The French disease, for it was that, remained in me more than four months dormant before it showed itself, and then it broke out over my whole body at one instant…with certain blisters, of the size of six-pence, and rose colored.
—Benvenuto Cellini (1500-1571): The Life of Benvenuto Cellini
Spirochetes are bacteria with a spiral morphology ranging from loose coils to a rigid corkscrew shape. The three medically important genera include the cause of syphilis, the ancient scourge of sexual indiscretion, and Lyme disease, a more recently discovered consequence of an innocent walk in the woods.
The spiral morphology of spirochetes (Figure 37–1) is produced by a flexible, peptidoglycan cell wall around which several axial fibrils are wound. The cell wall and axial fibrils are completely covered by an outer bilayered membrane similar to the outer membrane of other gram-negative bacteria. In some species, a hyaluronic acid slime layer forms around the exterior of the organism and may contribute to its virulence. Spirochetes are motile, exhibiting rotation and flexion; this motility is believed to result from movement of the axial filaments, although the mechanism is not clear.
Spiral structure is wound around axial filaments
Motility includes rotation and flexion
Spirochete morphology. A1. Longitudinal surface view of typical spirochete. A2. Electron micrograph of Treponema with axial filaments extending most of cell length. B. Cross-section of typical spirochete. (Reproduced with permission from Willey JM: Prescott, Harley, & Klein’s Microbiology, 7th edition. McGraw-Hill, 2008.)
Many spirochetes are difficult to see by routine microscopy. Although they are gram negative, many either take stains poorly or are too thin (0.15 μm or less) to fall within the resolving power of the light microscope. Only darkfield microscopy (Figure 37–2), immunofluorescence, or special staining techniques can demonstrate these spirochetes. Other spirochetes such as Borrelia are wider and readily visible in stained preparations, even routine blood smears.
Many are thin and take stains poorly
Darkfield demonstrates spirochetes
Treponema pallidum seen by darkfield microscopy. The darkfield method creates a bright halo around the corkscrew-shaped spirochetes. (Reproduced with permission from Nester EW: Microbiology: A Human Perspective, 6th edition. 2009.)
GROWTH AND CLASSIFICATION
Parasitic spirochetes grow more slowly in vitro than most other disease-causing bacteria. Some species, including the causative agent of syphilis, have not been grown beyond a few generations in cell culture. Some are strict anaerobes, others require low concentrations of oxygen, and still others are aerobic. Compared with other bacterial groups, the taxonomy of the spirochetes is underdeveloped. Because spirochetes are difficult to grow and ...