Factors influencing the in-vitro sensitivity of Ureaplasma urealyticum to tetracyclines.

The sensitivity of 23 strains of Ureaplasma urealyticum to doxycycline was measured by a metabolism-inhibition technique. The minimal inhibitory concentrations were not influenced by using, as inocula in the tests, organisms either directly from the patient or after culture, providing that the numbers of organisms in the inocula were about the same in both. All the strains were sensitive to doxycycline but an apparent increase in the resistance of the cultured organisms occurred when the number, expressed as colour-changing units (ccu), in the inoculum was 10(5) or more/ml. Tests may be undertaken on cultured organisms of U. urealyticum but it is recommended that a standard inoculum of 10(3)-10(4) ccu/ml should be used.