Standardized method for determining antimicrobial susceptibility of strains of Ureaplasma urealyticum and their response to tetracycline, erythromycin, and rosaramicin.

We describe a method for determining the minimal inhibitory concentrations (MICs) of antibiotics for Ureaplasma urealyticum which is compatible with current standard of susceptibility testing. A presumptive MIC is available after 24 h of incubation, and the definitive MIC is available at 48 h. The MICs for 9 serotype strains and 27 clinical isolates ranged from less than or equal to 0.5 to 256 microgram of tetracycline per ml, greater than or equal to 1 to 64 microgram of erythromycin per ml, and 0.031 to 4.0 microgram of rosaramicin per ml. Of an additional 555 isolates screened for their response to tetracycline, 2% required MICs of greater than 64 microgram/ml, which we believe is near the concentration at which in vivo resistance to this agent is expressed. After prolonged exposure to rosaramicin, the resistance of two of three serotype strains of U. urealyticum was increased 8- and 16-fold, but the MICs still did not exceed 1.0 microgram/ml.