In tissue cultures grown at 32°C., typhus Rickettsiae increase rapidly within the cytoplasm of infected cells up to about the 14th day. At this time practically every cell is infected and the majority of cells are distended with organisms.

This condition remains constant as long as successful cultures of the cells can be maintained (up to 52 days).

Loss in virulence does not take place during this period in vitro.

The number of Rickettsia-filled cells found in sections and the incubation period of the infection resulting from inoculation of cultures from each age group are definitely correlated.

The behavior of typhus Rickettsiae in dividing cells is described and methods of spread of the infection other than by mitosis of cells are discussed.

Normal tissues do not become infected in vitroto any considerable extent in spite of prolonged proximity to heavily infected cultures of scrotal sac exudate.

Complete anaerobiosis and alterations in pH do not alter the intracellular location of the organism in tissue cultures.

The organisms are not seen within nuclei of infected cells. They remain intact and infective for several weeks in cells which are kept alive but not multiplying. They disappear in less than 1 week, however, when the cells undergo degeneration.

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