A sexually transmitted infection of humans caused by Treponema pallidum ssp. pallidum, a corkscrew-shaped motile bacterium (spirochete). Due to its narrow width, T. pallidum cannot be seen by light microscopy but can be observed with staining procedures (silver stain or immunofluorescence) and with dark-field, phase-contrast, or electron microscopy. Treponema pallidum cannot be continuously cultured in vitro by standard bacteriological methods. For antigen production and experimental studies, the organism is propagated by inoculation of laboratory rabbits. Limited multiplication of T. pallidum has been obtained in a tissue culture system at a temperature of 91–95°F (33–35°C) in an atmosphere containing 1–5% oxygen and in media containing reducing agents. The organism is very sensitive to environmental conditions and to physical and chemical agents. The complete genome sequence of the T. pallidum Nichols strain (1.13 × 10⁶ base pairs) has been determined. Analysis of the nucleotide sequence of the small, circular treponemal chromosome indicates that T. pallidum lacks the genetic information for many of the metabolic activities found in other bacteria. Thus, this spirochete is dependent upon the host for most of its nutritional requirements.