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Three genera of spirochetes cause human infection: (1) Treponema, which causes syphilis and the nonvenereal treponematoses; (2) Borrelia, which causes Lyme disease and relapsing fever; and (3) Leptospira, which causes leptospirosis (Table 24–1).

TABLE 24–1Spirochetes of Medical Importance

Spirochetes are thin-walled, flexible, spiral rods (Figure 24–1). They are motile through the undulation of axial filaments that lie under the outer sheath. Treponemes and leptospirae are so thin that they are seen only by dark field microscopy, silver impregnation, or immunofluorescence. Borreliae are larger, accept Giemsa and other blood stains, and can be seen in the standard light microscope.

Additional information regarding the clinical aspects of infections caused by the organisms in this chapter is provided in Part IX entitled Infectious Diseases beginning on page 593.


Treponema pallidum—dark field microscopy. The coiled form of this spirochete is in the center of the field. (Source: Dr. Schwartz, Centers for Disease Control and Prevention.)


1. Treponema pallidum


Treponema pallidum causes syphilis.

Important Properties

Treponema pallidum has not been grown on bacteriologic media or in cell culture. Nonpathogenic treponemes, which are part of the normal flora of human mucous membranes, can be cultured.

Treponema pallidum grows very slowly. The medical importance of that fact is that antibiotics must be present at an effective level for several weeks to kill the organisms and cure the disease (see “Treatment” section later). For example, benzathine penicillin is the form of penicillin used to treat primary and secondary syphilis because the penicillin is released very slowly from this depot preparation, and bactericidal concentrations are present for weeks after administration of the antibiotic.

The antigens of T. pallidum induce specific antibodies, which can be detected by immunofluorescence or hemagglutination tests in the clinical laboratory. They also induce nonspecific antibodies (reagin),1 which can ...

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