Characterization of tetracycline-resistant strains of Ureaplasma urealyticum

Gel electrophoresis revealed no plasmids in cloned strains of Ureaplasma urealyticum for which minimal inhibitory concentration (MICs) of tetracycline were greater than 64 mg/l. However, DNA from these strains hybridized with the tetM sequence from Streptococcus agalactiae in dot blot hybridization, whilst DNA from more-susceptible strains did not do so. Our results confirm and extend previous work, in which the tetM sequence was associated with resistance to the tetracycline antibiotics in strains of U. urealyticum. The strains examined in this study were isolated primarily in western North America but included representatives from Europe and the United Kingdom. Serotyping showed an increase in strains with the serotype 9 determinant and a decrease in those with the serotype 14 determinant together with a shift to the biotype 2 cluster (P = less than 0.02). To characterize these strains and to identify alternative antibiotics for therapeutic and basic research applications, 26 tetracycline-resistant strains were tested against 25 diverse antimicrobial agents. All demonstrated in-vitro susceptibility to rosaramicin but were resistant to 2 mg/l erythromycin. Only two of the strains were resistant to 16 mg/l erythromycin. Most antibiotics were not active against these isolates, but low concentrations of filipin and unconventional agents such as flurofamide and hydroquinone inhibited the growth of some strains.