Susceptibility of Chlamydia pneumoniae to azithromycin and doxycycline: methodological aspects on the determination of minimal inhibitory and minimal bactericidal concentrations.

An in vitro assay for measuring and comparing the efficacy of different antimicrobial agents against Chlamydia pneumoniae was developed. Azithromycin, a representative of the new azalide group of antibiotics, and doxycycline were evaluated with respect to their antibacterial effect and capacity for intracellular killing under different experimental conditions. For both study drugs, the minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC) values increased significantly with longer bacterial preincubation time. The effect of different exposure times of antibiotics on the bacteria was also studied.