity of norfloxacin against C. trachomatis, combined with its favorable pharmacokinetic profile and topical application at the site of infection, suggests the potential value of this agent in the clinical management of chlamydial ocular infections.