In vitro susceptibilities of urogenital Chlamydia trachomatis clinical isolates to azithromycin alone and in combination with other antimicrobial agents
Objective To test the in vitro activity of azithromycin against recent clinical isolates of urogenital Chlamydia trachomatis, and combined activity of azithromycin with moxifloxacin, rifampicin, doxycycline and minocyline. Methods When more than 90% McCoy cells were infected, the 41 strains tested were collected to investigate minimal inhibitory concentrations (MICs) of 5 antimicrobials alone. Checkerboard array was used to calculate the fractional inhibitory concentrations(FICs) and then detected the interactions among the various combinations. Results In vitro, synergism or additivity effect of 51.22%,53.66% and 58.54% strains was found in azithromycin-moxifloxacin, azithromycin-doxycycline and asithromycin-rifampicin combinations, respectively. No difference was observed in all of the combinations ( P ＞0.05). However, antagonism effect of 90.24% strains was observed in azithromycin-minocyline combination. Conclusion This study indicates that the combination of azithromycin with moxifloxacin, doxycycline or rifampicin is more effective against Chlamydia trachomatis than individual antimicrobials. Therefore, these antimicrobials combinations might be recommended against Chlamydia trachomatis recurrent or persisitent infection. However, the combination of azithromycin with minocyline exhibited a markedly antagonism activity against Chlamydia trachomatis. Combined sensitivity test to a certain extent can compensate for some shortcomings of individual susceptibility test. 
 
Key words: 
Chlamydia trachomatis; Azithromycin; Minimal inhibitory concentration; Fraction inhibitory concentration; Drug interactions