When chick embryo tissues cultivated for 13 days in Hanks's balanced salt solution (BSS) were infected with psittacosis virus (6BC), they did not support active viral multiplication until synthetic medium 199 of Parker (3) was added. By testing various combinations of the substances in this and other synthetic media, it was found that the minimum number of compounds required to effectively stimulate virus growth in the presence of BSS comprised the amino acids and water-soluble vitamins found in medium 199. Addition of either amino acids or water-soluble vitamins alone to BSS resulted in only slight stimulation of viral proliferation.

Many constituents of the synthetic media were found not to be essential to the stimulation of viral multiplication. The following substances added to a medium containing amino acids and water-soluble vitamins in BSS failed to increase the quantity of virus produced: diphosphopyridine nucleotide (DPN), triphosphopyridine nucleotide (TPN), coenzyme A, the fat-soluble vitamins, ribose sugars, and three biological reducing agents: cysteine, glutathione, and ascorbic acid. Among other substances that proved to be not essential a group of purines and pyrimidines present in medium 199 were found to be probably toxic to cells in the concentrations used, since virus titers were lower in media containing these compounds than in those from which they were absent.

A change in the nutritional status of these cells involving amino acids and water-soluble vitamins has thus permitted to transform a latent, undetectable viral infection to an inactive infection in vitro.

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