The growth of psittacosis virus (6BC) was studied in cultures of minced whole chick embryo tissue maintained in either Hanks-Simms solution or Hanks's balanced salt solution (BSS), and in neither medium could sustained, long-term virus growth take place. Addition of beef embryo extract (BEE) to cultures at a time when virus multiplication was declining reversed this general trend and resulted in greater virus growth. This virus-stimulating action of BEE was only partially diminished by colchicine, a mitotic inhibitor, indicating that the action of BEE was not due entirely to the development of a larger population of cells as a result of its enhancement of cell proliferation.

Chick embryo tissue cultivated for 13 days in BSS prior to infection lost its ability to support the growth of psittacosis virus, but this capacity could be restored by the addition of BEE, alone or with colchicine, at the time of infection. A significant amount of virus was adsorbed to tissue in BSS alone, indicating that the failure of virus to grow in depleted tissue maintained only in BSS after infection was not due entirely to failure of virus to attach to and invade the cells.

It was found that an ultrafiltrate and a dialysate of BEE contained the major part of the stimulating capacity of the whole extract, indicating that the active materials were substances of low molecular weights. Autoclaved lactalbumin hydrolysate was an active stimulator, suggesting that the materials responsible for its activity were relatively heat-stable.

Since a chemically defined medium (Parker 199) was equally effective in stimulating viral growth, it should be possible eventually to define the chemical nature of the virus stimulators.

The implications of the findings are discussed with special reference to their application in the study of tissue tropisms and of latency in viral infections of cells.

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