Electron microscopy of treponemes subjected to the Treponema pallidum immobilization (TPI) test. I. Comparison of immunoimmobilized cells and control cells.

The ultrastructure of cells of T. pallidum Nichols subjected to the TPI test was studied in negatively stained specimens. Cells incubated in basal medium to which was added either human syphilis serum or heated guineapig serum (GPS) showed a normal morphology. This was also the cause for cells incubated with basal medium to which was added either human syphilis serum and heated GPS or normal human serum and unheated GPS. By dark-field microscopy cells obtained from these different incubation mixtures were found to be motile. In contrast, cells incubated in basal medium to which was added human syphilis serum and unheated GPS were all immobilized, and in the electron microscope they presented a morphology strikingly different from that of normal cells. The immunoimmobilized cells were swollen and their surface was completely covered with a layer of fuzzy material. The nature of this material and its possible role in rendering the treponemes immobile is discussed.