Ultrastructural changes observed in Rickettsia rickettsii incubated at different temperatures in cell-free medium.

When cells of Rickettsia rickettsii were suspended at different temperatures in growth medium free of host cells, ultrastructural changes were observed in some of these organisms. Depending upon the temperature and length of incubation, loosely organized cells developed into compact, intensely stained, rod-shaped organisms. These compact cells closely resembled the morphology of the original culture of Rickettsia used for inoculation. Morphological changes were primarily noted in cells maintained at 21 degrees C. The viability of the cells was also affected by the temperature of incubation.