Definition

Syphilis is caused by the gram-negative spirochete T. pallidum. As referred to in the introduction, there are several subspecies of T. pallidum. Although there is striking antigenic and genetic overlap within the species, the other subspecies including endemicum, pertenue, and carateum are generally of little virulence and produce nonvenereal disease. The name T. pallidum is commonly used to refer to subspecies pallidum, the etiology of syphilis. These spiral-shaped bacteria are approximately 5 to 15 pm long and roughly 0.2 pm in width (Fig. 1). The bacterium is surrounded by an outer membrane rich in phospholipids but relatively free of exposed surface proteins. It demonstrates corkscrew motility by way of endoflagella, which attach to either end of the organism and wrap around the body between the cytoplasmic membrane and the outer membrane. Darkfield microscopy is utilized for visualization of the bacteria, and the characteristic movement patterns of T. pallidum allow visual differentiation from other nonpathogenic treponemes (2).

T. pallidum has one of the smallest genomes among known bacteria and lacks many genes required for the synthesis of necessary compounds. It achieves glycolysis but scavenges many of its metabolic requirements from the host. The organism is actually quite fragile and does not live outside of a mammalian host; humans are the only known natural vectors. It cannot be cultured on artificial media, though there has been some success with growth in tissue culture, typically using rabbit testis. The difficulty with growing T. pallidum in the laboratory has translated to a relative paucity of information regarding its pathogenic features. There are no known virulence factors to distinguish T. pallidum from other nonpathogenic treponeme species. It is known to be capable of invasion and survival

FIGURE 1 This photomicrograph shows the unique spiral morphology of Treponema pallidum. Source: Courtesy of the CDC Public Health Image Library; special thanks to Bill Schwartz.

FIGURE 1 This photomicrograph shows the unique spiral morphology of Treponema pallidum. Source: Courtesy of the CDC Public Health Image Library; special thanks to Bill Schwartz.

in a wide variety of tissues and organs, based on its various clinical manifestations as well as findings in animal models. Nonetheless, the nature of this invasion is not yet understood. It may be related to the organism's ability to attach to a large diversity of cell types including epithelial, fibroblastic, and endothelial by way of adhesion molecules. Nonpathologic treponemal species do not adhere to cultured cells (3).

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