Neisseria and Spirochetes

The sexually transmitted diseases are perhaps the most important infections acquired through the urogenital tract, from the social as well as medical points of view. Three frequent infectious diseases of this type are gonorrhea, syphilis, and chlamydial urethritis/cervicitis. All three infections are caused by bacteria. Gonorrhea is caused by Neisseria gonorrhoeae; syphilis by Treponema pallidum, a spirochete; and chlamydial infection by Chlamydia trachomatis. Neisseria gonorrhoeae can be grown on special laboratory culture media, but chlamydiae are obligate intracellular parasites (once considered viruses, in part for this reason) and require special laboratory techniques for isolation. Treponema pallidum, on the other hand, has not yet been grown in any laboratory culture system and is cultivated only in certain animals, such as the rabbit.

The bacterial groups to which these sexually transmitted agents belong contain other pathogenic species associated with nonsexually transmitted disease, that is, infections acquired through other entry portals. Still other species of Neisseria and Treponema are nonpathogenic, including some that are frequent members of the normal flora of various mucous membrane surfaces, particularly of the respiratory tract.