Chlamydia trachomatis: in vitro susceptibility to antibiotics singly and in combination

Decreased susceptibility in vitro to erythromycin has been demonstrated for few C. trachomatis isolates outside Scandinavia, making local susceptibility-screening indicated. Eleven recent isolates of C. trachomatis found in a Danish hospital have been examined for susceptibility, expressed as minimal inhibitory concentration (MIC) to antibacterial agents commonly used in genito-urinary infections. Full susceptibility to doxycycline and erythromycin was demonstrated. Clindamycin and ampicillin showed moderate activity, and sulfamethizole had a MIC value in the border area of what is needed for therapeutic effect in non-urinary infections. C. trachomatis, being a major pathogen in pelvic inflammatory disease, makes combination chemotherapy desirable in order to protect against resistance development, to obtain synergistic effect and to ensure effect in infections of mixed etiology - provided antagonism could not be anticipated. In three checkerboard trials, with the combinations doxycycline plus ampicillin, erythromycin plus sulfamethizole and ampicillin plus sulfamethizole, using MIC as end-point, neither synergism nor antagonism could be demonstrated in the concentration range from 1/8 to 4 times the MIC values of each drug.