In vitro evaluation of activities of azithromycin, erythromycin, and tetracycline against Chlamydia trachomatis and Chlamydia pneumoniae

The in vitro activities of azithromycin (CP-62,993; Pfizer), erythromycin, and tetracycline were evaluated by inhibiting Chlamydia trachomatis and Chlamydia pneumoniae, formerly TWAR, propagation in vitro in McCoy cells, HeLa cells, and HL cells. Eleven clinical isolates of C. trachomatis (serovars D, E, F, J, K, and L₂) and four strains of C. pneumoniae were tested with an inoculum of 10³ inclusion-forming units in a 96-well microtiter plate. The MIC ranges of these antimicrobial agents against C. trachomatis were as follows: azithromycin, 0.125 to 0.5 μg/ml; erythromycin, 0.25 to 0.1 μg/ml; and tetracycline, 0.0625 to 1.0 μg/ml. The MBC ranges, calculated from passage into antibiotic-free medium, were as follows: azithromycin, 0.125 to 4.0 μg/ml; erythromycin, 0.5 to 8.0 μg/ml; and tetracycline, 0.0625 to 4.0 μg/ml. The MIC ranges for C. pneumoniae in both HeLa and HL cells were as follows: azithromycin, 0.125 to 1.0 μg/ml; erythromycin, 0.0625 to 1.0 μg/ml; and tetracycline, 0.125 to 1.0 μg/ml. The MBC ranges were as follows: azithromycin, 0.25 to 1.0 μg/ml; erythromycin, 0.25 to 1.0 μg/ml; and tetracycline, 0.125 to 4.0 μg/ml. From the results of this in vitro study, azithromycin appears to be an effective antibiotic comparable to tetracycline and erythromycin for use in the treatment of both C. trachomatis and C. pneumoniae infections.