Abstract: Objective To investigate the in vitro effects of various antibiotics (spectinomycin, ceftriaxone, erythromycin, ofloxacin and doxycycline) against 12 isolates of C. trachomatis. Methods Minimal inhibitory concentrations (MICs) and fractional inhibitory concentrations (FICs) of the antimicrobials against all C. trachomatis were calculated. Checkerboard method was used for the determination of FICs and Ridit test for the comparison of the interactions among the various combinations. Results No difference was observed in most of the combinations. No antagonism was found in all except for ceftriaxone-doxycycline combination. Synergism was observed in 42% (5 of 12) and 50% (6 of 12) of the chlamydial isolates for erythromycin-spectinomycin and doxycycline-spectinomycin combination, respectively. No significant difference was observed among triple combinations with spectinomycin or with ceftriaxone. When interactions of erythromycin, ofloxacin and doxycycline with spectinomycin were compared to those with ceftriaxone respectively, both interactions of erythromycin (U=2.46, P=0.014) and doxycycline (U=2.83, P=0.002) were more synergistic with spectinomycin than those with ceftriaxone. Conclusions This study indicates that the combination of spectinomycin with erythromycin or doxycycline is more effective against C. trachomatis than that of ceftriaxone. Therefore, spectinomycin rather than ceftriaxone might be recommended in the dual therapy against C. trachomatis and N. gonorrhoeae.

Key words: Chlamydia trachomatis, Antibiotics, Drug interactions