In vitro activities of azithromycin, clarithromycin, L-ofloxacin, and other antibiotics against Chlamydia pneumoniae.

The in vitro susceptibilities of 11 strains of Chlamydia pneumoniae to azithromycin, clarithromycin, erythromycin, L-oflaxacin, and doxycycline were determined. Clarithromycin was the most active agent tested, with an MIC for 90% of strains and minimal chlamydiacidal concentration for 90% of strains of 0.03 microns/ml. The activity of azithromycin was similar to those of erythromycin and doxycycline, with MICs for 90% of strains of 0.125 to 0.25 microns/ml. However, the prolonged half-life and enhanced tissue penetration of azithromycin should allow for less frequent dosing and shorter duration of therapy than with erythromycin or clarithromycin. L-Ofloxacin had activity similar to that of ofloxacin, with MICs of 0.125 to 0.5 micron/ml. From the results of this in vitro study, azithromycin and clarithromycin appear to be effective antibiotics that may have a role in the treatment of infections due to C. pneumoniae.