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Freeze fracture and deep etching were used to investigate the ultrastructural basis for the observation that anti-treponemal antibodies bind poorly to the surface of virulent Treponema pallidum. Fractures of T. pallidum outer membranes contained scarce, uniformly sized intramembranous particles (IMPs). IMPs on the convex faces often appeared to form linear arrays that wound in spirals about the organism. In contrast to the outer membrane, IMPs of the cytoplasmic membrane were randomly distributed, numerous, and heterogeneous in size. In Escherichia coli and T. pallidum cofractures, IMPs of the E. coli outer membranes were densely packed within the concave fracture faces, while the T. pallidum fractures were identical to the experiments lacking the E. coli internal controls. Outer membranes of two representative nonpathogenic treponemes, Treponema phagedenis biotype Reiter and Treponema denticola, contained numerous IMPs, which segregated preferentially with the concave halves. Examination of apposed replicas and deep-etched specimens indicated that at least some of the IMPs extend through the T. pallidum outer membrane and are exposed on the surface of the organism. The outer membrane of intact T. pallidum appears to contain a paucity of integral membrane proteins that can serve as targets for specific antibodies. These rmdings appear to represent an unusual parasitic strategy for evasion of host humoral defenses.Syphilis, a sexually transmitted disease caused by the bacterium Treponema pallidum subsp. pallidum, continues to be a major global public health problem. In untreated individuals, syphilis is a chronic infection that progresses through stages with characteristic clinical manifestations. The mechanisms that enable virulent treponemes to survive for years in the face of vigorous humoral and cellular immune responses by the host are among the most poorly understood aspects of syphilis pathogenesis.Like all spirochetes, T. pallidum morphologically consists of an outer membrane that surrounds the periplasmic endoflagella, the cytoplasmic membrane, and the protoplasmic cylinder (1). Investigators have presumed that, as with other bacterial pathogens, surface-exposed molecules mediate the interactions between treponemes and their human hosts (2). For this reason, characterization of the outer membrane of T. pallidum and analysis of the structure's protein constituents have become major goals of treponemal research. Complicating such investigations is the fact that T. pallidum remains one of the few important pathogens of humans that cannot be maintained by continuous in vitro cultivation. Recombinant DNA and monoclonal antibody methodologies have facilitated analyses of a number of treponemal immunogens, although the precise cellular locations of most ofthem remain uncertain (3-7).Despite these limitations, recent investigations have demonstrated that the T. pallidum outer membrane differs significantly from the analogous structure of conventional Gram-negative bacteria. It does not contain lipopolysa...

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