ABSTRACT
Treponema pallidum is an elongated spiral organism, the morphology of which differs markedly from that of other bacteria. The spirochete is 5 to 20 µm long and approximately 0.2m thick. Electron microscopy reveals an outer enveloping membrane, the periplast, contoured to give between 4 and 14 spirals.Within the underlying protoplasmic cylinder is an axial filament composed of several fibrils. The outer envelope is between 70 and 90 µm thick, lying between two membrane bundles of fibrils that are stretched from one end of the treponeme to the other and wind in helical coils. The marked thinness of the organism renders it relatively nondetectable by light microscopy. The presence of muramic acid suggests the existence of a cell wall component, which is thought to be the reason for the therapeutic effectiveness of penicillin. T. pallidum grows best at 34C to 35C both in vitro and in vivo. The sensitivity of the organism to elevated temperature [T. pallidum is destroyed at 105F (40.5C)] had long been clinically noted and constituted the basis for the efficacy of artificial fever therapy in the treatment of tertiary syphilis.
