T.pallidum can be grown in

Virulent Treponema pallidum strains cannot be cultivated in aificial media but have been maintained for many decades by a serial testicular passage in rabbits. One such strain (Nichol's strain), isolated from the brain of a fatal case of general paralysis of the insane in 1912, is still being propagated and used for diagnostic and research purposes.(Ref: Ananthanarayan 9th edition, p372)