Which bacterium is the causal agent of syphilis?

The bacterium that causes syphilis is Treponema pallidum. It is a slender, corkscrew-shaped bacterium called a spirochete, and the syphilis-causing form is classified as the subspecies T. pallidum pallidum.

Fritz Schaudinn and Erich Hoffmann first identified the organism in syphilitic lesions in Berlin in 1905. Its helical shape and internal flagella help it move through mucous membranes and tiny breaks in skin. The bacterium is an obligate parasite with limited metabolic abilities, so it depends heavily on its host for nutrients.

The other options are associated with different diseases: Haemophilus ducreyi causes chancroid, Chlamydia trachomatis causes several infections including chlamydia, and Neisseria gonorrhoeae causes gonorrhea. Syphilis can progress through primary, secondary, latent, and tertiary stages, and infection during pregnancy can cause congenital syphilis. Penicillin G remains a highly effective treatment.