In vitro activity of antibiotics against Ureaplasma urealyticum and Chlamydia trachomatis strains from patients with nongonococcal urethritis.

The activity of minocycline, doxycycline, tetracycline, erythromycin, mepartricin and lincomycin against 35 freshly isolated Ureaplasma urealyticum strains was tested. Doxycycline was the most active. Twelve strains were resistant to minocycline and four of these were sensitive to erythromycin. Mepartricin showed no activity against the organisms at a concentration of 10 micrograms/ml. The susceptibility of 30 low-laboratory-passage Chlamydia trachomatis strains against tetracycline and erythromycin was tested. A variable degree of sensitivity to tetracycline and erythromycin was found, the median MIC values being 0.13 micrograms/ml and 0.025 micrograms/ml respectively. No resistant Chlamydia trachomatis strain was found.