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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 one genus of medical importance: Leptospira.

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.


Electron micrograph of whole-mounted T. pallidum subspecies pallidum. The endoflagella are clearly visible. Inset: Electron micrograph of thin-sectioned T. 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.)


Morphology and Identification

A. 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.

B. Culture

Pathogenic T. pallidum has never been cultured continuously on artificial media, in fertile eggs, or in tissue culture.

In proper suspending fluids and in the presence of reducing substances, T. pallidum may remain motile for 3–6 days at 25°C. In whole blood or plasma stored at 4°C, organisms remain viable for at least 24 hours, which is of potential importance in blood transfusions.

D. Reactions to Physical and Chemical Agents

Drying kills the spirochete rapidly, as does elevation ...

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