In Vitro Susceptibility of Treponema pallidum subsp. pallidum to Doxycycline

Doxycycline is regarded as an effective therapy for early syphilis, and there is increasing interest in using doxycycline for prophylaxis of this infection. However, the MIC of doxycycline for subsp. has not been reported previously. In this study, an culture system was utilized to determine that the MIC of doxycycline is 0.06 to 0.10 μg/ml for four strains of subsp. (Nichols, SS14, UW231B, and UW249B). The Nichols strain cultured with doxycycline was also tested for infectivity in rabbits, and the minimum bactericidal concentration (MBC) was found to be ≤0.1 μg/ml using this method. The low MIC and MBC values are consistent with the previously demonstrated clinical efficacy of doxycycline for the treatment of early syphilis. This study represents the first report of the susceptibility of to doxycycline, and the resulting information may be useful in the consideration of doxycycline for use in prevention of syphilis.