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The spirochetes are a large, heterogeneous group of spiral, motile bacteria. One family (Spirochaetaceae) of the order Spirochaetales consists of two genera whose members are human pathogens, Borrelia and Treponema. The other family (Leptospiraceae) includes three genera: Leptospira, Leptonema, and Turneriella.

The spirochetes have many structural characteristics in common, as typified by Treponema pallidum (Figure 24-1). They are long, slender, helically coiled, spiral or corkscrew-shaped bacilli. T pallidum has an outer sheath or glycosaminoglycan coating. Inside the sheath is the outer membrane, which contains peptidoglycan and maintains the structural integrity of the organisms. Endoflagella (axial filaments) are the flagella-like organelles in the periplasmic space encased by the outer membrane. The endoflagella begin at each end of the organism and wind around it, extending to and overlapping at the midpoint. Inside the endoflagella is the inner membrane (cytoplasmic membrane) that provides osmotic stability and covers the protoplasmic cylinder. A series of cytoplasmic tubules (body fibrils) are inside the cell near the inner membrane. Treponemes reproduce by transverse fission.

Figure 24-1

Electron micrograph of whole-mounted Treponema pallidum subspecies pallidum. The endoflagella are clearly visible. Inset: Electron micrograph of thin-sectioned Treponema pallidum. Note the position of the endoflagella (EF) in the periplasmic space between the inner membrane (IM) and the outer membrane (OM). (Courtesy of EM Walker.)

The genus Treponema includes T pallidum subspecies pallidum, which causes syphilis; T pallidum subspecies pertenue, which causes yaws; T pallidum subspecies endemicum, which causes endemic syphilis (also called bejel); and Treponema carateum, which causes pinta.

Treponema Pallidum and Syphilis

Morphology and Identification

Typical Organisms

T pallidum are slender spirals measuring about 0.2 μm in width and 5–15 μm in length. The spiral coils are regularly spaced at a distance of 1 μm from one another. The organisms are actively motile, rotating steadily around their endoflagella even after attaching to cells by their tapered ends. The long axis of the spiral is ordinarily straight but may sometimes bend so that the organism forms a complete circle for moments at a time, returning then to its normal straight position.

The spirals are so thin that they are not readily seen unless immunofluorescent stain or dark-field illumination is used. They do not stain well with aniline dyes, but they can be seen in tissues when stained by a silver impregnation method.


Pathogenic T pallidum has never been cultured continuously on artificial media, in fertile eggs, or in tissue culture. Nonpathogenic treponemes (eg, Reiter strain) can be cultured anaerobically in vitro. They are saprophytes antigenically related to T pallidum.

Growth Characteristics

T pallidum is a microaerophilic organism; it survives best ...

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