T. pallidum is thecausativeagent of syphilis, avenereal disease first recognized in the16th century as the “great pox” that rapidly spread through Europe in association with ur-banization and military campaigns. Some argue that it was brought back from the New World by the sailors with Christopher Columbus. Its extended course and the protean, of-ten dramatic nature of its findings (genital ulcer, ataxia, dementia, ruptured aorta) are due to a state of balanced parasitism which spans decades. The cause of syphilis is actually a subspecies (T. pallidum subsp. pallidum) closely related to other agents which cause rare nonvenereal treponematoses.
T. pallidum is a slim (0.15μm) spirochete5 to 15 mlong withregular spirals thatresemble corkscrews with a wavelength (1μm) and amplitude (0.3μm). The organism is readily seen only by immunofluorescence, darkfield microscopy, or silver impregna-tion histologic techniques. Live T. pallidum cells show characteristic slow, rotating motility with sudden 90-degree angle flexion that suggests a gentleman quickly bowing at the waist.
T. pallidum has notbeengrownin theabsenceof cultured mammalian cells. Although itprefers low oxygen tensions, it is not a strict anaerobe. With careful control of oxygen tension and pH, the organism has now been shown to multiply through several genera-tions in primary cell culture, but is difficult to subculture. Growth is slow, with a mean generation time of about 30 hours. Information about its metabolic properties is limited because of the extreme difficulty in obtaining sufficient organisms for study. [In vivo growth is usually achieved by injection into rabbit testes — a source of antigens for spe-cific antibody testing.]
T. pallidum isextremelysusceptibleto any deviation from physiologic conditions.Itdies rapidly on drying and is readily killed by a wide range of detergents and disinfec-tants. The lethal effect of even modest elevations of temperature (41° to 42°C) was the basis of fever therapy early in the last century. These fragile properties account for its al-most exclusive transmission by direct contact.
Many studies of T. pallidum suggest its surface is inert lacking proteins and other exposed antigens. The search for such structures has been hampered by the inability to grow the organism free of animals or cell culture. This not only makes potential antigens difficult to isolate and purify but also introduces the issues of whether a component has been derived from the host or the bacteria. The outer membrane of T. pallidum contains antigenic transmembraneproteins and lipoproteinsbutin quanti-ties that are approximately 100-fold less than other Gram-negative bacteria such asEscherichia coli.