Simple, direct broth-disk method for antibiotic susceptibility testing of Ureaplasma urealyticum

A simple, direct broth-disk test, utilizing urine sediment as the inoculum and impregnated paper disks as the source of antibiotic, was developed and used to test the susceptibility of 54 isolates of Ureaplasma urealyticum (T-strain mycoplasmas) to minocycline, doxycycline, demeclocycline, tetracycline, and erythromycin. The concentration of each antibiotic was calculated to approximate the attainable blood level. Resistance or susceptibility to each antibiotic was determined by growth, indicated by a color change of the medium in each tube, comparable to that of a control culture without antibiotic. Of the 54 T-mycoplasmas tested, 46 (85.2%) were inhibited by 1 mug of minocycline per ml, 45 (83.3%) were inhibited by 1 mug of doxycycline per ml, 38 (72.2%) were inhibited by 1 mug of demeclocycline per ml, 18 (33.3%) were inhibited by 1 mug of tetracycline per ml, and only 2 (3.7%) were inhibited by 3 mug of erythromycin per ml. Seven (13%) of the 54 T-mycoplasmas tested were resistant to all five antibiotics. There was good correlation between results obtained by this direct broth-disk method and minimal inhibitory concentrations obtained by the direct broth dilution method.