The static effect of rosaramicin on Ureaplasma urealyticum and the development of antibiotic resistance

Abstract

The activity of rosaramicin was tested against serotypes 3, 4, 6 and 8 of Ureaplasma urealyticum and against three untyped ureaplasma strains isolated from the human urogenital tract. In the metabolism-inhibition test, the initial inhibitory concentration of rosaramicin was 0·004 mg or less/l but the final inhibitory concentration was greater by up to 512-fold. This indicated that the antibiotic did not kill the organisms. Verification of this was obtained by withdrawing samples from organism-antibiotic mixtures, diluting them serially to reduce the effect of the antibiotic and examining for viable organisms; rosaramicin at a concentration of 125 mg/l was not lethal. The effect of rosaramicin on serotype 6 of U. urealyticum was similar to that of doxycycline. Rosaramicin was active also against a tetracycline-resistant ureaplasma strain isolated from the urethra of a hypogammaglobulinaemic patient, the final inhibitory concentration of rosaramicin being 0·00015 mg/l. Nevertheless, the organisms were not eliminated by two courses of this antibiotic and the final inhibitory concentration of rosaramicin for those recovered from the patient after treatment was 125 mg/l. This illustrates not only the rapid development of resistance but also the importance of the competent immune system in helping to eliminate ureaplasmas from patients who are treated with antibiotics that do not have cidal activity.