In vitro comparison of the activity of doxycycline, tetracycline, erythromycin and a new macrolide, CP 62993, against Mycoplasma pneumoniae, Mycoplasma hominis and Ureaplasma urealyticum.

The in vitro activities of doxycycline, tetracycline, erythromycin and a new macrolide, CP 62993, against clinical isolates of Mycoplasma pneumoniae (n = 3), Mycoplasma hominis (n = 64) and Ureaplasma urealyticum (n = 33), were compared using an agar dilution method. M. pneumoniae seemed equally susceptible to the two tetracyclines, while the minimum inhibitory concentrations (MICs) of doxycycline were one or two dilution steps lower than the MICs of tetracycline for most strains of M. hominis and U. urealyticum. Ten of the 64 M. hominis strains required greater than or equal to 32 mg/l of tetracycline and 4 to 32 mg/l of doxycycline for inhibition. The MICs of CP 62993 were lower than those of erythromycin for all species tested although as much as 2 to 4 mg/l were required for inhibition of M. hominis. There is microbiological evidence that doxycycline as well as tetracycline might be effective against mycoplasmas and ureaplasmas. For erythromycin this is only true for M. pneumoniae and U. urealyticum.